[ViewVC] Annotation of: cvs/Guard/README

NAME
    Guard - safe cleanup blocks

SYNOPSIS
       use Guard;
   
       # temporarily chdir to "/etc" directory, but make sure
       # to go back to "/" no matter how myfun exits:
       sub myfun {
          scope_guard { chdir "/" };
          chdir "/etc";
   
          code_that_might_die_or_does_other_fun_stuff;
       }

       # create an object that, when the last reference to it is gone,
       # invokes the given codeblock:
       my $guard = guard { print "destroyed!\n" };
       undef $guard; # probably destroyed here

DESCRIPTION
    This module implements so-called "guards". A guard is something (usually
    an object) that "guards" a resource, ensuring that it is cleaned up when
    expected.

    Specifically, this module supports two different types of guards: guard
    objects, which execute a given code block when destroyed, and scoped
    guards, which are tied to the scope exit.

FUNCTIONS
    This module currently exports the "scope_guard" and "guard" functions by
    default.

    scope_guard BLOCK
        Registers a block that is executed when the current scope (block,
        function, method, eval etc.) is exited.

        See the EXCEPTIONS section for an explanation of how exceptions
        (i.e. "die") are handled inside guard blocks.

        The description below sounds a bit complicated, but that's just
        because "scope_guard" tries to get even corner cases "right": the
        goal is to provide you with a rock solid clean up tool.

        The behaviour is similar to this code fragment:

           eval ... code following scope_guard ...
           {
              local $@;
              eval BLOCK;
              eval { $Guard::DIED->() } if $@;
           }
           die if $@;

        Except it is much faster, and the whole thing gets executed even
        when the BLOCK calls "exit", "goto", "last" or escapes via other
        means.

        If multiple BLOCKs are registered to the same scope, they will be
        executed in reverse order. Other scope-related things such as
        "local" are managed via the same mechanism, so variables "local"ised
        *after* calling "scope_guard" will be restored when the guard runs.

        Example: temporarily change the timezone for the current process,
        ensuring it will be reset when the "if" scope is exited:

           use Guard;
           use POSIX ();

           if ($need_to_switch_tz) {
              # make sure we call tzset after $ENV{TZ} has been restored
              scope_guard { POSIX::tzset };

              # localise after the scope_guard, so it gets undone in time
              local $ENV{TZ} = "Europe/London";
              POSIX::tzset;

              # do something with the new timezone
           }

    my $guard = guard BLOCK
        Behaves the same as "scope_guard", except that instead of executing
        the block on scope exit, it returns an object whose lifetime
        determines when the BLOCK gets executed: when the last reference to
        the object gets destroyed, the BLOCK gets executed as with
        "scope_guard".

        The returned object can be copied as many times as you want.

        See the EXCEPTIONS section for an explanation of how exceptions
        (i.e. "die") are handled inside guard blocks.

        Example: acquire a Coro::Semaphore for a second by registering a
        timer. The timer callback references the guard used to unlock it
        again. (Please ignore the fact that "Coro::Semaphore" has a "guard"
        method that does this already):

           use Guard;
           use AnyEvent;
           use Coro::Semaphore;

           my $sem = new Coro::Semaphore;

           sub lock_for_a_second {
              $sem->down;
              my $guard = guard { $sem->up };

              my $timer;
              $timer = AnyEvent->timer (after => 1, sub {
                 # do something
                 undef $sem;
                 undef $timer;
              });
           }

        The advantage of doing this with a guard instead of simply calling
        "$sem->down" in the callback is that you can opt not to create the
        timer, or your code can throw an exception before it can create the
        timer, or you can create multiple timers or other event watchers and
        only when the last one gets executed will the lock be unlocked.
        Using the "guard", you do not have to worry about catching all the
        places where you have to unlock the semaphore.

    $guard->cancel
        Calling this function will "disable" the guard object returned by
        the "guard" function, i.e. it will free the BLOCK originally passed
        to "guard "and will arrange for the BLOCK not to be executed.

