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Revision 1.6 by root, Sat Dec 13 18:43:01 2008 UTC vs.
Revision 1.20 by root, Mon Jun 1 10:33:16 2009 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
18 # create an object that, when the last reference to it is gone,
19 # invokes the given codeblock:
20 my $guard = guard { print "destroyed!\n" };
21 undef $guard; # probably destroyed here
17 22
18=head1 DESCRIPTION 23=head1 DESCRIPTION
19 24
20This module implements so-called "guards". A guard is something (usually 25This module implements so-called "guards". A guard is something (usually
21an object) that "guards" a resource, ensuring that it is cleaned up when 26an object) that "guards" a resource, ensuring that it is cleaned up when
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34 39
35=cut 40=cut
36 41
37package Guard; 42package Guard;
38 43
44no warnings;
45
39BEGIN { 46BEGIN {
40 $VERSION = '0.1'; 47 $VERSION = '1.02';
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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52=item scope_guard BLOCK 59=item scope_guard BLOCK
53 60
54Registers a block that is executed when the current scope (block, 61Registers a block that is executed when the current scope (block,
55function, method, eval etc.) is exited. 62function, method, eval etc.) is exited.
56 63
64See the EXCEPTIONS section for an explanation of how exceptions
65(i.e. C<die>) are handled inside guard blocks.
66
57The description below sounds a bit complicated, but that's just because 67The 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 68C<scope_guard> tries to get even corner cases "right": the goal is to
59provide you with a rock solid clean up tool. 69provide you with a rock solid clean up tool.
60 70
61This is similar to this code fragment: 71The behaviour is similar to this code fragment:
62 72
63 eval ... code following scope_guard ... 73 eval ... code following scope_guard ...
64 { 74 {
65 local $@; 75 local $@;
66 eval BLOCK; 76 eval BLOCK;
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70 80
71Except it is much faster, and the whole thing gets executed even when the 81Except 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. 82BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means.
73 83
74If multiple BLOCKs are registered to the same scope, they will be executed 84If 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, 85in 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 86via the same mechanism, so variables C<local>ised I<after> calling
77when the guard runs. 87C<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 88
82Example: temporarily change the timezone for the current process, 89Example: temporarily change the timezone for the current process,
83ensuring it will be reset when the C<if> scope is exited: 90ensuring it will be reset when the C<if> scope is exited:
84 91
85 use Guard; 92 use Guard;
… …
103when the BLOCK gets executed: when the last reference to the object gets 110when the BLOCK gets executed: when the last reference to the object gets
104destroyed, the BLOCK gets executed as with C<scope_guard>. 111destroyed, the BLOCK gets executed as with C<scope_guard>.
105 112
106The returned object can be copied as many times as you want. 113The returned object can be copied as many times as you want.
107 114
108See B<EXCEPTIONS>, below, for an explanation of exception handling 115See the EXCEPTIONS section for an explanation of how exceptions
109(C<die>) within guard blocks. 116(i.e. C<die>) are handled inside guard blocks.
110 117
111Example: acquire a Coro::Semaphore for a second by registering a 118Example: acquire a Coro::Semaphore for a second by registering a
112timer. The timer callback references the guard used to unlock it again. 119timer. The timer callback references the guard used to unlock it
120again. (Please ignore the fact that C<Coro::Semaphore> has a C<guard>
121method that does this already):
113 122
123 use Guard;
114 use AnyEvent; 124 use AnyEvent;
115 use Coro::Semaphore; 125 use Coro::Semaphore;
116 126
117 my $sem = new Coro::Semaphore; 127 my $sem = new Coro::Semaphore;
118 128
119 sub lock_1s { 129 sub lock_for_a_second {
120 $sem->down; 130 $sem->down;
121 my $guard = guard { $sem->up }; 131 my $guard = guard { $sem->up };
122 132
123 my $timer; 133 my $timer;
124 $timer = AnyEvent->timer (after => 1, sub { 134 $timer = AnyEvent->timer (after => 1, sub {
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130 140
131The advantage of doing this with a guard instead of simply calling C<< 141The advantage of doing this with a guard instead of simply calling C<<
132$sem->down >> in the callback is that you can opt not to create the timer, 142$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 143or 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 144can create multiple timers or other event watchers and only when the last
135one gets executed will the lock be unlocked. 145one gets executed will the lock be unlocked. Using the C<guard>, you do
146not have to worry about catching all the places where you have to unlock
147the semaphore.
136 148
137=item Guard::cancel $guard 149=item $guard->cancel
138 150
139Calling this function will "disable" the guard object returned by the 151Calling this function will "disable" the guard object returned by the
140C<guard> function, i.e. it will free the BLOCK originally passed to 152C<guard> function, i.e. it will free the BLOCK originally passed to
141C<guard >and will arrange for the BLOCK not to be executed. 153C<guard >and will arrange for the BLOCK not to be executed.
142 154
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155all, they are usually used to clean up after such exceptions. However, if 167all, they are usually used to clean up after such exceptions. However, if
156something truly exceptional is happening, a guard block should be allowed 168something truly exceptional is happening, a guard block should be allowed
157to die. Also, programming errors are a large source of exceptions, and the 169to die. Also, programming errors are a large source of exceptions, and the
158programmer certainly wants to know about those. 170programmer certainly wants to know about those.
159 171
160Since in most cases, the block executing when the guard gets executes does 172Since in most cases, the block executing when the guard gets executed does
161not know or does not care about the guard blocks, it makes little sense to 173not know or does not care about the guard blocks, it makes little sense to
162let containing code handle the exception. 174let containing code handle the exception.
163 175
164Therefore, whenever a guard block throws an exception, it will be caught, 176Therefore, whenever a guard block throws an exception, it will be caught,
165and this module will call the code reference stored in C<$Guard::DIED> 177followed 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 178C<$@> set to the actual exception), which is similar to how most event
167event loops handle this case. 179loops handle this case.
180
181The default for C<$Guard::DIED> is to call C<warn "$@">.
182
183The C<$@> variable will be restored to its value before the guard call in
184all cases, so guards will not disturb C<$@> in any way.
168 185
169The code reference stored in C<$Guard::DIED> should not die (behaviour is 186The code reference stored in C<$Guard::DIED> should not die (behaviour is
170not guaranteed, but right now, the exception will simply be ignored). 187not guaranteed, but right now, the exception will simply be ignored).
171 188
172The default for C<$Guard::DIED> is to call C<warn "$@">.
173
174=head1 AUTHOR 189=head1 AUTHOR
175 190
176 Marc Lehmann <schmorp@schmorp.de> 191 Marc Lehmann <schmorp@schmorp.de>
177 http://home.schmorp.de/ 192 http://home.schmorp.de/
178 193
179=head1 THANKS 194=head1 THANKS
180 195
181Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED> 196Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED>
182solution to the problem of exceptions. 197solution to the problem of exceptions.
183 198
199=head1 SEE ALSO
200
201L<Scope::Guard> and L<Sub::ScopeFinalizer>, which actually implement
202dynamic, not scoped guards, and have a lot higher CPU, memory and typing
203overhead.
204
205L<Hook::Scope>, which has apparently never been finished and corrupts
206memory when used.
207
184=cut 208=cut
185 209

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