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13 | chdir "/etc"; |
13 | chdir "/etc"; |
14 | |
14 | |
15 | code_that_might_die_or_does_other_fun_stuff; |
15 | code_that_might_die_or_does_other_fun_stuff; |
16 | } |
16 | } |
17 | |
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 |
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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 |
22 | expected. |
27 | expected. |
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37 | package Guard; |
42 | package Guard; |
38 | |
43 | |
39 | no warnings; |
44 | no warnings; |
40 | |
45 | |
41 | BEGIN { |
46 | BEGIN { |
42 | $VERSION = '1.02'; |
47 | $VERSION = '1.021'; |
43 | @ISA = qw(Exporter); |
48 | @ISA = qw(Exporter); |
44 | @EXPORT = qw(guard scope_guard); |
49 | @EXPORT = qw(guard scope_guard); |
45 | |
50 | |
46 | require Exporter; |
51 | require Exporter; |
47 | |
52 | |
… | |
… | |
50 | } |
55 | } |
51 | |
56 | |
52 | our $DIED = sub { warn "$@" }; |
57 | our $DIED = sub { warn "$@" }; |
53 | |
58 | |
54 | =item scope_guard BLOCK |
59 | =item scope_guard BLOCK |
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60 | |
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61 | =item scope_guard ($coderef) |
55 | |
62 | |
56 | Registers a block that is executed when the current scope (block, |
63 | Registers a block that is executed when the current scope (block, |
57 | function, method, eval etc.) is exited. |
64 | function, method, eval etc.) is exited. |
58 | |
65 | |
59 | See the EXCEPTIONS section for an explanation of how exceptions |
66 | See the EXCEPTIONS section for an explanation of how exceptions |
… | |
… | |
98 | # do something with the new timezone |
105 | # do something with the new timezone |
99 | } |
106 | } |
100 | |
107 | |
101 | =item my $guard = guard BLOCK |
108 | =item my $guard = guard BLOCK |
102 | |
109 | |
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110 | =item my $guard = guard ($coderef) |
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111 | |
103 | 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 |
104 | 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 |
105 | 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 |
106 | destroyed, the BLOCK gets executed as with C<scope_guard>. |
115 | destroyed, the BLOCK gets executed as with C<scope_guard>. |
107 | |
116 | |
108 | The returned object can be copied as many times as you want. |
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109 | |
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110 | See the EXCEPTIONS section for an explanation of how exceptions |
117 | See the EXCEPTIONS section for an explanation of how exceptions |
111 | (i.e. C<die>) are handled inside guard blocks. |
118 | (i.e. C<die>) are handled inside guard blocks. |
112 | |
119 | |
113 | Example: acquire a Coro::Semaphore for a second by registering a |
120 | Example: acquire a Coro::Semaphore for a second by registering a |
114 | timer. The timer callback references the guard used to unlock it |
121 | timer. The timer callback references the guard used to unlock it |
115 | again. (Please ignore the fact that C<Coro::Semaphore> has a C<guard> |
122 | again. (Please ignore the fact that C<Coro::Semaphore> has a C<guard> |
116 | method that does this already): |
123 | method that does this already): |
117 | |
124 | |
118 | use Guard; |
125 | use Guard; |
119 | use AnyEvent; |
126 | use Coro::AnyEvent; |
120 | use Coro::Semaphore; |
127 | use Coro::Semaphore; |
121 | |
128 | |
122 | my $sem = new Coro::Semaphore; |
129 | my $sem = new Coro::Semaphore; |
123 | |
130 | |
124 | sub lock_for_a_second { |
131 | sub lock_for_a_second { |
125 | $sem->down; |
132 | $sem->down; |
126 | my $guard = guard { $sem->up }; |
133 | my $guard = guard { $sem->up }; |
127 | |
134 | |
128 | my $timer; |
135 | Coro::AnyEvent::sleep 1; |
129 | $timer = AnyEvent->timer (after => 1, sub { |
136 | |
130 | # do something |
137 | # $sem->up gets executed when returning |
131 | undef $sem; |
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132 | undef $timer; |
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133 | }); |
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134 | } |
138 | } |
135 | |
139 | |
136 | 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<< |
