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Revision 1.2 by root, Sat Dec 13 17:49:12 2008 UTC vs.
Revision 1.21 by root, Sun Jul 19 05:44:10 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
9 # temporarily chdir to "/etc" directory, but make sure
10 # to go back to "/" no matter how myfun exits:
11 sub myfun {
12 scope_guard { chdir "/" };
13 chdir "/etc";
14
15 code_that_might_die_or_does_other_fun_stuff;
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
8 22
9=head1 DESCRIPTION 23=head1 DESCRIPTION
10 24
11This module implements so-called "guards". A guard is something (usually 25This module implements so-called "guards". A guard is something (usually
12an 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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25 39
26=cut 40=cut
27 41
28package Guard; 42package Guard;
29 43
44no warnings;
45
30BEGIN { 46BEGIN {
31 $VERSION = '0.01'; 47 $VERSION = '1.021';
32 @ISA = qw(Exporter); 48 @ISA = qw(Exporter);
33 @EXPORT = qw(guard scope_guard cancel); 49 @EXPORT = qw(guard scope_guard);
34 50
35 require Exporter; 51 require Exporter;
36 52
37 require XSLoader; 53 require XSLoader;
38 XSLoader::load Guard, $VERSION; 54 XSLoader::load Guard, $VERSION;
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43=item scope_guard BLOCK 59=item scope_guard BLOCK
44 60
45Registers a block that is executed when the current scope (block, 61Registers a block that is executed when the current scope (block,
46function, method, eval etc.) is exited. 62function, method, eval etc.) is exited.
47 63
64See the EXCEPTIONS section for an explanation of how exceptions
65(i.e. C<die>) are handled inside guard blocks.
66
48The description below sounds a bit complicated, but that's just because 67The description below sounds a bit complicated, but that's just because
49C<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
50provide you with a rock solid clean up tool. 69provide you with a rock solid clean up tool.
51 70
52This is similar to this code fragment: 71The behaviour is similar to this code fragment:
53 72
54 eval ... code following scope_guard ... 73 eval ... code following scope_guard ...
55 { 74 {
56 local $@; 75 local $@;
57 eval BLOCK; 76 eval BLOCK;
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60 die if $@; 79 die if $@;
61 80
62Except it is much faster, and the whole thing gets executed even when the 81Except it is much faster, and the whole thing gets executed even when the
63BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means. 82BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means.
64 83
65See B<EXCEPTIONS>, below, for an explanation of exception handling 84If multiple BLOCKs are registered to the same scope, they will be executed
66(C<die>) within guard blocks. 85in reverse order. Other scope-related things such as C<local> are managed
86via the same mechanism, so variables C<local>ised I<after> calling
87C<scope_guard> will be restored when the guard runs.
67 88
68Example: Temporarily change the directory to F</etc> and make sure it's 89Example: temporarily change the timezone for the current process,
69set back to F</> when the function returns: 90ensuring it will be reset when the C<if> scope is exited:
70 91
71 sub dosomething { 92 use Guard;
72 scope_guard { chdir "/" }; 93 use POSIX ();
73 chdir "/etc";
74 94
75 ... 95 if ($need_to_switch_tz) {
96 # make sure we call tzset after $ENV{TZ} has been restored
97 scope_guard { POSIX::tzset };
98
99 # localise after the scope_guard, so it gets undone in time
100 local $ENV{TZ} = "Europe/London";
101 POSIX::tzset;
102
103 # do something with the new timezone
76 } 104 }
77 105
78=item my $guard = guard BLOCK 106=item my $guard = guard BLOCK
79 107
80Behaves the same as C<scope_guard>, except that instead of executing 108Behaves the same as C<scope_guard>, except that instead of executing
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82when 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
83destroyed, the BLOCK gets executed as with C<scope_guard>. 111destroyed, the BLOCK gets executed as with C<scope_guard>.
84 112
85The returned object can be copied as many times as you want. 113The returned object can be copied as many times as you want.
