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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 1.15 by root, Sat Dec 13 22:09:25 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 code_that_might_die_or_does_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
39no warnings;
40
25BEGIN { 41BEGIN {
26 $VERSION = '0.01'; 42 $VERSION = '0.5';
27 @ISA = qw(Exporter); 43 @ISA = qw(Exporter);
28 @EXPORT = qw(guard scope_guard cancel); 44 @EXPORT = qw(guard scope_guard);
29 45
30 require Exporter; 46 require Exporter;
31 47
32 require XSLoader; 48 require XSLoader;
33 XSLoader::load Guard, $VERSION; 49 XSLoader::load Guard, $VERSION;
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38=item scope_guard BLOCK 54=item scope_guard BLOCK
39 55
40Registers a block that is executed when the current scope (block, 56Registers a block that is executed when the current scope (block,
41function, method, eval etc.) is exited. 57function, method, eval etc.) is exited.
42 58
59See the EXCEPTIONS section for an explanation of how exceptions
60(i.e. C<die>) are handled inside guard blocks.
61
62The description below sounds a bit complicated, but that's just because
63C<scope_guard> tries to get even corner cases "right": the goal is to
64provide you with a rock solid clean up tool.
65
43This is similar to this code fragment: 66The behaviour is similar to this code fragment:
44 67
45 eval ... code following scope_guard ... 68 eval ... code following scope_guard ...
46 { 69 {
47 local $@; 70 local $@;
48 eval BLOCK; 71 eval BLOCK;
49 eval { $Guard::DIED->() } if $@; 72 eval { $Guard::DIED->() } if $@;
50 } 73 }
74 die if $@;
51 75
52Except it is much faster, and the whole thing gets executed even when the 76Except it is much faster, and the whole thing gets executed even when the
53BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means. 77BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means.
54 78
55See B<EXCEPTIONS>, below, for an explanation of exception handling 79If multiple BLOCKs are registered to the same scope, they will be executed
56(C<die>) within guard blocks. 80in reverse order. Other scope-related things such as C<local> are managed
81via the same mechanism, so variables C<local>ised I<after> calling
82C<scope_guard> will be restored when the guard runs.
57 83
58Example: Temporarily change the directory to F</etc> and make sure it's 84Example: temporarily change the timezone for the current process,
59set back to F</> when the function returns: 85ensuring it will be reset when the C<if> scope is exited:
60 86
61 sub dosomething { 87 use Guard;
62 scope_guard { chdir "/" }; 88 use POSIX ();
63 chdir "/etc";
64 89
65 ... 90 if ($need_to_switch_tz) {
91 # make sure we call tzset after $ENV{TZ} has been restored
92 scope_guard { POSIX::tzset };
93
94 # localise after the scope_guard, so it gets undone in time
95 local $ENV{TZ} = "Europe/London";
96 POSIX::tzset;
97
98 # do something with the new timezone
66 } 99 }
67 100
68=item my $guard = guard BLOCK 101=item my $guard = guard BLOCK
69 102
70Behaves the same as C<scope_guard>, except that instead of executing 103Behaves 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 105when the BLOCK gets executed: when the last reference to the object gets
73destroyed, the BLOCK gets executed as with C<scope_guard>. 106destroyed, the BLOCK gets executed as with C<scope_guard>.
74 107
75The returned object can be copied as many times as you want. 108The returned object can be copied as many times as you want.
76 109
77See B<EXCEPTIONS>, below, for an explanation of exception handling 110See the EXCEPTIONS section for an explanation of how exceptions
78(C<die>) within guard blocks. 111(i.e. C<die>) are handled inside guard blocks.
79 112
80Example: acquire a Coro::Semaphore for a second by registering a 113Example: acquire a Coro::Semaphore for a second by registering a
81timer. 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):
82 117
118 use Guard;
83 use AnyEvent; 119 use AnyEvent;
84 use Coro::Semaphore; 120 use Coro::Semaphore;
85 121
86 my $sem = new Coro::Semaphore; 122 my $sem = new Coro::Semaphore;
87 123
88 sub lock_1s { 124 sub lock_for_a_second {
89 $sem->down; 125 $sem->down;
90 my $guard = guard { $sem->up }; 126 my $guard = guard { $sem->up };
91 127
92 my $timer; 128 my $timer;
93 $timer = AnyEvent->timer (after => 1, sub { 129 $timer = AnyEvent->timer (after => 1, sub {
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99 135
100The advantage of doing this with a guard instead of simply calling C<< 136The advantage of doing this with a guard instead of simply calling C<<
101$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,
102or 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
103can 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
104one 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.
105 143
106=item Guard::cancel $guard 144=item $guard->cancel
107 145
108Calling this function will "disable" the guard object returned by the 146Calling this function will "disable" the guard object returned by the
109C<guard> function, i.e. it will free the BLOCK originally passed to 147C<guard> function, i.e. it will free the BLOCK originally passed to
110C<guard >and will arrange for the BLOCK not to be executed. 148C<guard >and will arrange for the BLOCK not to be executed.
111 149
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118 156
119=back 157=back
120 158
121=head1 EXCEPTIONS 159=head1 EXCEPTIONS
122 160
123Guard blocks should not normally throw exceptions (e.g. C<die>), after 161Guard blocks should not normally throw exceptions (that is, C<die>). After
124all, they are usually used to clean up after such exceptions. However, if 162all, they are usually used to clean up after such exceptions. However, if
125something truly exceptional is happening, a guard block should be allowed 163something truly exceptional is happening, a guard block should be allowed
126to die. Also, programming errors are a large source of exceptions, and the 164to die. Also, programming errors are a large source of exceptions, and the
127programmer certainly wants to know about those. 165programmer certainly wants to know about those.
128 166
129Since in most cases, the block executing when the guard gets executes does 167Since in most cases, the block executing when the guard gets executed does
130not know or does not care about the guard blocks, it makes little sense to 168not know or does not care about the guard blocks, it makes little sense to
131let containing code handle the exception. 169let containing code handle the exception.
132 170
133Therefore, whenever a guard block throws an exception, it will be caught, 171Therefore, whenever a guard block throws an exception, it will be caught,
134and this module will call the code reference stored in C<$Guard::DIED> 172followed 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 173C<$@> set to the actual exception), which is similar to how most event
136event loops handle this case. 174loops handle this case.
175
176The default for C<$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<$@> in any way.
137 180
138The code reference stored in C<$Guard::DIED> should not die (behaviour is 181The code reference stored in C<$Guard::DIED> should not die (behaviour is
139not guaranteed, but right now, the exception will simply be ignored). 182not guaranteed, but right now, the exception will simply be ignored).
140
141The default for C<$Guard::DIED> is to call C<warn "$@">.
142 183
143=head1 AUTHOR 184=head1 AUTHOR
144 185
145 Marc Lehmann <schmorp@schmorp.de> 186 Marc Lehmann <schmorp@schmorp.de>
146 http://home.schmorp.de/ 187 http://home.schmorp.de/
147 188
148=head1 THANKS 189=head1 THANKS
149 190
150To Marco Maisenhelder, who reminded me of the C<$Guard::DIED> solution to 191Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED>
151the problem of exceptions. 192solution to the problem of exceptions.
152 193
153=cut 194=cut
154 195

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