1 | \chapter{Exception Features} |
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2 | |
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3 | This chapter covers the design and user interface of the \CFA |
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4 | exception-handling mechanism (EHM). % or exception system. |
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5 | |
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6 | We will begin with an overview of EHMs in general. It is not a strict |
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7 | definition of all EHMs nor an exaustive list of all possible features. |
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8 | However it does cover the most common structure and features found in them. |
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9 | |
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10 | % We should cover what is an exception handling mechanism and what is an |
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11 | % exception before this. Probably in the introduction. Some of this could |
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12 | % move there. |
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13 | \paragraph{Raise / Handle} |
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14 | An exception operation has two main parts: raise and handle. |
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15 | These terms are sometimes also known as throw and catch but this work uses |
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16 | throw/catch as a particular kind of raise/handle. |
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17 | These are the two parts that the user will write themselves and may |
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18 | be the only two pieces of the EHM that have any syntax in the language. |
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19 | |
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20 | \subparagraph{Raise} |
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21 | The raise is the starting point for exception handling. It marks the beginning |
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22 | of exception handling by raising an excepion, which passes it to |
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23 | the EHM. |
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24 | |
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25 | Some well known examples include the @throw@ statements of \Cpp and Java and |
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26 | the \codePy{raise} statement from Python. In real systems a raise may preform |
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27 | some other work (such as memory management) but for the purposes of this |
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28 | overview that can be ignored. |
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29 | |
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30 | \subparagraph{Handle} |
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31 | The purpose of most exception operations is to run some user code to handle |
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32 | that exception. This code is given, with some other information, in a handler. |
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33 | |
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34 | A handler has three common features: the previously mentioned user code, a |
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35 | region of code they cover and an exception label/condition that matches |
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36 | certain exceptions. |
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37 | Only raises inside the covered region and raising exceptions that match the |
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38 | label can be handled by a given handler. |
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39 | Different EHMs will have different rules to pick a handler |
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40 | if multipe handlers could be used such as ``best match" or ``first found". |
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41 | |
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42 | The @try@ statements of \Cpp, Java and Python are common examples. All three |
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43 | also show another common feature of handlers, they are grouped by the covered |
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44 | region. |
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45 | |
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46 | \paragraph{Propagation} |
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47 | After an exception is raised comes what is usually the biggest step for the |
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48 | EHM: finding and setting up the handler. The propogation from raise to |
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49 | handler can be broken up into three different tasks: searching for a handler, |
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50 | matching against the handler and installing the handler. |
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51 | |
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52 | \subparagraph{Searching} |
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53 | The EHM begins by searching for handlers that might be used to handle |
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54 | the exception. Searching is usually independent of the exception that was |
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55 | thrown as it looks for handlers that have the raise site in their covered |
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56 | region. |
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57 | This includes handlers in the current function, as well as any in callers |
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58 | on the stack that have the function call in their covered region. |
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59 | |
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60 | \subparagraph{Matching} |
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61 | Each handler found has to be matched with the raised exception. The exception |
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62 | label defines a condition that be use used with exception and decides if |
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63 | there is a match or not. |
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64 | |
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65 | In languages where the first match is used this step is intertwined with |
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66 | searching, a match check is preformed immediately after the search finds |
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67 | a possible handler. |
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68 | |
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69 | \subparagraph{Installing} |
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70 | After a handler is chosen it must be made ready to run. |
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71 | The implementation can vary widely to fit with the rest of the |
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72 | design of the EHM. The installation step might be trivial or it could be |
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73 | the most expensive step in handling an exception. The latter tends to be the |
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74 | case when stack unwinding is involved. |
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75 | |
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76 | If a matching handler is not guarantied to be found the EHM will need a |
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77 | different course of action here in the cases where no handler matches. |
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78 | This is only required with unchecked exceptions as checked exceptions |
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79 | (such as in Java) can make than guaranty. |
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80 | This different action can also be installing a handler but it is usually an |
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81 | implicat and much more general one. |
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82 | |
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83 | \subparagraph{Hierarchy} |
