[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";
       
      call_function_that_might_die_or_other_fun_stuff;
       }

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 executes
    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, and this module will call 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.

Revision:	1.3
Committed:	Sat Dec 13 21:47:07 2008 UTC (15 years, 5 months ago) by root
Branch:	MAIN
CVS Tags:	rel-0_5
Changes since 1.2:	+53 -25 lines
Log Message:	0.5
#	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
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			}
15	root	1.1
16			DESCRIPTION
17	root	1.2	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
19			expected.
20
21			Specifically, this module supports two different types of guards: guard
22			objects, which execute a given code block when destroyed, and scoped
23			guards, which are tied to the scope exit.
24
25			FUNCTIONS
26			This module currently exports the "scope_guard" and "guard" functions by
27			default.
28
29			scope_guard BLOCK
30			Registers a block that is executed when the current scope (block,
31			function, method, eval etc.) is exited.
32
33	root	1.3	See the EXCEPTIONS section for an explanation of how exceptions
34			(i.e. "die") are handled inside guard blocks.
35
36	root	1.2	The description below sounds a bit complicated, but that's just
37			because "scope_guard" tries to get even corner cases "right": the
38			goal is to provide you with a rock solid clean up tool.
39
40	root	1.3	The behaviour is similar to this code fragment:
41	root	1.2
42			eval ... code following scope_guard ...
43			{
44			local $@;
45			eval BLOCK;
46			eval { $Guard::DIED->() } if $@;
47			}
48			die if $@;
49
50			Except it is much faster, and the whole thing gets executed even
51			when the BLOCK calls "exit", "goto", "last" or escapes via other
52			means.
53
54	root	1.3	If multiple BLOCKs are registered to the same scope, they will be
55			executed in reverse order. Other scope-related things such as
56			"local" are managed via the same mechanism, so variables "local"ised
57			after calling "scope_guard" will be restored when the guard runs.
58
59			Example: temporarily change the timezone for the current process,
60			ensuring it will be reset when the "if" scope is exited:
61
62			use Guard;
63			use POSIX ();
64
65			if ($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	root	1.2
73	root	1.3	# do something with the new timezone
74	root	1.2	}
75
76			my $guard = guard BLOCK
77			Behaves the same as "scope_guard", except that instead of executing
78			the block on scope exit, it returns an object whose lifetime
79			determines when the BLOCK gets executed: when the last reference to
80			the object gets destroyed, the BLOCK gets executed as with
81			"scope_guard".
82
83			The returned object can be copied as many times as you want.
84
85	root	1.3	See the EXCEPTIONS section for an explanation of how exceptions
86			(i.e. "die") are handled inside guard blocks.
87	root	1.2
88			Example: acquire a Coro::Semaphore for a second by registering a
89			timer. The timer callback references the guard used to unlock it
90	root	1.3	again. (Please ignore the fact that "Coro::Semaphore" has a "guard"
91			method that does this already):
92	root	1.2
93	root	1.3	use Guard;
94	root	1.2	use AnyEvent;
95			use Coro::Semaphore;
96
97			my $sem = new Coro::Semaphore;
98
99	root	1.3	sub lock_for_a_second {
100	root	1.2	$sem->down;
101			my $guard = guard { $sem->up };
102
103			my $timer;
104			$timer = AnyEvent->timer (after => 1, sub {
105			# do something
106			undef $sem;
107			undef $timer;
108			});
109			}
110
111			The advantage of doing this with a guard instead of simply calling
112			"$sem->down" in the callback is that you can opt not to create the
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
115			only when the last one gets executed will the lock be unlocked.
116	root	1.3	Using the "guard", you do not have to worry about catching all the
117			places where you have to unlock the semaphore.
118	root	1.2
119	root	1.3	$guard->cancel
120	root	1.2	Calling this function will "disable" the guard object returned by
121			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
124			This can be useful when you use "guard" to create a fatal cleanup
125			handler and later decide it is no longer needed.
126
127			EXCEPTIONS
128	root	1.3	Guard blocks should not normally throw exceptions (that is, "die").
129			After all, they are usually used to clean up after such exceptions.
130			However, if something truly exceptional is happening, a guard block
131			should be allowed to die. Also, programming errors are a large source of
132	root	1.2	exceptions, and the programmer certainly wants to know about those.
133
134			Since in most cases, the block executing when the guard gets executes
135			does not know or does not care about the guard blocks, it makes little
136			sense to let containing code handle the exception.
137
138			Therefore, whenever a guard block throws an exception, it will be
139			caught, and this module will call the code reference stored in
140			$Guard::DIED (with $@ set to the actual exception), which is similar to
141			how most event loops handle this case.
142
143	root	1.3	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
148	root	1.2	The code reference stored in $Guard::DIED should not die (behaviour is
149			not guaranteed, but right now, the exception will simply be ignored).
150
151	root	1.1	AUTHOR
152			Marc Lehmann <schmorp@schmorp.de>
153			http://home.schmorp.de/
154
155	root	1.2	THANKS
156	root	1.3	Thanks to Marco Maisenhelder, who reminded me of the $Guard::DIED
157			solution to the problem of exceptions.
158	root	1.2