86 114
87See B<EXCEPTIONS>, below, for an explanation of exception handling 115See the EXCEPTIONS section for an explanation of how exceptions
88(C<die>) within guard blocks. 116(i.e. C<die>) are handled inside guard blocks.
89 117
90Example: acquire a Coro::Semaphore for a second by registering a 118Example: acquire a Coro::Semaphore for a second by registering a
91timer. 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):
92 122
123 use Guard;
93 use AnyEvent; 124 use AnyEvent;
94 use Coro::Semaphore; 125 use Coro::Semaphore;
95 126
96 my $sem = new Coro::Semaphore; 127 my $sem = new Coro::Semaphore;
97 128
98 sub lock_1s { 129 sub lock_for_a_second {
99 $sem->down; 130 $sem->down;
100 my $guard = guard { $sem->up }; 131 my $guard = guard { $sem->up };
101 132
102 my $timer; 133 my $timer;
103 $timer = AnyEvent->timer (after => 1, sub { 134 $timer = AnyEvent->timer (after => 1, sub {
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109 140
110The advantage of doing this with a guard instead of simply calling C<< 141The advantage of doing this with a guard instead of simply calling C<<
111$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,
112or 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
113can 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
114one 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.
115 148
116=item Guard::cancel $guard 149=item $guard->cancel
117 150
118Calling this function will "disable" the guard object returned by the 151Calling this function will "disable" the guard object returned by the
119C<guard> function, i.e. it will free the BLOCK originally passed to 152C<guard> function, i.e. it will free the BLOCK originally passed to
120C<guard >and will arrange for the BLOCK not to be executed. 153C<guard >and will arrange for the BLOCK not to be executed.
121 154
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128 161
129=back 162=back
130 163
131=head1 EXCEPTIONS 164=head1 EXCEPTIONS
132 165
133Guard blocks should not normally throw exceptions (e.g. C<die>), after 166Guard blocks should not normally throw exceptions (that is, C<die>). After
134all, they are usually used to clean up after such exceptions. However, if 167all, they are usually used to clean up after such exceptions. However, if
135something truly exceptional is happening, a guard block should be allowed 168something truly exceptional is happening, a guard block should be allowed
136to die. Also, programming errors are a large source of exceptions, and the 169to die. Also, programming errors are a large source of exceptions, and the
137programmer certainly wants to know about those. 170programmer certainly wants to know about those.
138 171
139Since in most cases, the block executing when the guard gets executes does 172Since in most cases, the block executing when the guard gets executed does
140not know or does not care about the guard blocks, it makes little sense to 173not know or does not care about the guard blocks, it makes little sense to
141let containing code handle the exception. 174let containing code handle the exception.
142 175
143Therefore, whenever a guard block throws an exception, it will be caught, 176Therefore, whenever a guard block throws an exception, it will be caught,
144and this module will call the code reference stored in C<$Guard::DIED> 177followed by calling the code reference stored in C<$Guard::DIED> (with
145(with C<$@> set to the actual exception), which is similar to how most 178C<$@> set to the actual exception), which is similar to how most event
146event loops handle this case. 179loops handle this case.
180
181The default for C<$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<$@> in any way.
147 185
148The code reference stored in C<$Guard::DIED> should not die (behaviour is 186The code reference stored in C<$Guard::DIED> should not die (behaviour is
149not guaranteed, but right now, the exception will simply be ignored). 187not guaranteed, but right now, the exception will simply be ignored).
150 188
151The default for C<$Guard::DIED> is to call C<warn "$@">.
152
153=head1 AUTHOR 189=head1 AUTHOR
154 190
155 Marc Lehmann <schmorp@schmorp.de> 191 Marc Lehmann <schmorp@schmorp.de>
156 http://home.schmorp.de/ 192 http://home.schmorp.de/
157 193
158=head1 THANKS 194=head1 THANKS
159 195
160To Marco Maisenhelder, who reminded me of the C<$Guard::DIED> solution to 196Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED>
161the problem of exceptions. 197solution to the problem of exceptions.
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.
162 207
163=cut 208=cut
164 209

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