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Revision 1.2 by root, Sat Dec 13 17:50:29 2008 UTC vs.
Revision 1.6 by root, Fri Mar 12 17:25:58 2010 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 code_that_might_die_or_does_other_fun_stuff;
14 }
15
16 # create an object that, when the last reference to it is gone,
17 # invokes the given codeblock:
18 my $guard = guard { print "destroyed!\n" };
19 undef $guard; # probably destroyed here
6 20
7DESCRIPTION 21DESCRIPTION
8 This module implements so-called "guards". A guard is something (usually 22 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 23 an object) that "guards" a resource, ensuring that it is cleaned up when
10 expected. 24 expected.
… …
19 33
20 scope_guard BLOCK 34 scope_guard BLOCK
21 Registers a block that is executed when the current scope (block, 35 Registers a block that is executed when the current scope (block,
22 function, method, eval etc.) is exited. 36 function, method, eval etc.) is exited.
23 37
38 See the EXCEPTIONS section for an explanation of how exceptions
39 (i.e. "die") are handled inside guard blocks.
40
24 The description below sounds a bit complicated, but that's just 41 The description below sounds a bit complicated, but that's just
25 because "scope_guard" tries to get even corner cases "right": the 42 because "scope_guard" tries to get even corner cases "right": the
26 goal is to provide you with a rock solid clean up tool. 43 goal is to provide you with a rock solid clean up tool.
27 44
28 This is similar to this code fragment: 45 The behaviour is similar to this code fragment:
29 46
30 eval ... code following scope_guard ... 47 eval ... code following scope_guard ...
31 { 48 {
32 local $@; 49 local $@;
33 eval BLOCK; 50 eval BLOCK;
… …
37 54
38 Except it is much faster, and the whole thing gets executed even 55 Except it is much faster, and the whole thing gets executed even
39 when the BLOCK calls "exit", "goto", "last" or escapes via other 56 when the BLOCK calls "exit", "goto", "last" or escapes via other
40 means. 57 means.
41 58
42 See EXCEPTIONS, below, for an explanation of exception handling 59 If multiple BLOCKs are registered to the same scope, they will be
43 ("die") within guard blocks. 60 executed in reverse order. Other scope-related things such as
61 "local" are managed via the same mechanism, so variables "local"ised
62 *after* calling "scope_guard" will be restored when the guard runs.
44 63
45 Example: Temporarily change the directory to /etc and make sure it's 64 Example: temporarily change the timezone for the current process,
46 set back to / when the function returns: 65 ensuring it will be reset when the "if" scope is exited:
47 66
48 sub dosomething { 67 use Guard;
49 scope_guard { chdir "/" }; 68 use POSIX ();
50 chdir "/etc";
51 69
52 ... 70 if ($need_to_switch_tz) {
71 # make sure we call tzset after $ENV{TZ} has been restored
72 scope_guard { POSIX::tzset };
73
74 # localise after the scope_guard, so it gets undone in time
75 local $ENV{TZ} = "Europe/London";
76 POSIX::tzset;
77
78 # do something with the new timezone
53 } 79 }
54 80
55 my $guard = guard BLOCK 81 my $guard = guard BLOCK
56 Behaves the same as "scope_guard", except that instead of executing 82 Behaves the same as "scope_guard", except that instead of executing
57 the block on scope exit, it returns an object whose lifetime 83 the block on scope exit, it returns an object whose lifetime
58 determines when the BLOCK gets executed: when the last reference to 84 determines when the BLOCK gets executed: when the last reference to
59 the object gets destroyed, the BLOCK gets executed as with 85 the object gets destroyed, the BLOCK gets executed as with
60 "scope_guard". 86 "scope_guard".
61 87
62 The returned object can be copied as many times as you want.
63
64 See EXCEPTIONS, below, for an explanation of exception handling 88 See the EXCEPTIONS section for an explanation of how exceptions
65 ("die") within guard blocks. 89 (i.e. "die") are handled inside guard blocks.
