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Revision 1.4 by root, Sat Dec 13 18:34:18 2008 UTC vs.
Revision 1.16 by root, Mon Dec 15 02:38:21 2008 UTC

… …
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 code_that_might_die_or_does_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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34 34
35=cut 35=cut
36 36
37package Guard; 37package Guard;
38 38
39no warnings;
40
39BEGIN { 41BEGIN {
40 $VERSION = '0.1'; 42 $VERSION = '0.5';
41 @ISA = qw(Exporter); 43 @ISA = qw(Exporter);
42 @EXPORT = qw(guard scope_guard); 44 @EXPORT = qw(guard scope_guard);
43 45
44 require Exporter; 46 require Exporter;
45 47
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52=item scope_guard BLOCK 54=item scope_guard BLOCK
53 55
54Registers a block that is executed when the current scope (block, 56Registers a block that is executed when the current scope (block,
55function, method, eval etc.) is exited. 57function, method, eval etc.) is exited.
56 58
59See the EXCEPTIONS section for an explanation of how exceptions
60(i.e. C<die>) are handled inside guard blocks.
61
57The description below sounds a bit complicated, but that's just because 62The 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 63C<scope_guard> tries to get even corner cases "right": the goal is to
59provide you with a rock solid clean up tool. 64provide you with a rock solid clean up tool.
60 65
61This is similar to this code fragment: 66The behaviour is similar to this code fragment:
62 67
63 eval ... code following scope_guard ... 68 eval ... code following scope_guard ...
64 { 69 {
65 local $@; 70 local $@;
66 eval BLOCK; 71 eval BLOCK;
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70 75
71Except it is much faster, and the whole thing gets executed even when the 76Except 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. 77BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means.
73 78
74If multiple BLOCKs are registered to the same scope, they will be executed 79If 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, 80in 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 81via the same mechanism, so variables C<local>ised I<after> calling
77when the guard runs. 82C<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 83
82Example: temporarily change the timezone for the current process, 84Example: temporarily change the timezone for the current process,
83ensuring it will be reset when the C<if> scope is exited: 85ensuring it will be reset when the C<if> scope is exited:
84 86
85 use Guard; 87 use Guard;
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103when the BLOCK gets executed: when the last reference to the object gets 105when the BLOCK gets executed: when the last reference to the object gets
104destroyed, the BLOCK gets executed as with C<scope_guard>. 106destroyed, the BLOCK gets executed as with C<scope_guard>.
105 107
106The returned object can be copied as many times as you want. 108The returned object can be copied as many times as you want.
107 109
108See B<EXCEPTIONS>, below, for an explanation of exception handling 110See the EXCEPTIONS section for an explanation of how exceptions
109(C<die>) within guard blocks. 111(i.e. C<die>) are handled inside guard blocks.
110 112
111Example: acquire a Coro::Semaphore for a second by registering a 113Example: acquire a Coro::Semaphore for a second by registering a
112timer. The timer callback references the guard used to unlock it again. 114timer. The timer callback references the guard used to unlock it
115again. (Please ignore the fact that C<Coro::Semaphore> has a C<guard>
116method that does this already):
113 117
118 use Guard;
114 use AnyEvent; 119 use AnyEvent;
115 use Coro::Semaphore; 120 use Coro::Semaphore;
116 121
117 my $sem = new Coro::Semaphore; 122 my $sem = new Coro::Semaphore;
118 123
119 sub lock_1s { 124 sub lock_for_a_second {
120 $sem->down; 125 $sem->down;
121 my $guard = guard { $sem->up }; 126 my $guard = guard { $sem->up };
122 127
123 my $timer; 128 my $timer;
124 $timer = AnyEvent->timer (after => 1, sub { 129 $timer = AnyEvent->timer (after => 1, sub {
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130 135
131The advantage of doing this with a guard instead of simply calling C<< 136The advantage of doing this with a guard instead of simply calling C<<
132$sem->down >> in the callback is that you can opt not to create the timer, 137$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 138or 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 139can create multiple timers or other event watchers and only when the last
135one gets executed will the lock be unlocked. 140one gets executed will the lock be unlocked. Using the C<guard>, you do
141not have to worry about catching all the places where you have to unlock
142the semaphore.
136 143
137=item Guard::cancel $guard 144=item $guard->cancel
138 145
139Calling this function will "disable" the guard object returned by the 146Calling this function will "disable" the guard object returned by the
140C<guard> function, i.e. it will free the BLOCK originally passed to 147C<guard> function, i.e. it will free the BLOCK originally passed to
141C<guard >and will arrange for the BLOCK not to be executed. 148C<guard >and will arrange for the BLOCK not to be executed.
142 149
… …
149 156
150=back 157=back
151 158
152=head1 EXCEPTIONS 159=head1 EXCEPTIONS
153 160
154Guard blocks should not normally throw exceptions (e.g. C<die>), after 161Guard blocks should not normally throw exceptions (that is, C<die>). After
155all, they are usually used to clean up after such exceptions. However, if 162all, they are usually used to clean up after such exceptions. However, if
156something truly exceptional is happening, a guard block should be allowed 163something truly exceptional is happening, a guard block should be allowed
157to die. Also, programming errors are a large source of exceptions, and the 164to die. Also, programming errors are a large source of exceptions, and the
158programmer certainly wants to know about those. 165programmer certainly wants to know about those.
159 166
160Since in most cases, the block executing when the guard gets executes does 167Since in most cases, the block executing when the guard gets executed does
161not know or does not care about the guard blocks, it makes little sense to 168not know or does not care about the guard blocks, it makes little sense to
162let containing code handle the exception. 169let containing code handle the exception.
163 170
164Therefore, whenever a guard block throws an exception, it will be caught, 171Therefore, whenever a guard block throws an exception, it will be caught,
165and this module will call the code reference stored in C<$Guard::DIED> 172followed by calling the code reference stored in C<$Guard::DIED> (with
166(with C<$@> set to the actual exception), which is similar to how most 173C<$@> set to the actual exception), which is similar to how most event
167event loops handle this case. 174loops handle this case.
175
176The default for C<$Guard::DIED> is to call C<warn "$@">.
177
178The C<$@> variable will be restored to its value before the guard call in
179all cases, so guards will not disturb C<$@> in any way.
168 180
169The code reference stored in C<$Guard::DIED> should not die (behaviour is 181The code reference stored in C<$Guard::DIED> should not die (behaviour is
170not guaranteed, but right now, the exception will simply be ignored). 182not guaranteed, but right now, the exception will simply be ignored).
171 183
172The default for C<$Guard::DIED> is to call C<warn "$@">.
173
174=head1 AUTHOR 184=head1 AUTHOR
175 185
176 Marc Lehmann <schmorp@schmorp.de> 186 Marc Lehmann <schmorp@schmorp.de>
177 http://home.schmorp.de/ 187 http://home.schmorp.de/
178 188
179=head1 THANKS 189=head1 THANKS
180 190
181To Marco Maisenhelder, who reminded me of the C<$Guard::DIED> solution to 191Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED>
182the problem of exceptions. 192solution to the problem of exceptions.
193
194=head1 SEE ALSO
195
196L<Scope::Guard> and L<Sub::ScopeFinalizer>, which actually implement
197dynamic, not scoped guards, and have a lot higher CPU, memory and typing
198overhead.
199
200L<Hook::Scope>, which has apparently never been finished and corrupts
201memory when used.
183 202
184=cut 203=cut
185 204

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