        This can be useful when you use "guard" to create a fatal cleanup
        handler and later decide it is no longer needed.

EXCEPTIONS
    Guard blocks should not normally throw exceptions (that is, "die").
    After all, they are usually used to clean up after such exceptions.
    However, if something truly exceptional is happening, a guard block
    should be allowed to die. Also, programming errors are a large source of
    exceptions, and the programmer certainly wants to know about those.

    Since in most cases, the block executing when the guard gets executed
    does not know or does not care about the guard blocks, it makes little
    sense to let containing code handle the exception.

    Therefore, whenever a guard block throws an exception, it will be
    caught, followed by calling the code reference stored in $Guard::DIED
    (with $@ set to the actual exception), which is similar to how most
    event loops handle this case.

    The default for $Guard::DIED is to call "warn "$@"".

    The $@ variable will be restored to its value before the guard call in
    all cases, so guards will not disturb $@ in any way.

    The code reference stored in $Guard::DIED should not die (behaviour is
    not guaranteed, but right now, the exception will simply be ignored).

AUTHOR
     Marc Lehmann <schmorp@schmorp.de>
     http://home.schmorp.de/

THANKS
    Thanks to Marco Maisenhelder, who reminded me of the $Guard::DIED
    solution to the problem of exceptions.

SEE ALSO
    Scope::Guard and Sub::ScopeFinalizer, which actually implement dynamic,
    not scoped guards, and have a lot higher CPU, memory and typing
    overhead.

    Hook::Scope, which has apparently never been finished and corrupts
    memory when used.

