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Comparing Guard/Guard.pm (file contents):
Revision 1.3 by root, Sat Dec 13 17:50:29 2008 UTC vs.
Revision 1.14 by root, Sat Dec 13 22:05:20 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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25 34
26=cut 35=cut
27 36
28package Guard; 37package Guard;
29 38
39no warnings;
40
30BEGIN { 41BEGIN {
31 $VERSION = '0.1'; 42 $VERSION = '0.5';
32 @ISA = qw(Exporter); 43 @ISA = qw(Exporter);
33 @EXPORT = qw(guard scope_guard); 44 @EXPORT = qw(guard scope_guard);
34 45
35 require Exporter; 46 require Exporter;
36 47
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43=item scope_guard BLOCK 54=item scope_guard BLOCK
44 55
45Registers a block that is executed when the current scope (block, 56Registers a block that is executed when the current scope (block,
46function, method, eval etc.) is exited. 57function, method, eval etc.) is exited.
47 58
59See the EXCEPTIONS section for an explanation of how exceptions
60(i.e. C<die>) are handled inside guard blocks.
61
48The description below sounds a bit complicated, but that's just because 62The 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 63C<scope_guard> tries to get even corner cases "right": the goal is to
50provide you with a rock solid clean up tool. 64provide you with a rock solid clean up tool.
51 65
52This is similar to this code fragment: 66The behaviour is similar to this code fragment:
53 67
54 eval ... code following scope_guard ... 68 eval ... code following scope_guard ...
55 { 69 {
56 local $@; 70 local $@;
57 eval BLOCK; 71 eval BLOCK;
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60 die if $@; 74 die if $@;
61 75
62Except it is much faster, and the whole thing gets executed even when the 76Except it is much faster, and the whole thing gets executed even when the
63BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means. 77BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means.
64 78
65See B<EXCEPTIONS>, below, for an explanation of exception handling 79If multiple BLOCKs are registered to the same scope, they will be executed
66(C<die>) within guard blocks. 80in reverse order. Other scope-related things such as C<local> are managed
81via the same mechanism, so variables C<local>ised I<after> calling
82C<scope_guard> will be restored when the guard runs.
67 83
68Example: Temporarily change the directory to F</etc> and make sure it's 84Example: temporarily change the timezone for the current process,
69set back to F</> when the function returns: 85ensuring it will be reset when the C<if> scope is exited:
70 86
71 sub dosomething { 87 use Guard;
72 scope_guard { chdir "/" }; 88 use POSIX ();
73 chdir "/etc";
74 89
75 ... 90 if ($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
76 } 99 }
77 100
78=item my $guard = guard BLOCK 101=item my $guard = guard BLOCK
79 102
80Behaves 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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82when 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
83destroyed, the BLOCK gets executed as with C<scope_guard>. 106destroyed, the BLOCK gets executed as with C<scope_guard>.
84 107
85The returned object can be copied as many times as you want. 108The returned object can be copied as many times as you want.
86 109
87See B<EXCEPTIONS>, below, for an explanation of exception handling 110See the EXCEPTIONS section for an explanation of how exceptions
88(C<die>) within guard blocks. 111(i.e. C<die>) are handled inside guard blocks.
89 112
90Example: acquire a Coro::Semaphore for a second by registering a 113Example: acquire a Coro::Semaphore for a second by registering a
91timer. 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):
92 117
118 use Guard;
93 use AnyEvent; 119 use AnyEvent;
94 use Coro::Semaphore; 120 use Coro::Semaphore;
95 121
96 my $sem = new Coro::Semaphore; 122 my $sem = new Coro::Semaphore;
97 123
98 sub lock_1s { 124 sub lock_for_a_second {
99 $sem->down; 125 $sem->down;
100 my $guard = guard { $sem->up }; 126 my $guard = guard { $sem->up };
101 127
102 my $timer; 128 my $timer;
103 $timer = AnyEvent->timer (after => 1, sub { 129 $timer = AnyEvent->timer (after => 1, sub {
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109 135
110The advantage of doing this with a guard instead of simply calling C<< 136The advantage of doing this with a guard instead of simply calling C<<
111$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,
112or 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
113can 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
114one 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.
115 143
116=item Guard::cancel $guard 144=item $guard->cancel
117 145
118Calling this function will "disable" the guard object returned by the 146Calling this function will "disable" the guard object returned by the
119C<guard> function, i.e. it will free the BLOCK originally passed to 147C<guard> function, i.e. it will free the BLOCK originally passed to
120C<guard >and will arrange for the BLOCK not to be executed. 148C<guard >and will arrange for the BLOCK not to be executed.
121 149
… …
128 156
129=back 157=back
130 158
131=head1 EXCEPTIONS 159=head1 EXCEPTIONS
132 160
133Guard blocks should not normally throw exceptions (e.g. C<die>), after 161Guard blocks should not normally throw exceptions (that is, C<die>). After
134all, they are usually used to clean up after such exceptions. However, if 162all, they are usually used to clean up after such exceptions. However, if
135something truly exceptional is happening, a guard block should be allowed 163something truly exceptional is happening, a guard block should be allowed
136to die. Also, programming errors are a large source of exceptions, and the 164to die. Also, programming errors are a large source of exceptions, and the
137programmer certainly wants to know about those. 165programmer certainly wants to know about those.
138 166
139Since in most cases, the block executing when the guard gets executes does 167Since in most cases, the block executing when the guard gets executed does
140not know or does not care about the guard blocks, it makes little sense to 168not know or does not care about the guard blocks, it makes little sense to
141let containing code handle the exception. 169let containing code handle the exception.
142 170
143Therefore, whenever a guard block throws an exception, it will be caught, 171Therefore, whenever a guard block throws an exception, it will be caught,
144and this module will call the code reference stored in C<$Guard::DIED> 172followed 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 173C<$@> set to the actual exception), which is similar to how most event
146event loops handle this case. 174loops handle this case.
175
176The default for C<$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<$@> in any way.
147 180
148The code reference stored in C<$Guard::DIED> should not die (behaviour is 181The code reference stored in C<$Guard::DIED> should not die (behaviour is
149not guaranteed, but right now, the exception will simply be ignored). 182not guaranteed, but right now, the exception will simply be ignored).
150
151The default for C<$Guard::DIED> is to call C<warn "$@">.
152 183
153=head1 AUTHOR 184=head1 AUTHOR
154 185
155 Marc Lehmann <schmorp@schmorp.de> 186 Marc Lehmann <schmorp@schmorp.de>
156 http://home.schmorp.de/ 187 http://home.schmorp.de/
157 188
158=head1 THANKS 189=head1 THANKS
159 190
160To Marco Maisenhelder, who reminded me of the C<$Guard::DIED> solution to 191Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED>
161the problem of exceptions. 192solution to the problem of exceptions.
162 193
163=cut 194=cut
164 195

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