137 | $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, |
138 | 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 |
139 | 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 |
140 | one gets executed will the lock be unlocked. Using the C<guard>, you do |
144 | event watchers and only when the last one gets executed will the lock be |
141 | not have to worry about catching all the places where you have to unlock |
145 | unlocked. Using the C<guard>, you do not have to worry about catching all |
142 | the semaphore. |
146 | the places where you have to unlock the semaphore. |
143 | |
147 | |
144 | =item $guard->cancel |
148 | =item $guard->cancel |
145 | |
149 | |
146 | Calling this function will "disable" the guard object returned by the |
150 | Calling this function will "disable" the guard object returned by the |
147 | 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 |
148 | 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. |
149 | |
153 | |
150 | 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 |
151 | and later decide it is no longer needed. |
155 | called under fatal conditions and later decide it is no longer needed. |
152 | |
156 | |
153 | =cut |
157 | =cut |
154 | |
158 | |
155 | 1; |
159 | 1; |
156 | |
160 | |
157 | =back |
161 | =back |
158 | |
162 | |
159 | =head1 EXCEPTIONS |
163 | =head1 EXCEPTIONS |
160 | |
164 | |
161 | 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 |
162 | 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, |
163 | something truly exceptional is happening, a guard block should be allowed |
167 | if something truly exceptional is happening, a guard block should of |
164 | 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 |
165 | programmer certainly wants to know about those. |
169 | exceptions, and the programmer certainly wants to know about those. |
166 | |
170 | |
167 | Since in most cases, the block executing when the guard gets executed does |
171 | Since in most cases, the block executing when the guard gets executed does |
168 | 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 |
169 | let containing code handle the exception. |
173 | let containing code handle the exception. |
170 | |
174 | |
171 | 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 |
172 | followed by calling the code reference stored in C<$Guard::DIED> (with |
176 | by Guard, followed by calling the code reference stored in C<$Guard::DIED> |
173 | C<$@> set to the actual exception), which is similar to how most event |
177 | (with C<$@> set to the actual exception), which is similar to how most |
174 | loops handle this case. |
178 | event loops handle this case. |
175 | |
179 | |
176 | The default for C<$Guard::DIED> is to call C<warn "$@">. |
180 | The default for C<$Guard::DIED> is to call C<warn "$@">, i.e. the error is |
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181 | printed as a warning and the program continues. |
177 | |
182 | |
178 | The C<$@> variable will be restored to its value before the guard call in |
183 | The C<$@> variable will be restored to its value before the guard call in |
179 | all cases, so guards will not disturb C<$@> in any way. |
184 | all cases, so guards will not disturb C<$@> in any way. |
180 | |
185 | |
181 | The code reference stored in C<$Guard::DIED> should not die (behaviour is |
186 | The code reference stored in C<$Guard::DIED> should not die (behaviour is |
… | |
… | |
192 | solution to the problem of exceptions. |
197 | solution to the problem of exceptions. |
193 | |
198 | |
194 | =head1 SEE ALSO |
199 | =head1 SEE ALSO |
195 | |
200 | |
196 | L<Scope::Guard> and L<Sub::ScopeFinalizer>, which actually implement |
201 | L<Scope::Guard> and L<Sub::ScopeFinalizer>, which actually implement |
197 | dynamic, not scoped guards, and have a lot higher CPU, memory and typing |
202 | dynamic guards only, not scoped guards, and have a lot higher CPU, memory |
198 | overhead. |
203 | and typing overhead. |
199 | |
204 | |
200 | L<Hook::Scope>, which has apparently never been finished and corrupts |
205 | L<Hook::Scope>, which has apparently never been finished and can corrupt |
201 | memory when used. |
206 | memory when used. |
202 | |
207 | |
203 | =cut |
208 | =cut |
204 | |
209 | |