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2 | |
2 | |
3 | Guard - safe cleanup blocks |
3 | Guard - safe cleanup blocks |
4 | |
4 | |
5 | =head1 SYNOPSIS |
5 | =head1 SYNOPSIS |
6 | |
6 | |
7 | use Guard; |
7 | use Guard; |
8 | |
8 | |
9 | # temporarily chdir to "/etc" directory, but make sure |
9 | # temporarily chdir to "/etc" directory, but make sure |
10 | # to go back to "/" no matter how myfun exits: |
10 | # to go back to "/" no matter how myfun exits: |
11 | sub myfun { |
11 | sub myfun { |
12 | scope_guard { chdir "/" }; |
12 | scope_guard { chdir "/" }; |
13 | chdir "/etc"; |
13 | chdir "/etc"; |
14 | |
14 | |
15 | call_function_that_might_die_or_other_fun_stuff; |
15 | code_that_might_die_or_does_other_fun_stuff; |
16 | } |
16 | } |
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17 | |
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18 | # create an object that, when the last reference to it is gone, |
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19 | # invokes the given codeblock: |
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20 | my $guard = guard { print "destroyed!\n" }; |
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21 | undef $guard; # probably destroyed here |
17 | |
22 | |
18 | =head1 DESCRIPTION |
23 | =head1 DESCRIPTION |
19 | |
24 | |
20 | This module implements so-called "guards". A guard is something (usually |
25 | This module implements so-called "guards". A guard is something (usually |
21 | an object) that "guards" a resource, ensuring that it is cleaned up when |
26 | an object) that "guards" a resource, ensuring that it is cleaned up when |
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34 | |
39 | |
35 | =cut |
40 | =cut |
36 | |
41 | |
37 | package Guard; |
42 | package Guard; |
38 | |
43 | |
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44 | no warnings; |
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45 | |
39 | BEGIN { |
46 | BEGIN { |
40 | $VERSION = '0.1'; |
47 | $VERSION = '1.021'; |
41 | @ISA = qw(Exporter); |
48 | @ISA = qw(Exporter); |
42 | @EXPORT = qw(guard scope_guard); |
49 | @EXPORT = qw(guard scope_guard); |
43 | |
50 | |
44 | require Exporter; |
51 | require Exporter; |
45 | |
52 | |
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49 | |
56 | |
50 | our $DIED = sub { warn "$@" }; |
57 | our $DIED = sub { warn "$@" }; |
51 | |
58 | |
52 | =item scope_guard BLOCK |
59 | =item scope_guard BLOCK |
53 | |
60 | |
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61 | =item scope_guard ($coderef) |
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62 | |
54 | Registers a block that is executed when the current scope (block, |
63 | Registers a block that is executed when the current scope (block, |
55 | function, method, eval etc.) is exited. |
64 | function, method, eval etc.) is exited. |
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65 | |
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66 | See the EXCEPTIONS section for an explanation of how exceptions |
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67 | (i.e. C<die>) are handled inside guard blocks. |
56 | |
68 | |
57 | The description below sounds a bit complicated, but that's just because |
69 | The description below sounds a bit complicated, but that's just because |
58 | C<scope_guard> tries to get even corner cases "right": the goal is to |
70 | C<scope_guard> tries to get even corner cases "right": the goal is to |
59 | provide you with a rock solid clean up tool. |
71 | provide you with a rock solid clean up tool. |
60 | |
72 | |
61 | This is similar to this code fragment: |
73 | The behaviour is similar to this code fragment: |
62 | |
74 | |
63 | eval ... code following scope_guard ... |
75 | eval ... code following scope_guard ... |
64 | { |
76 | { |
65 | local $@; |
77 | local $@; |
66 | eval BLOCK; |
78 | eval BLOCK; |
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70 | |
82 | |
71 | Except it is much faster, and the whole thing gets executed even when the |
83 | Except it is much faster, and the whole thing gets executed even when the |
72 | BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means. |
84 | BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means. |
73 | |
85 | |
74 | If multiple BLOCKs are registered to the same scope, they will be executed |
86 | If multiple BLOCKs are registered to the same scope, they will be executed |
75 | in reverse order. Stuff like C<local> is managed via the same mechanism, |
87 | in reverse order. Other scope-related things such as C<local> are managed |
76 | so variables C<local>ised after calling C<scope_guard> will be restored |
88 | via the same mechanism, so variables C<local>ised I<after> calling |
77 | when the guard runs. |
89 | C<scope_guard> will be restored when the guard runs. |
78 | |
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79 | See B<EXCEPTIONS>, below, for an explanation of exception handling |