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84 | A common way to organize exceptions is in a hierarchical structure. |
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85 | This is especially true in object-orientated languages where the |
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86 | exception hierarchy is a natural extension of the object hierarchy. |
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87 | |
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88 | Consider the following hierarchy of exceptions: |
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89 | \begin{center} |
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90 | \input{exception-hierarchy} |
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91 | \end{center} |
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92 | |
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93 | A handler labelled with any given exception can handle exceptions of that |
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94 | type or any child type of that exception. The root of the exception hierarchy |
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95 | (here \codeC{exception}) acts as a catch-all, leaf types catch single types |
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96 | and the exceptions in the middle can be used to catch different groups of |
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97 | related exceptions. |
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98 | |
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99 | This system has some notable advantages, such as multiple levels of grouping, |
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100 | the ability for libraries to add new exception types and the isolation |
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101 | between different sub-hierarchies. |
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102 | This design is used in \CFA even though it is not a object-orientated |
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103 | language; so different tools are used to create the hierarchy. |
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104 | |
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105 | % Could I cite the rational for the Python IO exception rework? |
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106 | |
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107 | \paragraph{Completion} |
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108 | After the handler has finished the entire exception operation has to complete |
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109 | and continue executing somewhere else. This step is usually simple, |
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110 | both logically and in its implementation, as the installation of the handler |
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111 | is usually set up to do most of the work. |
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112 | |
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113 | The EHM can return control to many different places, |
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114 | the most common are after the handler definition and after the raise. |
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115 | |
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116 | \paragraph{Communication} |
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117 | For effective exception handling, additional information is usually passed |
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118 | from the raise to the handler. |
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119 | So far only communication of the exceptions' identity has been covered. |
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120 | A common method is putting fields into the exception instance and giving the |
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121 | handler access to them. |
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122 | |
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123 | \section{Virtuals} |
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124 | Virtual types and casts are not part of \CFA's EHM nor are they required for |
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125 | any EHM. |
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126 | However the \CFA uses a hierarchy built with the virtual system as the basis |
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127 | for exceptions and exception matching. |
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128 | |
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129 | The virtual system would have ideally been part of \CFA before the work |
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130 | on exception handling began, but unfortunately it was not. |
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131 | Because of this only the features and framework needed for the EHM were |
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132 | designed and implemented. Other features were considered to ensure that |
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133 | the structure could accomidate other desirable features but they were not |
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134 | implemented. |
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135 | The rest of this section will only discuss the finalized portion of the |
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136 | virtual system. |
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137 | |
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138 | The virtual system supports multiple ``trees" of types. Each tree is |
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139 | a simple hierarchy with a single root type. Each type in a tree has exactly |
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140 | one parent -- except for the root type which has zero parents -- and any |
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141 | number of children. |
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142 | Any type that belongs to any of these trees is called a virtual type. |
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143 | |
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144 | % A type's ancestors are its parent and its parent's ancestors. |
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145 | % The root type has no ancestors. |
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146 | % A type's decendents are its children and its children's decendents. |
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147 | |
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148 | Every virtual type also has a list of virtual members. Children inherit |
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149 | their parent's list of virtual members but may add new members to it. |
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150 | It is important to note that these are virtual members, not virtual methods |
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151 | of object-orientated programming, and can be of any type. |
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152 | However, since \CFA has function pointers and they are allowed, virtual |
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153 | members can be used to mimic virtual methods. |
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154 | |
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155 | Each virtual type has a unique id. |
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156 | This unique id and all the virtual members are combined |
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157 | into a virtual table type. Each virtual type has a pointer to a virtual table |
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158 | as a hidden field. |
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159 | |
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160 | Up until this point the virtual system is similar to ones found in |
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161 | object-orientated languages but this where \CFA diverges. Objects encapsulate a |
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162 | single set of behaviours in each type, universally across the entire program, |
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163 | and indeed all programs that use that type definition. In this sense the |
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164 | types are ``closed" and cannot be altered. |
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165 | |
