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Revision 1.4 by root, Fri Dec 26 13:04:04 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 6
7 # temporarily chdir to "/etc" directory, but make sure 7 # temporarily chdir to "/etc" directory, but make sure
8 # to go back to "/" no matter how myfun exits: 8 # to go back to "/" no matter how myfun exits:
9 sub myfun { 9 sub myfun {
10 scope_guard { chdir "/" }; 10 scope_guard { chdir "/" };
11 chdir "/etc"; 11 chdir "/etc";
12 12
13 code_that_might_die_or_does_other_fun_stuff; 13 code_that_might_die_or_does_other_fun_stuff;
14 } 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
15 20
16DESCRIPTION 21DESCRIPTION
17 This module implements so-called "guards". A guard is something (usually 22 This module implements so-called "guards". A guard is something (usually
18 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
19 expected. 24 expected.
… …
78 the block on scope exit, it returns an object whose lifetime 83 the block on scope exit, it returns an object whose lifetime
79 determines when the BLOCK gets executed: when the last reference to 84 determines when the BLOCK gets executed: when the last reference to
80 the object gets destroyed, the BLOCK gets executed as with 85 the object gets destroyed, the BLOCK gets executed as with
81 "scope_guard". 86 "scope_guard".
82 87
83 The returned object can be copied as many times as you want.
84
85 See the EXCEPTIONS section for an explanation of how exceptions 88 See the EXCEPTIONS section for an explanation of how exceptions
86 (i.e. "die") are handled inside guard blocks. 89 (i.e. "die") are handled inside guard blocks.
87 90
88 Example: acquire a Coro::Semaphore for a second by registering a 91 Example: acquire a Coro::Semaphore for a second by registering a
89 timer. The timer callback references the guard used to unlock it 92 timer. The timer callback references the guard used to unlock it
90 again. (Please ignore the fact that "Coro::Semaphore" has a "guard" 93 again. (Please ignore the fact that "Coro::Semaphore" has a "guard"
91 method that does this already): 94 method that does this already):
92 95
93 use Guard; 96 use Guard;
94 use AnyEvent; 97 use Coro::AnyEvent;
95 use Coro::Semaphore; 98 use Coro::Semaphore;
96 99
97 my $sem = new Coro::Semaphore; 100 my $sem = new Coro::Semaphore;
98 101
99 sub lock_for_a_second { 102 sub lock_for_a_second {
100 $sem->down; 103 $sem->down;
101 my $guard = guard { $sem->up }; 104 my $guard = guard { $sem->up };
102 105
103 my $timer; 106 Coro::AnyEvent::sleep 1;
104 $timer = AnyEvent->timer (after => 1, sub { 107
105 # do something 108 # $sem->up gets executed when returning
106 undef $sem;
107 undef $timer;
108 });
109 } 109 }
110 110
111 The advantage of doing this with a guard instead of simply calling 111 The advantage of doing this with a guard instead of simply calling
112 "$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
113 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
114 timer, or you can create multiple timers or other event watchers and 114 timer (or the thread gets canceled), or you can create multiple
115 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 Using the "guard", you do not have to worry about catching all the 116 executed will the lock be unlocked. Using the "guard", you do not
117 places where you have to unlock the semaphore. 117 have to worry about catching all the places where you have to unlock
118 the semaphore.
118 119
119 $guard->cancel 120 $guard->cancel
120 Calling this function will "disable" the guard object returned by 121 Calling this function will "disable" the guard object returned by
121 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
122 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.
123 124
124 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
125 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.
126 128
127EXCEPTIONS 129EXCEPTIONS
128 Guard blocks should not normally throw exceptions (that is, "die"). 130 Guard blocks should not normally throw exceptions (that is, "die").
129 After all, they are usually used to clean up after such exceptions. 131 After all, they are usually used to clean up after such exceptions.
130 However, if something truly exceptional is happening, a guard block 132 However, if something truly exceptional is happening, a guard block
131 should be 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
132 exceptions, and the programmer certainly wants to know about those. 134 source of exceptions, and the programmer certainly wants to know about
135 those.
133 136
134 Since in most cases, the block executing when the guard gets executed 137 Since in most cases, the block executing when the guard gets executed
135 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
136 sense to let containing code handle the exception. 139 sense to let containing code handle the exception.
137 140
138 Therefore, whenever a guard block throws an exception, it will be 141 Therefore, whenever a guard block throws an exception, it will be caught
139 caught, followed by calling the code reference stored in $Guard::DIED 142 by Guard, followed by calling the code reference stored in $Guard::DIED
140 (with $@ set to the actual exception), which is similar to how most 143 (with $@ set to the actual exception), which is similar to how most
141 event loops handle this case. 144 event loops handle this case.
142 145
143 The default for $Guard::DIED is to call "warn "$@"". 146 The default for $Guard::DIED is to call "warn "$@"", i.e. the error is
147 printed as a warning and the program continues.
144 148
145 The $@ variable will be restored to its value before the guard call in 149 The $@ variable will be restored to its value before the guard call in
146 all cases, so guards will not disturb $@ in any way. 150 all cases, so guards will not disturb $@ in any way.
147 151
148 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
… …
155THANKS 159THANKS
156 Thanks to Marco Maisenhelder, who reminded me of the $Guard::DIED 160 Thanks to Marco Maisenhelder, who reminded me of the $Guard::DIED
157 solution to the problem of exceptions. 161 solution to the problem of exceptions.
158 162
159SEE ALSO 163SEE ALSO
160 Scope::Guard and Sub::ScopeFinalizer, which actually implement dynamic, 164 Scope::Guard and Sub::ScopeFinalizer, which actually implement dynamic
161 not scoped guards, and have a lot higher CPU, memory and typing 165 guards only, not scoped guards, and have a lot higher CPU, memory and
162 overhead. 166 typing overhead.
163 167
164 Hook::Scope, which has apparently never been finished and corrupts 168 Hook::Scope, which has apparently never been finished and can corrupt
165 memory when used. 169 memory when used.
166 170

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