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80 | (C<die>) within guard blocks. |
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81 | |
90 | |
82 | Example: temporarily change the timezone for the current process, |
91 | Example: temporarily change the timezone for the current process, |
83 | ensuring it will be reset when the C<if> scope is exited: |
92 | ensuring it will be reset when the C<if> scope is exited: |
84 | |
93 | |
85 | use Guard; |
94 | use Guard; |
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95 | |
104 | |
96 | # do something with the new timezone |
105 | # do something with the new timezone |
97 | } |
106 | } |
98 | |
107 | |
99 | =item my $guard = guard BLOCK |
108 | =item my $guard = guard BLOCK |
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109 | |
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110 | =item my $guard = guard ($coderef) |
100 | |
111 | |
101 | Behaves the same as C<scope_guard>, except that instead of executing |
112 | Behaves the same as C<scope_guard>, except that instead of executing |
102 | the block on scope exit, it returns an object whose lifetime determines |
113 | the block on scope exit, it returns an object whose lifetime determines |
103 | when the BLOCK gets executed: when the last reference to the object gets |
114 | when the BLOCK gets executed: when the last reference to the object gets |
104 | destroyed, the BLOCK gets executed as with C<scope_guard>. |
115 | destroyed, the BLOCK gets executed as with C<scope_guard>. |
105 | |
116 | |
106 | The returned object can be copied as many times as you want. |
117 | See the EXCEPTIONS section for an explanation of how exceptions |
107 | |
118 | (i.e. C<die>) are handled inside guard blocks. |
108 | See B<EXCEPTIONS>, below, for an explanation of exception handling |
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109 | (C<die>) within guard blocks. |
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110 | |
119 | |
111 | Example: acquire a Coro::Semaphore for a second by registering a |
120 | Example: acquire a Coro::Semaphore for a second by registering a |
112 | timer. The timer callback references the guard used to unlock it again. |
121 | timer. The timer callback references the guard used to unlock it |
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122 | again. (Please ignore the fact that C<Coro::Semaphore> has a C<guard> |
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123 | method that does this already): |
113 | |
124 | |
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125 | use Guard; |
114 | use AnyEvent; |
126 | use Coro::AnyEvent; |
115 | use Coro::Semaphore; |
127 | use Coro::Semaphore; |
116 | |
128 | |
117 | my $sem = new Coro::Semaphore; |
129 | my $sem = new Coro::Semaphore; |
118 | |
130 | |
119 | sub lock_1s { |
131 | sub lock_for_a_second { |
120 | $sem->down; |
132 | $sem->down; |
121 | my $guard = guard { $sem->up }; |
133 | my $guard = guard { $sem->up }; |
122 | |
134 | |
123 | my $timer; |
135 | Coro::AnyEvent::sleep 1; |
124 | $timer = AnyEvent->timer (after => 1, sub { |
136 | |
125 | # do something |
137 | # $sem->up gets executed when returning |
126 | undef $sem; |
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127 | undef $timer; |
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128 | }); |
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129 | } |
138 | } |
130 | |
139 | |
131 | The advantage of doing this with a guard instead of simply calling C<< |
140 | The advantage of doing this with a guard instead of simply calling C<< |
132 | $sem->down >> in the callback is that you can opt not to create the timer, |
141 | $sem->down >> in the callback is that you can opt not to create the timer, |
133 | or your code can throw an exception before it can create the timer, or you |
142 | or your code can throw an exception before it can create the timer (or |
134 | can create multiple timers or other event watchers and only when the last |
143 | the thread gets canceled), or you can create multiple timers or other |
135 | one gets executed will the lock be unlocked. |
144 | event watchers and only when the last one gets executed will the lock be |
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145 | unlocked. Using the C<guard>, you do not have to worry about catching all |
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146 | the places where you have to unlock the semaphore. |
136 | |
147 | |
137 | =item Guard::cancel $guard |
148 | =item $guard->cancel |
138 | |
149 | |
139 | Calling this function will "disable" the guard object returned by the |
150 | Calling this function will "disable" the guard object returned by the |
140 | C<guard> function, i.e. it will free the BLOCK originally passed to |
151 | C<guard> function, i.e. it will free the BLOCK originally passed to |
141 | C<guard >and will arrange for the BLOCK not to be executed. |