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166 | In \CFA types do not encapsulate any behaviour. Traits are local and |
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167 | types can begin to statify a trait, stop satifying a trait or satify the same |
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168 | trait in a different way at any lexical location in the program. |
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169 | In this sense they are ``open" as they can change at any time. This means it |
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170 | is implossible to pick a single set of functions that repersent the type's |
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171 | implementation across the program. |
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172 | |
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173 | \CFA side-steps this issue by not having a single virtual table for each |
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174 | type. A user can define virtual tables which are filled in at their |
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175 | declaration and given a name. Anywhere that name is visible, even if it was |
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176 | defined locally inside a function (although that means it will not have a |
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177 | static lifetime), it can be used. |
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178 | Specifically, a virtual type is ``bound" to a virtual table which |
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179 | sets the virtual members for that object. The virtual members can be accessed |
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180 | through the object. |
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181 | |
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182 | While much of the virtual infrastructure is created, it is currently only used |
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183 | internally for exception handling. The only user-level feature is the virtual |
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184 | cast, which is the same as the \Cpp \codeCpp{dynamic_cast}. |
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185 | \label{p:VirtualCast} |
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186 | \begin{cfa} |
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187 | (virtual TYPE)EXPRESSION |
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188 | \end{cfa} |
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189 | Note, the syntax and semantics matches a C-cast, rather than the function-like |
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190 | \Cpp syntax for special casts. Both the type of @EXPRESSION@ and @TYPE@ must be |
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191 | a pointer to a virtual type. |
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192 | The cast dynamically checks if the @EXPRESSION@ type is the same or a sub-type |
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193 | of @TYPE@, and if true, returns a pointer to the |
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194 | @EXPRESSION@ object, otherwise it returns @0p@ (null pointer). |
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195 | |
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196 | \section{Exception} |
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197 | % Leaving until later, hopefully it can talk about actual syntax instead |
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198 | % of my many strange macros. Syntax aside I will also have to talk about the |
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199 | % features all exceptions support. |
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200 | |
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201 | Exceptions are defined by the trait system; there are a series of traits, and |
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202 | if a type satisfies them, then it can be used as an exception. The following |
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203 | is the base trait all exceptions need to match. |
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204 | \begin{cfa} |
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205 | trait is_exception(exceptT &, virtualT &) { |
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206 | // Numerous imaginary assertions. |
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207 | }; |
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208 | \end{cfa} |
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209 | The trait is defined over two types, the exception type and the virtual table |
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210 | type. Each exception type should have but a single virtual table type. |
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211 | Now there are no actual assertions in this trait because the trait system |
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212 | actually can't express them (adding such assertions would be part of |
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213 | completing the virtual system). The imaginary assertions would probably come |
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214 | from a trait defined by the virtual system, and state that the exception type |
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215 | is a virtual type, is a decendent of @exception_t@ (the base exception type) |
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216 | and note its virtual table type. |
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217 | |
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218 | % I did have a note about how it is the programmer's responsibility to make |
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219 | % sure the function is implemented correctly. But this is true of every |
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220 | % similar system I know of (except Agda's I guess) so I took it out. |
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221 | |
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222 | There are two more traits for exceptions defined as follows: |
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223 | \begin{cfa} |
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224 | trait is_termination_exception( |
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225 | exceptT &, virtualT & | is_exception(exceptT, virtualT)) { |
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226 | void defaultTerminationHandler(exceptT &); |
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227 | }; |
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228 | |
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229 | trait is_resumption_exception( |
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230 | exceptT &, virtualT & | is_exception(exceptT, virtualT)) { |
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231 | void defaultResumptionHandler(exceptT &); |
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232 | }; |
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233 | \end{cfa} |
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234 | Both traits ensure a pair of types are an exception type and its virtual table |
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235 | and defines one of the two default handlers. The default handlers are used |
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236 | as fallbacks and are discussed in detail in \vref{s:ExceptionHandling}. |
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237 | |
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238 | However, all three of these traits can be tricky to use directly. |
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239 | While there is a bit of repetition required, |
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240 | the largest issue is that the virtual table type is mangled and not in a user |
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241 | facing way. So these three macros are provided to wrap these traits to |
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242 | simplify referring to the names: |
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243 | @IS_EXCEPTION@, @IS_TERMINATION_EXCEPTION@ and @IS_RESUMPTION_EXCEPTION@. |
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244 | |
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245 | All three take one or two arguments. The first argument is the name of the |
