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Comparing Guard/Guard.pm (file contents):
Revision 1.2 by root, Sat Dec 13 17:49:12 2008 UTC vs.
Revision 1.12 by root, Sat Dec 13 18:53:30 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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26=cut 35=cut
27 36
28package Guard; 37package Guard;
29 38
30BEGIN { 39BEGIN {
31 $VERSION = '0.01'; 40 $VERSION = '0.1';
32 @ISA = qw(Exporter); 41 @ISA = qw(Exporter);
33 @EXPORT = qw(guard scope_guard cancel); 42 @EXPORT = qw(guard scope_guard);
34 43
35 require Exporter; 44 require Exporter;
36 45
37 require XSLoader; 46 require XSLoader;
38 XSLoader::load Guard, $VERSION; 47 XSLoader::load Guard, $VERSION;
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43=item scope_guard BLOCK 52=item scope_guard BLOCK
44 53
45Registers a block that is executed when the current scope (block, 54Registers a block that is executed when the current scope (block,
46function, method, eval etc.) is exited. 55function, method, eval etc.) is exited.
47 56
57See the EXCEPTIONS section for an explanation of how exceptions
58(i.e. C<die>) are handled inside guard blocks.
59
48The description below sounds a bit complicated, but that's just because 60The 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 61C<scope_guard> tries to get even corner cases "right": the goal is to
50provide you with a rock solid clean up tool. 62provide you with a rock solid clean up tool.
51 63
52This is similar to this code fragment: 64The behaviour is similar to this code fragment:
53 65
54 eval ... code following scope_guard ... 66 eval ... code following scope_guard ...
55 { 67 {
56 local $@; 68 local $@;
57 eval BLOCK; 69 eval BLOCK;
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60 die if $@; 72 die if $@;
61 73
62Except it is much faster, and the whole thing gets executed even when the 74Except it is much faster, and the whole thing gets executed even when the
63BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means. 75BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means.
64 76
65See B<EXCEPTIONS>, below, for an explanation of exception handling 77If multiple BLOCKs are registered to the same scope, they will be executed
66(C<die>) within guard blocks. 78in reverse order. Other scope-related things such as C<local> are managed
79via the same mechanism, so variables C<local>ised I<after> calling
80C<scope_guard> will be restored when the guard runs.
67 81
68Example: Temporarily change the directory to F</etc> and make sure it's 82Example: temporarily change the timezone for the current process,
69set back to F</> when the function returns: 83ensuring it will be reset when the C<if> scope is exited:
70 84
71 sub dosomething { 85 use Guard;
72 scope_guard { chdir "/" }; 86 use POSIX ();
73 chdir "/etc";
74 87
75 ... 88 if ($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
76 } 97 }
77 98
78=item my $guard = guard BLOCK 99=item my $guard = guard BLOCK
79 100
80Behaves 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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82when 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
83destroyed, the BLOCK gets executed as with C<scope_guard>. 104destroyed, the BLOCK gets executed as with C<scope_guard>.
84 105
85The returned object can be copied as many times as you want. 106The returned object can be copied as many times as you want.
86 107
87See B<EXCEPTIONS>, below, for an explanation of exception handling 108See the EXCEPTIONS section for an explanation of how exceptions
88(C<die>) within guard blocks. 109(i.e. C<die>) are handled inside guard blocks.
89 110
90Example: acquire a Coro::Semaphore for a second by registering a 111Example: acquire a Coro::Semaphore for a second by registering a
91timer. 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):
92 115
116 use Guard;
93 use AnyEvent; 117 use AnyEvent;
94 use Coro::Semaphore; 118 use Coro::Semaphore;
95 119
96 my $sem = new Coro::Semaphore; 120 my $sem = new Coro::Semaphore;
97 121
98 sub lock_1s { 122 sub lock_for_a_second {
99 $sem->down; 123 $sem->down;
100 my $guard = guard { $sem->up }; 124 my $guard = guard { $sem->up };
101 125
102 my $timer; 126 my $timer;
103 $timer = AnyEvent->timer (after => 1, sub { 127 $timer = AnyEvent->timer (after => 1, sub {
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109 133
110The advantage of doing this with a guard instead of simply calling C<< 134The advantage of doing this with a guard instead of simply calling C<<
111$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,
112or 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
113can 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
114one 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.
115 141
116=item Guard::cancel $guard 142=item Guard::cancel $guard
117 143
118Calling this function will "disable" the guard object returned by the 144Calling this function will "disable" the guard object returned by the
119C<guard> function, i.e. it will free the BLOCK originally passed to 145C<guard> function, i.e. it will free the BLOCK originally passed to
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128 154
129=back 155=back
130 156
131=head1 EXCEPTIONS 157=head1 EXCEPTIONS
132 158
133Guard blocks should not normally throw exceptions (e.g. C<die>), after 159Guard blocks should not normally throw exceptions (that is, C<die>). After
134all, they are usually used to clean up after such exceptions. However, if 160all, they are usually used to clean up after such exceptions. However, if
135something truly exceptional is happening, a guard block should be allowed 161something truly exceptional is happening, a guard block should be allowed
136to die. Also, programming errors are a large source of exceptions, and the 162to die. Also, programming errors are a large source of exceptions, and the
137programmer certainly wants to know about those. 163programmer certainly wants to know about those.
138 164
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143Therefore, whenever a guard block throws an exception, it will be caught, 169Therefore, whenever a guard block throws an exception, it will be caught,
144and this module will call the code reference stored in C<$Guard::DIED> 170and this module will call the code reference stored in C<$Guard::DIED>
145(with C<$@> set to the actual exception), which is similar to how most 171(with C<$@> set to the actual exception), which is similar to how most
146event loops handle this case. 172event loops handle this case.
147 173
174The default for C<$Guard::DIED> is to call C<warn "$@">.
175
176The C<$@> variable will be restored to its value before the guard call in
177all cases, so guards will not disturb C<$@> in any way.
178
148The code reference stored in C<$Guard::DIED> should not die (behaviour is 179The code reference stored in C<$Guard::DIED> should not die (behaviour is
149not guaranteed, but right now, the exception will simply be ignored). 180not guaranteed, but right now, the exception will simply be ignored).
150
151The default for C<$Guard::DIED> is to call C<warn "$@">.
152 181
153=head1 AUTHOR 182=head1 AUTHOR
154 183
155 Marc Lehmann <schmorp@schmorp.de> 184 Marc Lehmann <schmorp@schmorp.de>
156 http://home.schmorp.de/ 185 http://home.schmorp.de/
157 186
158=head1 THANKS 187=head1 THANKS
159 188
160To Marco Maisenhelder, who reminded me of the C<$Guard::DIED> solution to 189Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED>
161the problem of exceptions. 190solution to the problem of exceptions.
162 191
163=cut 192=cut
164 193

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