Revision:	1.5
Committed:	Sun Jul 19 05:44:10 2009 UTC (14 years, 10 months ago) by root
Branch:	MAIN
CVS Tags:	rel-1_021
Changes since 1.4:	+9 -4 lines
Log Message:	1.021
#	User	Rev	Content
1	root	1.1	NAME
2	root	1.2	Guard - safe cleanup blocks
3	root	1.1
4			SYNOPSIS
5	root	1.3	use Guard;
6	root	1.5
7			# temporarily chdir to "/etc" directory, but make sure
8	root	1.3	# to go back to "/" no matter how myfun exits:
9			sub myfun {
10			scope_guard { chdir "/" };
11			chdir "/etc";
12	root	1.5
13			code_that_might_die_or_does_other_fun_stuff;
14	root	1.3	}
15	root	1.1
16	root	1.5	# 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
20
21	root	1.1	DESCRIPTION
22	root	1.2	This module implements so-called "guards". A guard is something (usually
23			an object) that "guards" a resource, ensuring that it is cleaned up when
24			expected.
25
26			Specifically, this module supports two different types of guards: guard
27			objects, which execute a given code block when destroyed, and scoped
28			guards, which are tied to the scope exit.
29
30			FUNCTIONS
31			This module currently exports the "scope_guard" and "guard" functions by
32			default.
33
34			scope_guard BLOCK
35			Registers a block that is executed when the current scope (block,
36			function, method, eval etc.) is exited.
37
38	root	1.3	See the EXCEPTIONS section for an explanation of how exceptions
39			(i.e. "die") are handled inside guard blocks.
40
41	root	1.2	The description below sounds a bit complicated, but that's just
42			because "scope_guard" tries to get even corner cases "right": the
43			goal is to provide you with a rock solid clean up tool.
44
45	root	1.3	The behaviour is similar to this code fragment:
46	root	1.2
47			eval ... code following scope_guard ...
48			{
49			local $@;
50			eval BLOCK;
51			eval { $Guard::DIED->() } if $@;
52			}
53			die if $@;
54
55			Except it is much faster, and the whole thing gets executed even
56			when the BLOCK calls "exit", "goto", "last" or escapes via other
57			means.
58
59	root	1.3	If multiple BLOCKs are registered to the same scope, they will be
60			executed in reverse order. Other scope-related things such as
61			"local" are managed via the same mechanism, so variables "local"ised
62			after calling "scope_guard" will be restored when the guard runs.
63
64			Example: temporarily change the timezone for the current process,
65			ensuring it will be reset when the "if" scope is exited:
66
67			use Guard;
68			use POSIX ();
69
70			if ($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	root	1.2
78	root	1.3	# do something with the new timezone
79	root	1.2	}
80
81			my $guard = guard BLOCK
82			Behaves the same as "scope_guard", except that instead of executing
83			the block on scope exit, it returns an object whose lifetime
84			determines when the BLOCK gets executed: when the last reference to
85			the object gets destroyed, the BLOCK gets executed as with
86			"scope_guard".
87
88			The returned object can be copied as many times as you want.
89
90	root	1.3	See the EXCEPTIONS section for an explanation of how exceptions
91			(i.e. "die") are handled inside guard blocks.
92	root	1.2
93			Example: acquire a Coro::Semaphore for a second by registering a
94			timer. The timer callback references the guard used to unlock it
95	root	1.3	again. (Please ignore the fact that "Coro::Semaphore" has a "guard"
96			method that does this already):
97	root	1.2
98	root	1.3	use Guard;
99	root	1.2	use AnyEvent;
100			use Coro::Semaphore;
101
102			my $sem = new Coro::Semaphore;
103
104	root	1.3	sub lock_for_a_second {
105	root	1.2	$sem->down;
106			my $guard = guard { $sem->up };
107
108			my $timer;
109			$timer = AnyEvent->timer (after => 1, sub {
110			# do something
111			undef $sem;
112			undef $timer;
113			});
114			}
115
116			The advantage of doing this with a guard instead of simply calling
117			"$sem->down" in the callback is that you can opt not to create the
118			timer, or your code can throw an exception before it can create the
119			timer, or you can create multiple timers or other event watchers and
120			only when the last one gets executed will the lock be unlocked.
121	root	1.3	Using the "guard", you do not have to worry about catching all the
122			places where you have to unlock the semaphore.
123	root	1.2
124	root	1.3	$guard->cancel
125	root	1.2	Calling this function will "disable" the guard object returned by
126			the "guard" function, i.e. it will free the BLOCK originally passed
127			to "guard "and will arrange for the BLOCK not to be executed.
128
129			This can be useful when you use "guard" to create a fatal cleanup
130			handler and later decide it is no longer needed.
131
132			EXCEPTIONS
133	root	1.3	Guard blocks should not normally throw exceptions (that is, "die").
134			After all, they are usually used to clean up after such exceptions.
135			However, if something truly exceptional is happening, a guard block
136			should be allowed to die. Also, programming errors are a large source of
137	root	1.2	exceptions, and the programmer certainly wants to know about those.
138
139	root	1.4	Since in most cases, the block executing when the guard gets executed
140	root	1.2	does not know or does not care about the guard blocks, it makes little
141			sense to let containing code handle the exception.
142
143			Therefore, whenever a guard block throws an exception, it will be
144	root	1.4	caught, followed by calling the code reference stored in $Guard::DIED
145			(with $@ set to the actual exception), which is similar to how most
146			event loops handle this case.
147	root	1.2
148	root	1.3	The default for $Guard::DIED is to call "warn "$@"".
149
150			The $@ variable will be restored to its value before the guard call in
151			all cases, so guards will not disturb $@ in any way.
152
153	root	1.2	The code reference stored in $Guard::DIED should not die (behaviour is
154			not guaranteed, but right now, the exception will simply be ignored).
155
156	root	1.1	AUTHOR
157			Marc Lehmann <schmorp@schmorp.de>
158			http://home.schmorp.de/
159
160	root	1.2	THANKS
161	root	1.3	Thanks to Marco Maisenhelder, who reminded me of the $Guard::DIED
162			solution to the problem of exceptions.
163	root	1.2
164	root	1.4	SEE ALSO
165			Scope::Guard and Sub::ScopeFinalizer, which actually implement dynamic,
166			not scoped guards, and have a lot higher CPU, memory and typing
167			overhead.
168
169			Hook::Scope, which has apparently never been finished and corrupts
170			memory when used.
171