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Revision 1.1 by root, Sat Dec 13 17:37:22 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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14 28
15Specifically, this module supports two different types of guards: guard 29Specifically, this module supports two different types of guards: guard
16objects, which execute a given code block when destroyed, and scoped 30objects, which execute a given code block when destroyed, and scoped
17guards, which are tied to the scope exit. 31guards, which are tied to the scope exit.
18 32
33=head1 FUNCTIONS
34
35This module currently exports the C<scope_guard> and C<guard> functions by
36default.
37
19=over 4 38=over 4
20 39
21=cut 40=cut
22 41
23package Guard; 42package Guard;
24 43
44no warnings;
45
25BEGIN { 46BEGIN {
26 $VERSION = '0.01'; 47 $VERSION = '1.021';
27 @ISA = qw(Exporter); 48 @ISA = qw(Exporter);
28 @EXPORT = qw(guard scope_guard cancel); 49 @EXPORT = qw(guard scope_guard);
29 50
30 require Exporter; 51 require Exporter;
31 52
32 require XSLoader; 53 require XSLoader;
33 XSLoader::load Guard, $VERSION; 54 XSLoader::load Guard, $VERSION;
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38=item scope_guard BLOCK 59=item scope_guard BLOCK
39 60
40Registers a block that is executed when the current scope (block, 61Registers a block that is executed when the current scope (block,
41function, method, eval etc.) is exited. 62function, method, eval etc.) is exited.
42 63
64See the EXCEPTIONS section for an explanation of how exceptions
65(i.e. C<die>) are handled inside guard blocks.
66
67The description below sounds a bit complicated, but that's just because
68C<scope_guard> tries to get even corner cases "right": the goal is to
69provide you with a rock solid clean up tool.
70
43This is similar to this code fragment: 71The behaviour is similar to this code fragment:
44 72
45 eval ... code following scope_guard ... 73 eval ... code following scope_guard ...
46 { 74 {
47 local $@; 75 local $@;
48 eval BLOCK; 76 eval BLOCK;
49 eval { $Guard::DIED->() } if $@; 77 eval { $Guard::DIED->() } if $@;
50 } 78 }
79 die if $@;
51 80
52Except it is much faster, and the whole thing gets executed even when the 81Except it is much faster, and the whole thing gets executed even when the
53BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means. 82BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means.
54 83
55See B<EXCEPTIONS>, below, for an explanation of exception handling 84If multiple BLOCKs are registered to the same scope, they will be executed
56(C<die>) within guard blocks. 85in reverse order. Other scope-related things such as C<local> are managed
86via the same mechanism, so variables C<local>ised I<after> calling
87C<scope_guard> will be restored when the guard runs.
57 88
58Example: Temporarily change the directory to F</etc> and make sure it's 89Example: temporarily change the timezone for the current process,
59set back to F</> when the function returns: 90ensuring it will be reset when the C<if> scope is exited:
60 91
61 sub dosomething { 92 use Guard;
62 scope_guard { chdir "/" }; 93 use POSIX ();
63 chdir "/etc";
64 94
65 ... 95 if ($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
66 } 104 }
67 105
68=item my $guard = guard BLOCK 106=item my $guard = guard BLOCK
69 107
70Behaves 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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72when 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
73destroyed, the BLOCK gets executed as with C<scope_guard>. 111destroyed, the BLOCK gets executed as with C<scope_guard>.
74 112
75The returned object can be copied as many times as you want. 113The returned object can be copied as many times as you want.
76 114
77See B<EXCEPTIONS>, below, for an explanation of exception handling 115See the EXCEPTIONS section for an explanation of how exceptions
78(C<die>) within guard blocks. 116(i.e. C<die>) are handled inside guard blocks.
79 117
80Example: acquire a Coro::Semaphore for a second by registering a 118Example: acquire a Coro::Semaphore for a second by registering a
81timer. 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):
82 122
123 use Guard;
83 use AnyEvent; 124 use AnyEvent;
84 use Coro::Semaphore; 125 use Coro::Semaphore;
85 126
86 my $sem = new Coro::Semaphore; 127 my $sem = new Coro::Semaphore;
87 128
88 sub lock_1s { 129 sub lock_for_a_second {
89 $sem->down; 130 $sem->down;
90 my $guard = guard { $sem->up }; 131 my $guard = guard { $sem->up };
91 132
92 my $timer; 133 my $timer;
93 $timer = AnyEvent->timer (after => 1, sub { 134 $timer = AnyEvent->timer (after => 1, sub {
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99 140
100The advantage of doing this with a guard instead of simply calling C<< 141The advantage of doing this with a guard instead of simply calling C<<
101$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,
102or 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
103can 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
104one 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.
105 148
106=item Guard::cancel $guard 149=item $guard->cancel
107 150
108Calling this function will "disable" the guard object returned by the 151Calling this function will "disable" the guard object returned by the
109C<guard> function, i.e. it will free the BLOCK originally passed to 152C<guard> function, i.e. it will free the BLOCK originally passed to
110C<guard >and will arrange for the BLOCK not to be executed. 153C<guard >and will arrange for the BLOCK not to be executed.
111 154
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118 161
119=back 162=back
120 163
121=head1 EXCEPTIONS 164=head1 EXCEPTIONS
122 165
123Guard blocks should not normally throw exceptions (e.g. C<die>), after 166Guard blocks should not normally throw exceptions (that is, C<die>). After
124all, they are usually used to clean up after such exceptions. However, if 167all, they are usually used to clean up after such exceptions. However, if
125something truly exceptional is happening, a guard block should be allowed 168something truly exceptional is happening, a guard block should be allowed
126to die. Also, programming errors are a large source of exceptions, and the 169to die. Also, programming errors are a large source of exceptions, and the
127programmer certainly wants to know about those. 170programmer certainly wants to know about those.
128 171
129Since in most cases, the block executing when the guard gets executes does 172Since in most cases, the block executing when the guard gets executed does
130not know or does not care about the guard blocks, it makes little sense to 173not know or does not care about the guard blocks, it makes little sense to
131let containing code handle the exception. 174let containing code handle the exception.
132 175
133Therefore, whenever a guard block throws an exception, it will be caught, 176Therefore, whenever a guard block throws an exception, it will be caught,
134and this module will call the code reference stored in C<$Guard::DIED> 177followed by calling the code reference stored in C<$Guard::DIED> (with
135(with C<$@> set to the actual exception), which is similar to how most 178C<$@> set to the actual exception), which is similar to how most event
136event loops handle this case. 179loops handle this case.
180
181The default for C<$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<$@> in any way.
137 185
138The code reference stored in C<$Guard::DIED> should not die (behaviour is 186The code reference stored in C<$Guard::DIED> should not die (behaviour is
139not guaranteed, but right now, the exception will simply be ignored). 187not guaranteed, but right now, the exception will simply be ignored).
140 188
141The default for C<$Guard::DIED> is to call C<warn "$@">.
142
143=head1 AUTHOR 189=head1 AUTHOR
144 190
145 Marc Lehmann <schmorp@schmorp.de> 191 Marc Lehmann <schmorp@schmorp.de>
146 http://home.schmorp.de/ 192 http://home.schmorp.de/
147 193
148=head1 THANKS 194=head1 THANKS
149 195
150To Marco Maisenhelder, who reminded me of the C<$Guard::DIED> solution to 196Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED>
151the 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.
152 207
153=cut 208=cut
154 209

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