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Revision 1.2 by root, Sat Dec 13 17:50:29 2008 UTC vs.
Revision 1.3 by root, Sat Dec 13 21:47:07 2008 UTC

1NAME 1NAME
2 Guard - safe cleanup blocks 2 Guard - safe cleanup blocks
3 3
4SYNOPSIS 4SYNOPSIS
5 use Guard; 5 use Guard;
6
7 # temporarily chdir to "/etc" directory, but make sure
8 # to go back to "/" no matter how myfun exits:
9 sub myfun {
10 scope_guard { chdir "/" };
11 chdir "/etc";
12
13 call_function_that_might_die_or_other_fun_stuff;
14 }
6 15
7DESCRIPTION 16DESCRIPTION
8 This module implements so-called "guards". A guard is something (usually 17 This module implements so-called "guards". A guard is something (usually
9 an object) that "guards" a resource, ensuring that it is cleaned up when 18 an object) that "guards" a resource, ensuring that it is cleaned up when
10 expected. 19 expected.
… …
19 28
20 scope_guard BLOCK 29 scope_guard BLOCK
21 Registers a block that is executed when the current scope (block, 30 Registers a block that is executed when the current scope (block,
22 function, method, eval etc.) is exited. 31 function, method, eval etc.) is exited.
23 32
33 See the EXCEPTIONS section for an explanation of how exceptions
34 (i.e. "die") are handled inside guard blocks.
35
24 The description below sounds a bit complicated, but that's just 36 The description below sounds a bit complicated, but that's just
25 because "scope_guard" tries to get even corner cases "right": the 37 because "scope_guard" tries to get even corner cases "right": the
26 goal is to provide you with a rock solid clean up tool. 38 goal is to provide you with a rock solid clean up tool.
27 39
28 This is similar to this code fragment: 40 The behaviour is similar to this code fragment:
29 41
30 eval ... code following scope_guard ... 42 eval ... code following scope_guard ...
31 { 43 {
32 local $@; 44 local $@;
33 eval BLOCK; 45 eval BLOCK;
… …
37 49
38 Except it is much faster, and the whole thing gets executed even 50 Except it is much faster, and the whole thing gets executed even
39 when the BLOCK calls "exit", "goto", "last" or escapes via other 51 when the BLOCK calls "exit", "goto", "last" or escapes via other
40 means. 52 means.
41 53
42 See EXCEPTIONS, below, for an explanation of exception handling 54 If multiple BLOCKs are registered to the same scope, they will be
43 ("die") within guard blocks. 55 executed in reverse order. Other scope-related things such as
56 "local" are managed via the same mechanism, so variables "local"ised
57 *after* calling "scope_guard" will be restored when the guard runs.
44 58
45 Example: Temporarily change the directory to /etc and make sure it's 59 Example: temporarily change the timezone for the current process,
46 set back to / when the function returns: 60 ensuring it will be reset when the "if" scope is exited:
47 61
48 sub dosomething { 62 use Guard;
49 scope_guard { chdir "/" }; 63 use POSIX ();
50 chdir "/etc";
51 64
52 ... 65 if ($need_to_switch_tz) {
66 # make sure we call tzset after $ENV{TZ} has been restored
67 scope_guard { POSIX::tzset };
68
69 # localise after the scope_guard, so it gets undone in time
70 local $ENV{TZ} = "Europe/London";
71 POSIX::tzset;
72
73 # do something with the new timezone
53 } 74 }
54 75
55 my $guard = guard BLOCK 76 my $guard = guard BLOCK
56 Behaves the same as "scope_guard", except that instead of executing 77 Behaves the same as "scope_guard", except that instead of executing
57 the block on scope exit, it returns an object whose lifetime 78 the block on scope exit, it returns an object whose lifetime
… …
59 the object gets destroyed, the BLOCK gets executed as with 80 the object gets destroyed, the BLOCK gets executed as with
60 "scope_guard". 81 "scope_guard".
61 82
62 The returned object can be copied as many times as you want. 83 The returned object can be copied as many times as you want.
63 84
64 See EXCEPTIONS, below, for an explanation of exception handling 85 See the EXCEPTIONS section for an explanation of how exceptions
65 ("die") within guard blocks. 86 (i.e. "die") are handled inside guard blocks.