66 90
67 Example: acquire a Coro::Semaphore for a second by registering a 91 Example: acquire a Coro::Semaphore for a second by registering a
68 timer. The timer callback references the guard used to unlock it 92 timer. The timer callback references the guard used to unlock it
69 again. 93 again. (Please ignore the fact that "Coro::Semaphore" has a "guard"
94 method that does this already):
70 95
96 use Guard;
71 use AnyEvent; 97 use Coro::AnyEvent;
72 use Coro::Semaphore; 98 use Coro::Semaphore;
73 99
74 my $sem = new Coro::Semaphore; 100 my $sem = new Coro::Semaphore;
75 101
76 sub lock_1s { 102 sub lock_for_a_second {
77 $sem->down; 103 $sem->down;
78 my $guard = guard { $sem->up }; 104 my $guard = guard { $sem->up };
79 105
80 my $timer; 106 Coro::AnyEvent::sleep 1;
81 $timer = AnyEvent->timer (after => 1, sub { 107
82 # do something 108 # $sem->up gets executed when returning
83 undef $sem;
84 undef $timer;
85 });
86 } 109 }
87 110
88 The advantage of doing this with a guard instead of simply calling 111 The advantage of doing this with a guard instead of simply calling
89 "$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 the thread gets canceled), or you can create multiple
92 only when the last one gets executed will the lock be unlocked. 115 timers or other event watchers and only when the last one gets
116 executed will the lock be unlocked. Using the "guard", you do not
117 have to worry about catching all the places where you have to unlock
118 the semaphore.
93 119
94 Guard::cancel $guard 120 $guard->cancel
95 Calling this function will "disable" the guard object returned by 121 Calling this function will "disable" the guard object returned by
96 the "guard" function, i.e. it will free the BLOCK originally passed 122 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. 123 to "guard "and will arrange for the BLOCK not to be executed.
98 124
99 This can be useful when you use "guard" to create a fatal cleanup 125 This can be useful when you use "guard" to create a cleanup handler
100 handler and later decide it is no longer needed. 126 to be called under fatal conditions and later decide it is no longer
127 needed.
101 128
102EXCEPTIONS 129EXCEPTIONS
103 Guard blocks should not normally throw exceptions (e.g. "die"), after 130 Guard blocks should not normally throw exceptions (that is, "die").
104 all, they are usually used to clean up after such exceptions. However, 131 After all, they are usually used to clean up after such exceptions.
105 if something truly exceptional is happening, a guard block should be 132 However, if something truly exceptional is happening, a guard block
106 allowed to die. Also, programming errors are a large source of 133 should of course be allowed to die. Also, programming errors are a large
107 exceptions, and the programmer certainly wants to know about those. 134 source of exceptions, and the programmer certainly wants to know about
135 those.
108 136
109 Since in most cases, the block executing when the guard gets executes 137 Since in most cases, the block executing when the guard gets executed
110 does not know or does not care about the guard blocks, it makes little 138 does not know or does not care about the guard blocks, it makes little
111 sense to let containing code handle the exception. 139 sense to let containing code handle the exception.
112 140
113 Therefore, whenever a guard block throws an exception, it will be 141 Therefore, whenever a guard block throws an exception, it will be caught
114 caught, and this module will call the code reference stored in 142 by Guard, followed by calling the code reference stored in $Guard::DIED
115 $Guard::DIED (with $@ set to the actual exception), which is similar to 143 (with $@ set to the actual exception), which is similar to how most
116 how most event loops handle this case. 144 event loops handle this case.
145
146 The default for $Guard::DIED is to call "warn "$@"", i.e. the error is
147 printed as a warning and the program continues.
148
149 The $@ variable will be restored to its value before the guard call in
150 all cases, so guards will not disturb $@ in any way.
117 151
118 The code reference stored in $Guard::DIED should not die (behaviour is 152 The code reference stored in $Guard::DIED should not die (behaviour is
119 not guaranteed, but right now, the exception will simply be ignored). 153 not guaranteed, but right now, the exception will simply be ignored).
120
121 The default for $Guard::DIED is to call "warn "$@"".
122 154
123AUTHOR 155AUTHOR
124 Marc Lehmann <schmorp@schmorp.de> 156 Marc Lehmann <schmorp@schmorp.de>
125 http://home.schmorp.de/ 157 http://home.schmorp.de/
126 158
127THANKS 159THANKS
128 To Marco Maisenhelder, who reminded me of the $Guard::DIED solution to 160 Thanks to Marco Maisenhelder, who reminded me of the $Guard::DIED
129 the problem of exceptions. 161 solution to the problem of exceptions.
130 162
163SEE ALSO
164 Scope::Guard and Sub::ScopeFinalizer, which actually implement dynamic
165 guards only, not scoped guards, and have a lot higher CPU, memory and
166 typing overhead.
167
168 Hook::Scope, which has apparently never been finished and can corrupt
169 memory when used.
170

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