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246 | exception type. The macro passes its unmangled and mangled form to the trait. |
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247 | The second (optional) argument is a parenthesized list of polymorphic |
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248 | arguments. This argument is only used with polymorphic exceptions and the |
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249 | list is be passed to both types. |
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250 | In the current set-up, the two types always have the same polymorphic |
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251 | arguments so these macros can be used without losing flexibility. |
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252 | |
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253 | For example consider a function that is polymorphic over types that have a |
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254 | defined arithmetic exception: |
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255 | \begin{cfa} |
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256 | forall(Num | IS_EXCEPTION(Arithmetic, (Num))) |
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257 | void some_math_function(Num & left, Num & right); |
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258 | \end{cfa} |
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259 | |
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260 | \section{Exception Handling} |
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261 | \label{s:ExceptionHandling} |
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262 | \CFA provides two kinds of exception handling: termination and resumption. |
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263 | These twin operations are the core of \CFA's exception handling mechanism. |
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264 | This section will cover the general patterns shared by the two operations and |
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265 | then go on to cover the details each individual operation. |
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266 | |
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267 | Both operations follow the same set of steps. |
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268 | Both start with the user preforming a raise on an exception. |
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269 | Then the exception propogates up the stack. |
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270 | If a handler is found the exception is caught and the handler is run. |
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271 | After that control returns to normal execution. |
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272 | If the search fails a default handler is run and then control |
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273 | returns to normal execution after the raise. |
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274 | |
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275 | This general description covers what the two kinds have in common. |
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276 | Differences include how propogation is preformed, where exception continues |
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277 | after an exception is caught and handled and which default handler is run. |
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278 | |
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279 | \subsection{Termination} |
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280 | \label{s:Termination} |
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281 | Termination handling is the familiar kind and used in most programming |
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282 | languages with exception handling. |
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283 | It is dynamic, non-local goto. If the raised exception is matched and |
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284 | handled the stack is unwound and control will (usually) continue the function |
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285 | on the call stack that defined the handler. |
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286 | Termination is commonly used when an error has occurred and recovery is |
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287 | impossible locally. |
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288 | |
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289 | % (usually) Control can continue in the current function but then a different |
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290 | % control flow construct should be used. |
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291 | |
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292 | A termination raise is started with the @throw@ statement: |
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293 | \begin{cfa} |
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294 | throw EXPRESSION; |
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295 | \end{cfa} |
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296 | The expression must return a reference to a termination exception, where the |
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297 | termination exception is any type that satisfies the trait |
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298 | @is_termination_exception@ at the call site. |
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299 | Through \CFA's trait system the trait functions are implicity passed into the |
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300 | throw code and the EHM. |
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301 | A new @defaultTerminationHandler@ can be defined in any scope to |
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302 | change the throw's behavior (see below). |
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303 | |
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304 | The throw will copy the provided exception into managed memory to ensure |
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305 | the exception is not destroyed if the stack is unwound. |
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306 | It is the user's responsibility to ensure the original exception is cleaned |
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307 | up wheither the stack is unwound or not. Allocating it on the stack is |
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308 | usually sufficient. |
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309 | |
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310 | Then propogation starts with the search. \CFA uses a ``first match" rule so |
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311 | matching is preformed with the copied exception as the search continues. |
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312 | It starts from the throwing function and proceeds to the base of the stack, |
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313 | from callee to caller. |
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314 | At each stack frame, a check is made for resumption handlers defined by the |
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315 | @catch@ clauses of a @try@ statement. |
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316 | \begin{cfa} |
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317 | try { |
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318 | GUARDED_BLOCK |
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319 | } catch (EXCEPTION_TYPE$\(_1\)$ * [NAME$\(_1\)$]) { |
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320 | HANDLER_BLOCK$\(_1\)$ |
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321 | } catch (EXCEPTION_TYPE$\(_2\)$ * [NAME$\(_2\)$]) { |
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322 | HANDLER_BLOCK$\(_2\)$ |
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323 | } |
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324 | \end{cfa} |
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325 | When viewed on its own, a try statement will simply execute the statements |
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326 | in @GUARDED_BLOCK@ and when those are finished the try statement finishes. |
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327 | |
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328 | However, while the guarded statements are being executed, including any |
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329 | invoked functions, all the handlers in the statement are now on the search |
