[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
    scope_guard ($coderef)
        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
    my $guard = guard ($coderef)
        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".

        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 Coro::AnyEvent;
           use Coro::Semaphore;

           my $sem = new Coro::Semaphore;

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

              Coro::AnyEvent::sleep 1;

              # $sem->up gets executed when returning
           }

        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 the thread gets canceled), 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 cleanup handler
        to be called under fatal conditions 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 of course 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
    by Guard, 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 $Guard::DIED is to call "warn "$@"", i.e. the error is
    printed as a warning and the program continues.

    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
    dynamically scoped guards only, not the lexically scoped guards that
    their documentation promises, and have a lot higher CPU, memory and
    typing overhead.

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

    Scope::Guard seems to have a big SEE ALSO section for even more modules
    like it.

Revision:	1.7
Committed:	Sat Jul 2 00:38:44 2011 UTC (12 years, 10 months ago) by root
Branch:	MAIN
CVS Tags:	rel-1_023, rel-1_022, HEAD
Changes since 1.6:	+8 -2 lines
Log Message:	1.022
#	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 $guard; # probably destroyed here
20
21	root	1.1	DESCRIPTION
22	root	1.2	This module implements so-called "guards". A guard is something (usually
23			an object) that "guards" a resource, ensuring that it is cleaned up when
24			expected.
25
26			Specifically, this module supports two different types of guards: guard
27			objects, which execute a given code block when destroyed, and scoped
28			guards, which are tied to the scope exit.
29
30			FUNCTIONS
31			This module currently exports the "scope_guard" and "guard" functions by
32			default.
33
34			scope_guard BLOCK
35	root	1.7	scope_guard ($coderef)
36	root	1.2	Registers a block that is executed when the current scope (block,
37			function, method, eval etc.) is exited.
38
39	root	1.3	See the EXCEPTIONS section for an explanation of how exceptions
40			(i.e. "die") are handled inside guard blocks.
41
42	root	1.2	The description below sounds a bit complicated, but that's just
43			because "scope_guard" tries to get even corner cases "right": the
44			goal is to provide you with a rock solid clean up tool.
45
46	root	1.3	The behaviour is similar to this code fragment:
47	root	1.2
48			eval ... code following scope_guard ...
49			{
50			local $@;
51			eval BLOCK;
52			eval { $Guard::DIED->() } if $@;
53			}
54			die if $@;
55
56			Except it is much faster, and the whole thing gets executed even
57			when the BLOCK calls "exit", "goto", "last" or escapes via other
58			means.
59
60	root	1.3	If multiple BLOCKs are registered to the same scope, they will be
61			executed in reverse order. Other scope-related things such as
62			"local" are managed via the same mechanism, so variables "local"ised
63			after calling "scope_guard" will be restored when the guard runs.
64
65			Example: temporarily change the timezone for the current process,
66			ensuring it will be reset when the "if" scope is exited:
67
68			use Guard;
69			use POSIX ();
70
71			if ($need_to_switch_tz) {
72			# make sure we call tzset after $ENV{TZ} has been restored
73			scope_guard { POSIX::tzset };
74
75			# localise after the scope_guard, so it gets undone in time
76			local $ENV{TZ} = "Europe/London";
77			POSIX::tzset;
78	root	1.2
79	root	1.3	# do something with the new timezone
80	root	1.2	}
81
82			my $guard = guard BLOCK
83	root	1.7	my $guard = guard ($coderef)
84	root	1.2	Behaves the same as "scope_guard", except that instead of executing
85			the block on scope exit, it returns an object whose lifetime
86			determines when the BLOCK gets executed: when the last reference to
87			the object gets destroyed, the BLOCK gets executed as with
88			"scope_guard".
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	1.6	use Coro::AnyEvent;
100	root	1.2	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	root	1.6	Coro::AnyEvent::sleep 1;
109
110			# $sem->up gets executed when returning
111	root	1.2	}
112
113			The advantage of doing this with a guard instead of simply calling
114			"$sem->down" in the callback is that you can opt not to create the
115			timer, or your code can throw an exception before it can create the
116	root	1.6	timer (or the thread gets canceled), or you can create multiple
117			timers or other event watchers and only when the last one gets
118			executed will the lock be unlocked. Using the "guard", you do not
119			have to worry about catching all the places where you have to unlock
120			the semaphore.
121	root	1.2
122	root	1.3	$guard->cancel
123	root	1.2	Calling this function will "disable" the guard object returned by
124			the "guard" function, i.e. it will free the BLOCK originally passed
125			to "guard "and will arrange for the BLOCK not to be executed.
126
127	root	1.6	This can be useful when you use "guard" to create a cleanup handler
128			to be called under fatal conditions and later decide it is no longer
129			needed.
130	root	1.2
131			EXCEPTIONS
132	root	1.3	Guard blocks should not normally throw exceptions (that is, "die").
133			After all, they are usually used to clean up after such exceptions.
134			However, if something truly exceptional is happening, a guard block
135	root	1.6	should of course be allowed to die. Also, programming errors are a large
136			source of exceptions, and the programmer certainly wants to know about
137			those.
138	root	1.2
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	root	1.6	Therefore, whenever a guard block throws an exception, it will be caught
144			by Guard, followed by calling the code reference stored in $Guard::DIED
145	root	1.4	(with $@ set to the actual exception), which is similar to how most
146			event loops handle this case.
147	root	1.2
148	root	1.6	The default for $Guard::DIED is to call "warn "$@"", i.e. the error is
149			printed as a warning and the program continues.
150	root	1.3
151			The $@ variable will be restored to its value before the guard call in
152			all cases, so guards will not disturb $@ in any way.
153
154	root	1.2	The code reference stored in $Guard::DIED should not die (behaviour is
155			not guaranteed, but right now, the exception will simply be ignored).
156
157	root	1.1	AUTHOR
158			Marc Lehmann <schmorp@schmorp.de>
159			http://home.schmorp.de/
160
161	root	1.2	THANKS
162	root	1.3	Thanks to Marco Maisenhelder, who reminded me of the $Guard::DIED
163			solution to the problem of exceptions.
164	root	1.2
165	root	1.4	SEE ALSO
166	root	1.7	Scope::Guard and Sub::ScopeFinalizer, which actually implement
167			dynamically scoped guards only, not the lexically scoped guards that
168			their documentation promises, and have a lot higher CPU, memory and
169	root	1.6	typing overhead.
170	root	1.4
171	root	1.6	Hook::Scope, which has apparently never been finished and can corrupt
172	root	1.4	memory when used.
173
174	root	1.7	Scope::Guard seems to have a big SEE ALSO section for even more modules
175			like it.
176