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Comparing Guard/Guard.pm (file contents):
Revision 1.1 by root, Sat Dec 13 17:37:22 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
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 call_function_that_might_die_or_other_fun_stuff;
16 }
8 17
9=head1 DESCRIPTION 18=head1 DESCRIPTION
10 19
11This module implements so-called "guards". A guard is something (usually 20This module implements so-called "guards". A guard is something (usually
12an 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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14 23
15Specifically, this module supports two different types of guards: guard 24Specifically, this module supports two different types of guards: guard
16objects, which execute a given code block when destroyed, and scoped 25objects, which execute a given code block when destroyed, and scoped
17guards, which are tied to the scope exit. 26guards, which are tied to the scope exit.
18 27
28=head1 FUNCTIONS
29
30This module currently exports the C<scope_guard> and C<guard> functions by
31default.
32
19=over 4 33=over 4
20 34
21=cut 35=cut
22 36
23package Guard; 37package Guard;
24 38
25BEGIN { 39BEGIN {
26 $VERSION = '0.01'; 40 $VERSION = '0.1';
27 @ISA = qw(Exporter); 41 @ISA = qw(Exporter);
28 @EXPORT = qw(guard scope_guard cancel); 42 @EXPORT = qw(guard scope_guard);
29 43
30 require Exporter; 44 require Exporter;
31 45
32 require XSLoader; 46 require XSLoader;
33 XSLoader::load Guard, $VERSION; 47 XSLoader::load Guard, $VERSION;
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38=item scope_guard BLOCK 52=item scope_guard BLOCK
39 53
40Registers a block that is executed when the current scope (block, 54Registers a block that is executed when the current scope (block,
41function, method, eval etc.) is exited. 55function, method, eval etc.) is exited.
42 56
57See the EXCEPTIONS section for an explanation of how exceptions
58(i.e. C<die>) are handled inside guard blocks.
59
60The description below sounds a bit complicated, but that's just because
61C<scope_guard> tries to get even corner cases "right": the goal is to
62provide you with a rock solid clean up tool.
63
43This is similar to this code fragment: 64The behaviour is similar to this code fragment:
44 65
45 eval ... code following scope_guard ... 66 eval ... code following scope_guard ...
46 { 67 {
47 local $@; 68 local $@;
48 eval BLOCK; 69 eval BLOCK;
49 eval { $Guard::DIED->() } if $@; 70 eval { $Guard::DIED->() } if $@;
50 } 71 }
72 die if $@;
51 73
52Except it is much faster, and the whole thing gets executed even when the 74Except it is much faster, and the whole thing gets executed even when the
53BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means. 75BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means.
54 76
55See B<EXCEPTIONS>, below, for an explanation of exception handling 77If multiple BLOCKs are registered to the same scope, they will be executed
56(C<die>) within guard blocks. 78in reverse order. Other scope-related things such as C<local> are managed
79via the same mechanism, so variables C<local>ised I<after> calling
80C<scope_guard> will be restored when the guard runs.
57 81
58Example: Temporarily change the directory to F</etc> and make sure it's 82Example: temporarily change the timezone for the current process,
59set back to F</> when the function returns: 83ensuring it will be reset when the C<if> scope is exited:
60 84
61 sub dosomething { 85 use Guard;
62 scope_guard { chdir "/" }; 86 use POSIX ();
63 chdir "/etc";
64 87
65 ... 88 if ($need_to_switch_tz) {
89 # make sure we call tzset after $ENV{TZ} has been restored
90 scope_guard { POSIX::tzset };
91
92 # localise after the scope_guard, so it gets undone in time
93 local $ENV{TZ} = "Europe/London";
94 POSIX::tzset;
95
96 # do something with the new timezone
66 } 97 }
67 98
68=item my $guard = guard BLOCK 99=item my $guard = guard BLOCK
69 100
70Behaves the same as C<scope_guard>, except that instead of executing 101Behaves 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 103when the BLOCK gets executed: when the last reference to the object gets
73destroyed, the BLOCK gets executed as with C<scope_guard>. 104destroyed, the BLOCK gets executed as with C<scope_guard>.
74 105
75The returned object can be copied as many times as you want. 106The returned object can be copied as many times as you want.
76 107
77See B<EXCEPTIONS>, below, for an explanation of exception handling 108See the EXCEPTIONS section for an explanation of how exceptions
78(C<die>) within guard blocks. 109(i.e. C<die>) are handled inside guard blocks.
79 110
80Example: acquire a Coro::Semaphore for a second by registering a 111Example: acquire a Coro::Semaphore for a second by registering a
81timer. 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):
82 115
116 use Guard;
83 use AnyEvent; 117 use AnyEvent;
84 use Coro::Semaphore; 118 use Coro::Semaphore;
85 119
86 my $sem = new Coro::Semaphore; 120 my $sem = new Coro::Semaphore;
87 121
88 sub lock_1s { 122 sub lock_for_a_second {
89 $sem->down; 123 $sem->down;
90 my $guard = guard { $sem->up }; 124 my $guard = guard { $sem->up };
91 125
92 my $timer; 126 my $timer;
93 $timer = AnyEvent->timer (after => 1, sub { 127 $timer = AnyEvent->timer (after => 1, sub {
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99 133
100The advantage of doing this with a guard instead of simply calling C<< 134The advantage of doing this with a guard instead of simply calling C<<
101$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,
102or 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
103can 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
104one 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.
105 141
106=item Guard::cancel $guard 142=item Guard::cancel $guard
107 143
108Calling this function will "disable" the guard object returned by the 144Calling this function will "disable" the guard object returned by the
109C<guard> function, i.e. it will free the BLOCK originally passed to 145C<guard> function, i.e. it will free the BLOCK originally passed to
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118 154
119=back 155=back
120 156
121=head1 EXCEPTIONS 157=head1 EXCEPTIONS
122 158
123Guard blocks should not normally throw exceptions (e.g. C<die>), after 159Guard blocks should not normally throw exceptions (that is, C<die>). After
124all, they are usually used to clean up after such exceptions. However, if 160all, they are usually used to clean up after such exceptions. However, if
125something truly exceptional is happening, a guard block should be allowed 161something truly exceptional is happening, a guard block should be allowed
126to die. Also, programming errors are a large source of exceptions, and the 162to die. Also, programming errors are a large source of exceptions, and the
127programmer certainly wants to know about those. 163programmer certainly wants to know about those.
128 164
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145 Marc Lehmann <schmorp@schmorp.de> 181 Marc Lehmann <schmorp@schmorp.de>
146 http://home.schmorp.de/ 182 http://home.schmorp.de/
147 183
148=head1 THANKS 184=head1 THANKS
149 185
150To Marco Maisenhelder, who reminded me of the C<$Guard::DIED> solution to 186Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED>
151the problem of exceptions. 187solution to the problem of exceptions.
152 188
153=cut 189=cut
154 190

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