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330 | path. If a termination exception is thrown and not handled further up the |
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331 | stack they will be matched against the exception. |
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332 | |
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333 | Exception matching checks the handler in each catch clause in the order |
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334 | they appear, top to bottom. If the representation of the thrown exception type |
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335 | is the same or a descendant of @EXCEPTION_TYPE@$_i$ then @NAME@$_i$ |
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336 | (if provided) is |
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337 | bound to a pointer to the exception and the statements in @HANDLER_BLOCK@$_i$ |
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338 | are executed. If control reaches the end of the handler, the exception is |
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339 | freed and control continues after the try statement. |
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340 | |
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341 | If no termination handler is found during the search then the default handler |
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342 | (@defaultTerminationHandler@) is run. |
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343 | Through \CFA's trait system the best match at the throw sight will be used. |
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344 | This function is run and is passed the copied exception. After the default |
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345 | handler is run control continues after the throw statement. |
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346 | |
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347 | There is a global @defaultTerminationHandler@ that is polymorphic over all |
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348 | exception types. Since it is so general a more specific handler can be |
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349 | defined and will be used for those types, effectively overriding the handler |
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350 | for particular exception type. |
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351 | The global default termination handler performs a cancellation |
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352 | (see \vref{s:Cancellation}) on the current stack with the copied exception. |
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353 | |
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354 | \subsection{Resumption} |
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355 | \label{s:Resumption} |
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356 | |
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357 | Resumption exception handling is less common than termination but is |
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358 | just as old~\cite{Goodenough75} and is simpler in many ways. |
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359 | It is a dynamic, non-local function call. If the raised exception is |
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360 | matched a closure will be taken from up the stack and executed, |
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361 | after which the raising function will continue executing. |
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362 | These are most often used when an error occurred and if the error is repaired |
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363 | then the function can continue. |
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364 | |
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365 | A resumption raise is started with the @throwResume@ statement: |
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366 | \begin{cfa} |
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367 | throwResume EXPRESSION; |
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368 | \end{cfa} |
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369 | It works much the same way as the termination throw. |
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370 | The expression must return a reference to a resumption exception, |
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371 | where the resumption exception is any type that satisfies the trait |
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372 | @is_resumption_exception@ at the call site. |
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373 | The assertions from this trait are available to |
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374 | the exception system while handling the exception. |
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375 | |
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376 | At run-time, no exception copy is made. |
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377 | As the stack is not unwound the exception and |
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378 | any values on the stack will remain in scope while the resumption is handled. |
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379 | |
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380 | The EHM then begins propogation. The search starts from the raise in the |
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381 | resuming function and proceeds to the base of the stack, from callee to caller. |
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382 | At each stack frame, a check is made for resumption handlers defined by the |
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383 | @catchResume@ clauses of a @try@ statement. |
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384 | \begin{cfa} |
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385 | try { |
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386 | GUARDED_BLOCK |
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387 | } catchResume (EXCEPTION_TYPE$\(_1\)$ * [NAME$\(_1\)$]) { |
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388 | HANDLER_BLOCK$\(_1\)$ |
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389 | } catchResume (EXCEPTION_TYPE$\(_2\)$ * [NAME$\(_2\)$]) { |
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390 | HANDLER_BLOCK$\(_2\)$ |
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391 | } |
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392 | \end{cfa} |
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393 | % I wonder if there would be some good central place for this. |
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394 | Note that termination handlers and resumption handlers may be used together |
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395 | in a single try statement, intermixing @catch@ and @catchResume@ freely. |
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396 | Each type of handler will only interact with exceptions from the matching |
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397 | type of raise. |
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398 | When a try statement is executed it simply executes the statements in the |
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399 | @GUARDED_BLOCK@ and then finishes. |
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400 | |
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401 | However, while the guarded statements are being executed, including any |
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402 | invoked functions, all the handlers in the statement are now on the search |
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403 | path. If a resumption exception is reported and not handled further up the |
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404 | stack they will be matched against the exception. |
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405 | |
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406 | Exception matching checks the handler in each catch clause in the order |
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407 | they appear, top to bottom. If the representation of the thrown exception type |
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408 | is the same or a descendant of @EXCEPTION_TYPE@$_i$ then @NAME@$_i$ |
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409 | (if provided) is bound to a pointer to the exception and the statements in |
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410 | @HANDLER_BLOCK@$_i$ are executed. |