152 | C<guard >and will arrange for the BLOCK not to be executed. |
142 | |
153 | |
143 | This can be useful when you use C<guard> to create a fatal cleanup handler |
154 | This can be useful when you use C<guard> to create a cleanup handler to be |
144 | and later decide it is no longer needed. |
155 | called under fatal conditions and later decide it is no longer needed. |
145 | |
156 | |
146 | =cut |
157 | =cut |
147 | |
158 | |
148 | 1; |
159 | 1; |
149 | |
160 | |
150 | =back |
161 | =back |
151 | |
162 | |
152 | =head1 EXCEPTIONS |
163 | =head1 EXCEPTIONS |
153 | |
164 | |
154 | Guard blocks should not normally throw exceptions (that is, C<die>). After |
165 | Guard blocks should not normally throw exceptions (that is, C<die>). After |
155 | all, they are usually used to clean up after such exceptions. However, if |
166 | all, they are usually used to clean up after such exceptions. However, |
156 | something truly exceptional is happening, a guard block should be allowed |
167 | if something truly exceptional is happening, a guard block should of |
157 | to die. Also, programming errors are a large source of exceptions, and the |
168 | course be allowed to die. Also, programming errors are a large source of |
158 | programmer certainly wants to know about those. |
169 | exceptions, and the programmer certainly wants to know about those. |
159 | |
170 | |
160 | Since in most cases, the block executing when the guard gets executes does |
171 | Since in most cases, the block executing when the guard gets executed does |
161 | not know or does not care about the guard blocks, it makes little sense to |
172 | not know or does not care about the guard blocks, it makes little sense to |
162 | let containing code handle the exception. |
173 | let containing code handle the exception. |
163 | |
174 | |
164 | Therefore, whenever a guard block throws an exception, it will be caught, |
175 | Therefore, whenever a guard block throws an exception, it will be caught |
165 | and this module will call the code reference stored in C<$Guard::DIED> |
176 | by Guard, followed by calling the code reference stored in C<$Guard::DIED> |
166 | (with C<$@> set to the actual exception), which is similar to how most |
177 | (with C<$@> set to the actual exception), which is similar to how most |
167 | event loops handle this case. |
178 | event loops handle this case. |
168 | |
179 | |
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180 | The default for C<$Guard::DIED> is to call C<warn "$@">, i.e. the error is |
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181 | printed as a warning and the program continues. |
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182 | |
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183 | The C<$@> variable will be restored to its value before the guard call in |
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184 | all cases, so guards will not disturb C<$@> in any way. |
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185 | |
169 | The code reference stored in C<$Guard::DIED> should not die (behaviour is |
186 | The code reference stored in C<$Guard::DIED> should not die (behaviour is |
170 | not guaranteed, but right now, the exception will simply be ignored). |
187 | not guaranteed, but right now, the exception will simply be ignored). |
171 | |
188 | |
172 | The default for C<$Guard::DIED> is to call C<warn "$@">. |
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173 | |
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174 | =head1 AUTHOR |
189 | =head1 AUTHOR |
175 | |
190 | |
176 | Marc Lehmann <schmorp@schmorp.de> |
191 | Marc Lehmann <schmorp@schmorp.de> |
177 | http://home.schmorp.de/ |
192 | http://home.schmorp.de/ |
178 | |
193 | |
179 | =head1 THANKS |
194 | =head1 THANKS |
180 | |
195 | |
181 | Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED> |
196 | Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED> |
182 | solution to the problem of exceptions. |
197 | solution to the problem of exceptions. |
183 | |
198 | |
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199 | =head1 SEE ALSO |
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200 | |
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201 | L<Scope::Guard> and L<Sub::ScopeFinalizer>, which actually implement |
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202 | dynamic guards only, not scoped guards, and have a lot higher CPU, memory |
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203 | and typing overhead. |
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204 | |
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205 | L<Hook::Scope>, which has apparently never been finished and can corrupt |
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206 | memory when used. |
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207 | |
184 | =cut |
208 | =cut |
185 | |
209 | |