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Comparing Guard/Guard.pm (file contents):
Revision 1.4 by root, Sat Dec 13 18:34:18 2008 UTC vs.
Revision 1.11 by root, Sat Dec 13 18:51:19 2008 UTC

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2 2
3Guard - safe cleanup blocks 3Guard - 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 dosomething { 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 call_function_that_might_die_or_other_fun_stuff;
16 } 16 }
17 17
18=head1 DESCRIPTION 18=head1 DESCRIPTION
19 19
20This module implements so-called "guards". A guard is something (usually 20This module implements so-called "guards". A guard is something (usually
21an object) that "guards" a resource, ensuring that it is cleaned up when 21an object) that "guards" a resource, ensuring that it is cleaned up when
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52=item scope_guard BLOCK 52=item scope_guard BLOCK
53 53
54Registers a block that is executed when the current scope (block, 54Registers a block that is executed when the current scope (block,
55function, method, eval etc.) is exited. 55function, method, eval etc.) is exited.
56 56
57See the EXCEPTIONS section for an explanation of how exceptions
58(i.e. C<die>) are handled inside guard blocks.
59
57The description below sounds a bit complicated, but that's just because 60The description below sounds a bit complicated, but that's just because
58C<scope_guard> tries to get even corner cases "right": the goal is to 61C<scope_guard> tries to get even corner cases "right": the goal is to
59provide you with a rock solid clean up tool. 62provide you with a rock solid clean up tool.
60 63
61This is similar to this code fragment: 64The behaviour is similar to this code fragment:
62 65
63 eval ... code following scope_guard ... 66 eval ... code following scope_guard ...
64 { 67 {
65 local $@; 68 local $@;
66 eval BLOCK; 69 eval BLOCK;
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70 73
71Except it is much faster, and the whole thing gets executed even when the 74Except it is much faster, and the whole thing gets executed even when the
72BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means. 75BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means.
73 76
74If multiple BLOCKs are registered to the same scope, they will be executed 77If multiple BLOCKs are registered to the same scope, they will be executed
75in reverse order. Stuff like C<local> is managed via the same mechanism, 78in reverse order. Other scope-related things such as C<local> are managed
76so variables C<local>ised after calling C<scope_guard> will be restored 79via the same mechanism, so variables C<local>ised I<after> calling
77when the guard runs. 80C<scope_guard> will be restored when the guard runs.
78
79See B<EXCEPTIONS>, below, for an explanation of exception handling
80(C<die>) within guard blocks.
81 81
82Example: temporarily change the timezone for the current process, 82Example: temporarily change the timezone for the current process,
83ensuring it will be reset when the C<if> scope is exited: 83ensuring it will be reset when the C<if> scope is exited:
84 84
85 use Guard; 85 use Guard;
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103when the BLOCK gets executed: when the last reference to the object gets 103when the BLOCK gets executed: when the last reference to the object gets
104destroyed, the BLOCK gets executed as with C<scope_guard>. 104destroyed, the BLOCK gets executed as with C<scope_guard>.
105 105
106The returned object can be copied as many times as you want. 106The returned object can be copied as many times as you want.
107 107
108See B<EXCEPTIONS>, below, for an explanation of exception handling 108See the EXCEPTIONS section for an explanation of how exceptions
109(C<die>) within guard blocks. 109(i.e. C<die>) are handled inside guard blocks.
110 110
111Example: acquire a Coro::Semaphore for a second by registering a 111Example: acquire a Coro::Semaphore for a second by registering a
112timer. The timer callback references the guard used to unlock it again. 112timer. The timer callback references the guard used to unlock it
113again. (Please ignore the fact that C<Coro::Semaphore> has a C<guard>
114method that does this already):
113 115
116 use Guard;
114 use AnyEvent; 117 use AnyEvent;
115 use Coro::Semaphore; 118 use Coro::Semaphore;
116 119
117 my $sem = new Coro::Semaphore; 120 my $sem = new Coro::Semaphore;
118 121
119 sub lock_1s { 122 sub lock_for_a_second {
120 $sem->down; 123 $sem->down;
121 my $guard = guard { $sem->up }; 124 my $guard = guard { $sem->up };
122 125
123 my $timer; 126 my $timer;
124 $timer = AnyEvent->timer (after => 1, sub { 127 $timer = AnyEvent->timer (after => 1, sub {
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130 133
131The advantage of doing this with a guard instead of simply calling C<< 134The advantage of doing this with a guard instead of simply calling C<<
132$sem->down >> in the callback is that you can opt not to create the timer, 135$sem->down >> in the callback is that you can opt not to create the timer,
133or your code can throw an exception before it can create the timer, or you 136or your code can throw an exception before it can create the timer, or you
134can create multiple timers or other event watchers and only when the last 137can create multiple timers or other event watchers and only when the last
135one gets executed will the lock be unlocked. 138one gets executed will the lock be unlocked. Using the C<guard>, you do
139not have to worry about catching all the places where you have to unlock
140the semaphore.
136 141
137=item Guard::cancel $guard 142=item Guard::cancel $guard
138 143
139Calling this function will "disable" the guard object returned by the 144Calling this function will "disable" the guard object returned by the
140C<guard> function, i.e. it will free the BLOCK originally passed to 145C<guard> function, i.e. it will free the BLOCK originally passed to
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149 154
150=back 155=back
151 156
152=head1 EXCEPTIONS 157=head1 EXCEPTIONS
153 158
154Guard blocks should not normally throw exceptions (e.g. C<die>), after 159Guard blocks should not normally throw exceptions (that is, C<die>). After
155all, they are usually used to clean up after such exceptions. However, if 160all, they are usually used to clean up after such exceptions. However, if
156something truly exceptional is happening, a guard block should be allowed 161something truly exceptional is happening, a guard block should be allowed
157to die. Also, programming errors are a large source of exceptions, and the 162to die. Also, programming errors are a large source of exceptions, and the
158programmer certainly wants to know about those. 163programmer certainly wants to know about those.
159 164
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176 Marc Lehmann <schmorp@schmorp.de> 181 Marc Lehmann <schmorp@schmorp.de>
177 http://home.schmorp.de/ 182 http://home.schmorp.de/
178 183
179=head1 THANKS 184=head1 THANKS
180 185
181To Marco Maisenhelder, who reminded me of the C<$Guard::DIED> solution to 186Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED>
182the problem of exceptions. 187solution to the problem of exceptions.
183 188
184=cut 189=cut
185 190

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