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411 | If control reaches the end of the handler, execution continues after the |
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412 | the raise statement that raised the handled exception. |
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413 | |
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414 | Like termination, if no resumption handler is found, the default handler |
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415 | visible at the throw statement is called. It will use the best match at the |
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416 | call sight according to \CFA's overloading rules. The default handler is |
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417 | passed the exception given to the throw. When the default handler finishes |
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418 | execution continues after the raise statement. |
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419 | |
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420 | There is a global @defaultResumptionHandler@ is polymorphic over all |
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421 | termination exceptions and preforms a termination throw on the exception. |
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422 | The @defaultTerminationHandler@ for that raise is matched at the original |
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423 | raise statement (the resumption @throwResume@) and it can be customized by |
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424 | introducing a new or better match as well. |
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425 | |
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426 | \subsubsection{Resumption Marking} |
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427 | \label{s:ResumptionMarking} |
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428 | A key difference between resumption and termination is that resumption does |
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429 | not unwind the stack. A side effect that is that when a handler is matched |
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430 | and run it's try block (the guarded statements) and every try statement |
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431 | searched before it are still on the stack. This can lead to the recursive |
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432 | resumption problem. |
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433 | |
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434 | The recursive resumption problem is any situation where a resumption handler |
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435 | ends up being called while it is running. |
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436 | Consider a trivial case: |
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437 | \begin{cfa} |
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438 | try { |
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439 | throwResume (E &){}; |
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440 | } catchResume(E *) { |
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441 | throwResume (E &){}; |
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442 | } |
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443 | \end{cfa} |
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444 | When this code is executed the guarded @throwResume@ will throw, start a |
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445 | search and match the handler in the @catchResume@ clause. This will be |
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446 | call and placed on the stack on top of the try-block. The second throw then |
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447 | throws and will search the same try block and put call another instance of the |
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448 | same handler leading to an infinite loop. |
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449 | |
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450 | This situation is trivial and easy to avoid, but much more complex cycles |
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451 | can form with multiple handlers and different exception types. |
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452 | |
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453 | To prevent all of these cases we mark try statements on the stack. |
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454 | A try statement is marked when a match check is preformed with it and an |
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455 | exception. The statement will be unmarked when the handling of that exception |
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456 | is completed or the search completes without finding a handler. |
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457 | While a try statement is marked its handlers are never matched, effectify |
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458 | skipping over it to the next try statement. |
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459 | |
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460 | \begin{center} |
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461 | \input{stack-marking} |
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462 | \end{center} |
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463 | |
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464 | These rules mirror what happens with termination. |
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465 | When a termination throw happens in a handler the search will not look at |
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466 | any handlers from the original throw to the original catch because that |
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467 | part of the stack has been unwound. |
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468 | A resumption raise in the same situation wants to search the entire stack, |
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469 | but it will not try to match the exception with try statements in the section |
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470 | that would have been unwound as they are marked. |
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471 | |
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472 | The symmetry between resumption termination is why this pattern was picked. |
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473 | Other patterns, such as marking just the handlers that caught, also work but |
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474 | lack the symmetry means there are more rules to remember. |
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475 | |
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476 | \section{Conditional Catch} |
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477 | Both termination and resumption handler clauses can be given an additional |
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478 | condition to further control which exceptions they handle: |
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479 | \begin{cfa} |
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480 | catch (EXCEPTION_TYPE * [NAME] ; CONDITION) |
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481 | \end{cfa} |
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482 | First, the same semantics is used to match the exception type. Second, if the |
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483 | exception matches, @CONDITION@ is executed. The condition expression may |
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484 | reference all names in scope at the beginning of the try block and @NAME@ |
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485 | introduced in the handler clause. If the condition is true, then the handler |
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486 | matches. Otherwise, the exception search continues as if the exception type |
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487 | did not match. |
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488 | |
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489 | The condition matching allows finer matching by allowing the match to check |
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490 | more kinds of information than just the exception type. |
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491 | \begin{cfa} |
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492 | try { |