66 87
67 Example: acquire a Coro::Semaphore for a second by registering a 88 Example: acquire a Coro::Semaphore for a second by registering a
68 timer. The timer callback references the guard used to unlock it 89 timer. The timer callback references the guard used to unlock it
69 again. 90 again. (Please ignore the fact that "Coro::Semaphore" has a "guard"
91 method that does this already):
70 92
93 use Guard;
71 use AnyEvent; 94 use AnyEvent;
72 use Coro::Semaphore; 95 use Coro::Semaphore;
73 96
74 my $sem = new Coro::Semaphore; 97 my $sem = new Coro::Semaphore;
75 98
76 sub lock_1s { 99 sub lock_for_a_second {
77 $sem->down; 100 $sem->down;
78 my $guard = guard { $sem->up }; 101 my $guard = guard { $sem->up };
79 102
80 my $timer; 103 my $timer;
81 $timer = AnyEvent->timer (after => 1, sub { 104 $timer = AnyEvent->timer (after => 1, sub {
… …
88 The advantage of doing this with a guard instead of simply calling 111 The advantage of doing this with a guard instead of simply calling
89 "$sem->down" in the callback is that you can opt not to create the 112 "$sem->down" in the callback is that you can opt not to create the
90 timer, or your code can throw an exception before it can create the 113 timer, or your code can throw an exception before it can create the
91 timer, or you can create multiple timers or other event watchers and 114 timer, or you can create multiple timers or other event watchers and
92 only when the last one gets executed will the lock be unlocked. 115 only when the last one gets executed will the lock be unlocked.
116 Using the "guard", you do not have to worry about catching all the
117 places where you have to unlock the semaphore.
93 118
94 Guard::cancel $guard 119 $guard->cancel
95 Calling this function will "disable" the guard object returned by 120 Calling this function will "disable" the guard object returned by
96 the "guard" function, i.e. it will free the BLOCK originally passed 121 the "guard" function, i.e. it will free the BLOCK originally passed
97 to "guard "and will arrange for the BLOCK not to be executed. 122 to "guard "and will arrange for the BLOCK not to be executed.
98 123
99 This can be useful when you use "guard" to create a fatal cleanup 124 This can be useful when you use "guard" to create a fatal cleanup
100 handler and later decide it is no longer needed. 125 handler and later decide it is no longer needed.
101 126
102EXCEPTIONS 127EXCEPTIONS
103 Guard blocks should not normally throw exceptions (e.g. "die"), after 128 Guard blocks should not normally throw exceptions (that is, "die").
104 all, they are usually used to clean up after such exceptions. However, 129 After all, they are usually used to clean up after such exceptions.
105 if something truly exceptional is happening, a guard block should be 130 However, if something truly exceptional is happening, a guard block
106 allowed to die. Also, programming errors are a large source of 131 should be allowed to die. Also, programming errors are a large source of
107 exceptions, and the programmer certainly wants to know about those. 132 exceptions, and the programmer certainly wants to know about those.
108 133
109 Since in most cases, the block executing when the guard gets executes 134 Since in most cases, the block executing when the guard gets executes
110 does not know or does not care about the guard blocks, it makes little 135 does not know or does not care about the guard blocks, it makes little
111 sense to let containing code handle the exception. 136 sense to let containing code handle the exception.
… …
113 Therefore, whenever a guard block throws an exception, it will be 138 Therefore, whenever a guard block throws an exception, it will be
114 caught, and this module will call the code reference stored in 139 caught, and this module will call the code reference stored in
115 $Guard::DIED (with $@ set to the actual exception), which is similar to 140 $Guard::DIED (with $@ set to the actual exception), which is similar to
116 how most event loops handle this case. 141 how most event loops handle this case.
117 142
143 The default for $Guard::DIED is to call "warn "$@"".
144
145 The $@ variable will be restored to its value before the guard call in
146 all cases, so guards will not disturb $@ in any way.
147
118 The code reference stored in $Guard::DIED should not die (behaviour is 148 The code reference stored in $Guard::DIED should not die (behaviour is
119 not guaranteed, but right now, the exception will simply be ignored). 149 not guaranteed, but right now, the exception will simply be ignored).
120
121 The default for $Guard::DIED is to call "warn "$@"".
122 150
123AUTHOR 151AUTHOR
124 Marc Lehmann <schmorp@schmorp.de> 152 Marc Lehmann <schmorp@schmorp.de>
125 http://home.schmorp.de/ 153 http://home.schmorp.de/
126 154
127THANKS 155THANKS
128 To Marco Maisenhelder, who reminded me of the $Guard::DIED solution to 156 Thanks to Marco Maisenhelder, who reminded me of the $Guard::DIED
129 the problem of exceptions. 157 solution to the problem of exceptions.
130 158

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