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493 | handle1 = open( f1, ... ); |
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494 | handle2 = open( f2, ... ); |
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495 | handle3 = open( f3, ... ); |
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496 | ... |
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497 | } catch( IOFailure * f ; fd( f ) == f1 ) { |
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498 | // Only handle IO failure for f1. |
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499 | } catch( IOFailure * f ; fd( f ) == f3 ) { |
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500 | // Only handle IO failure for f3. |
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501 | } |
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502 | // Can't handle a failure relating to f2 here. |
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503 | \end{cfa} |
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504 | In this example the file that experianced the IO error is used to decide |
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505 | which handler should be run, if any at all. |
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506 | |
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507 | \begin{comment} |
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508 | % I know I actually haven't got rid of them yet, but I'm going to try |
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509 | % to write it as if I had and see if that makes sense: |
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510 | \section{Reraising} |
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511 | \label{s:Reraising} |
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512 | Within the handler block or functions called from the handler block, it is |
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513 | possible to reraise the most recently caught exception with @throw@ or |
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514 | @throwResume@, respectively. |
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515 | \begin{cfa} |
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516 | try { |
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517 | ... |
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518 | } catch( ... ) { |
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519 | ... throw; |
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520 | } catchResume( ... ) { |
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521 | ... throwResume; |
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522 | } |
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523 | \end{cfa} |
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524 | The only difference between a raise and a reraise is that reraise does not |
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525 | create a new exception; instead it continues using the current exception, \ie |
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526 | no allocation and copy. However the default handler is still set to the one |
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527 | visible at the raise point, and hence, for termination could refer to data that |
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528 | is part of an unwound stack frame. To prevent this problem, a new default |
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529 | handler is generated that does a program-level abort. |
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530 | \end{comment} |
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531 | |
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532 | \subsection{Comparison with Reraising} |
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533 | A more popular way to allow handlers to match in more detail is to reraise |
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534 | the exception after it has been caught if it could not be handled here. |
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535 | On the surface these two features seem interchangable. |
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536 | |
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537 | If we used @throw;@ to start a termination reraise then these two statements |
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538 | would have the same behaviour: |
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539 | \begin{cfa} |
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540 | try { |
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541 | do_work_may_throw(); |
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542 | } catch(exception_t * exc ; can_handle(exc)) { |
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543 | handle(exc); |
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544 | } |
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545 | \end{cfa} |
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546 | |
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547 | \begin{cfa} |
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548 | try { |
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549 | do_work_may_throw(); |
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550 | } catch(exception_t * exc) { |
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551 | if (can_handle(exc)) { |
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552 | handle(exc); |
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553 | } else { |
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554 | throw; |
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555 | } |
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556 | } |
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557 | \end{cfa} |
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558 | If there are further handlers after this handler only the first version will |
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559 | check them. If multiple handlers on a single try block that could handle the |
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560 | same exception the translations get more complex but they are equivilantly |
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561 | powerful. |
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562 | |
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563 | Until stack unwinding comes into the picture. In termination handling, a |
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564 | conditional catch happens before the stack is unwound, but a reraise happens |
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565 | afterwards. Normally this might only cause you to loose some debug |
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566 | information you could get from a stack trace (and that can be side stepped |
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567 | entirely by collecting information during the unwind). But for \CFA there is |
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568 | another issue, if the exception isn't handled the default handler should be |
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569 | run at the site of the original raise. |
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570 | |
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571 | There are two problems with this: the site of the original raise doesn't |
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572 | exist anymore and the default handler might not exist anymore. The site will |
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573 | always be removed as part of the unwinding, often with the entirety of the |
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574 | function it was in. The default handler could be a stack allocated nested |
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575 | function removed during the unwind. |
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576 | |
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577 | This means actually trying to pretend the catch didn't happening, continuing |
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578 | the original raise instead of starting a new one, is infeasible. |
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579 | That is the expected behaviour for most languages and we can't replicate |
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580 | that behaviour. |
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581 | |
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582 | \section{Finally Clauses} |
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583 | \label{s:FinallyClauses} |
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584 | Finally clauses are used to preform unconditional clean-up when leaving a |
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585 | scope and are placed at the end of a try statement after any handler clauses: |
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586 | \begin{cfa} |
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587 | try { |
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588 | GUARDED_BLOCK |
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589 | } ... // any number or kind of handler clauses |
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590 | ... finally { |
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591 | FINALLY_BLOCK |
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592 | } |
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593 | \end{cfa} |
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594 | The @FINALLY_BLOCK@ is executed when the try statement is removed from the |
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595 | stack, including when the @GUARDED_BLOCK@ finishes, any termination handler |
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596 | finishes or during an unwind. |
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597 | The only time the block is not executed is if the program is exited before |
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598 | the stack is unwound. |
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599 | |
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600 | Execution of the finally block should always finish, meaning control runs off |
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601 | the end of the block. This requirement ensures control always continues as if |
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602 | the finally clause is not present, \ie finally is for cleanup not changing |
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603 | control flow. |
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604 | Because of this requirement, local control flow out of the finally block |
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605 | is forbidden. The compiler precludes any @break@, @continue@, @fallthru@ or |
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606 | @return@ that causes control to leave the finally block. Other ways to leave |
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607 | the finally block, such as a long jump or termination are much harder to check, |
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608 | and at best requiring additional run-time overhead, and so are only |
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609 | discouraged. |
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610 | |
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611 | Not all languages with unwinding have finally clauses. Notably \Cpp does |
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612 | without it as descructors serve a similar role. Although destructors and |
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613 | finally clauses can be used in many of the same areas they have their own |
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614 | use cases like top-level functions and lambda functions with closures. |
---|
615 | Destructors take a bit more work to set up but are much easier to reuse while |
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616 | finally clauses are good for one-off uses and |
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617 | can easily include local information. |
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618 | |
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619 | \section{Cancellation} |
---|
620 | \label{s:Cancellation} |
---|
621 | Cancellation is a stack-level abort, which can be thought of as as an |
---|
622 | uncatchable termination. It unwinds the entire current stack, and if |
---|
623 | possible forwards the cancellation exception to a different stack. |
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624 | |
---|
625 | Cancellation is not an exception operation like termination or resumption. |
---|
626 | There is no special statement for starting a cancellation; instead the standard |
---|
627 | library function @cancel_stack@ is called passing an exception. Unlike a |
---|
628 | raise, this exception is not used in matching only to pass information about |
---|
629 | the cause of the cancellation. |
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630 | (This also means matching cannot fail so there is no default handler.) |
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631 | |
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632 | After @cancel_stack@ is called the exception is copied into the EHM's memory |
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633 | and the current stack is |
---|
634 | unwound. After that it depends one which stack is being cancelled. |
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635 | |
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636 | \paragraph{Main Stack} |
---|
637 | The main stack is the one used by the program main at the start of execution, |
---|
638 | and is the only stack in a sequential program. |
---|
639 | After the main stack is unwound there is a program-level abort. |
---|
640 | |
---|
641 | There are two reasons for this. The first is that it obviously had to do this |
---|
642 | in a sequential program as there is nothing else to notify and the simplicity |
---|
643 | of keeping the same behaviour in sequential and concurrent programs is good. |
---|
644 | Also, even in concurrent programs there is no stack that an innate connection |
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645 | to, so it would have be explicitly managed. |
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646 | |
---|
647 | \paragraph{Thread Stack} |
---|
648 | A thread stack is created for a \CFA @thread@ object or object that satisfies |
---|
649 | the @is_thread@ trait. |
---|
650 | After a thread stack is unwound there exception is stored until another |
---|
651 | thread attempts to join with it. Then the exception @ThreadCancelled@, |
---|
652 | which stores a reference to the thread and to the exception passed to the |
---|
653 | cancellation, is reported from the join. |
---|
654 | There is one difference between an explicit join (with the @join@ function) |
---|
655 | and an implicit join (from a destructor call). The explicit join takes the |
---|
656 | default handler (@defaultResumptionHandler@) from its calling context while |
---|
657 | the implicit join provides its own which does a program abort if the |
---|
658 | @ThreadCancelled@ exception cannot be handled. |
---|
659 | |
---|
660 | Communication is done at join because a thread only has to have to points of |
---|
661 | communication with other threads: start and join. |
---|
662 | Since a thread must be running to perform a cancellation (and cannot be |
---|
663 | cancelled from another stack), the cancellation must be after start and |
---|
664 | before the join. So join is the one that we will use. |
---|
665 | |
---|
666 | % TODO: Find somewhere to discuss unwind collisions. |
---|
667 | The difference between the explicit and implicit join is for safety and |
---|
668 | debugging. It helps prevent unwinding collisions by avoiding throwing from |
---|
669 | a destructor and prevents cascading the error across multiple threads if |
---|
670 | the user is not equipped to deal with it. |
---|
671 | Also you can always add an explicit join if that is the desired behaviour. |
---|
672 | |
---|
673 | \paragraph{Coroutine Stack} |
---|
674 | A coroutine stack is created for a @coroutine@ object or object that |
---|
675 | satisfies the @is_coroutine@ trait. |
---|
676 | After a coroutine stack is unwound control returns to the resume function |
---|
677 | that most recently resumed it. The resume statement reports a |
---|
678 | @CoroutineCancelled@ exception, which contains a references to the cancelled |
---|
679 | coroutine and the exception used to cancel it. |
---|
680 | The resume function also takes the @defaultResumptionHandler@ from the |
---|
681 | caller's context and passes it to the internal report. |
---|
682 | |
---|
683 | A coroutine knows of two other coroutines, its starter and its last resumer. |
---|
684 | The starter has a much more distant connection while the last resumer just |
---|
685 | (in terms of coroutine state) called resume on this coroutine, so the message |
---|
686 | is passed to the latter. |
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