[ViewVC] Contents 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".

        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 dynamic
    guards only, not scoped guards, and have a lot higher CPU, memory and
    typing overhead.

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

Revision:	1.6
Committed:	Fri Mar 12 17:25:58 2010 UTC (14 years, 2 months ago) by root
Branch:	MAIN
Changes since 1.5:	+23 -24 lines
Log Message:	* empty log message *
#	Content
1	NAME
2	Guard - safe cleanup blocks
3
4	SYNOPSIS
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
20
21	DESCRIPTION
22	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	See the EXCEPTIONS section for an explanation of how exceptions
39	(i.e. "die") are handled inside guard blocks.
40
41	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	The behaviour is similar to this code fragment:
46
47	eval ... code following scope_guard ...
48	{
49	local $@;
50	eval BLOCK;
51	eval { $Guard::DIED->() } if $@;
52	}
53	die if $@;
54
55	Except it is much faster, and the whole thing gets executed even
56	when the BLOCK calls "exit", "goto", "last" or escapes via other
57	means.
58
59	If multiple BLOCKs are registered to the same scope, they will be
60	executed in reverse order. Other scope-related things such as
61	"local" are managed via the same mechanism, so variables "local"ised
62	after calling "scope_guard" will be restored when the guard runs.
63
64	Example: temporarily change the timezone for the current process,
65	ensuring it will be reset when the "if" scope is exited:
66
67	use Guard;
68	use POSIX ();
69
70	if ($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
78	# do something with the new timezone
79	}
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	See the EXCEPTIONS section for an explanation of how exceptions
89	(i.e. "die") are handled inside guard blocks.
90
91	Example: acquire a Coro::Semaphore for a second by registering a
92	timer. The timer callback references the guard used to unlock it
93	again. (Please ignore the fact that "Coro::Semaphore" has a "guard"
94	method that does this already):
95
96	use Guard;
97	use Coro::AnyEvent;
98	use Coro::Semaphore;
99
100	my $sem = new Coro::Semaphore;
101
102	sub lock_for_a_second {
103	$sem->down;
104	my $guard = guard { $sem->up };
105
106	Coro::AnyEvent::sleep 1;
107
108	# $sem->up gets executed when returning
109	}
110
111	The advantage of doing this with a guard instead of simply calling
112	"$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 the thread gets canceled), or you can create multiple
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.
119
120	$guard->cancel
121	Calling this function will "disable" the guard object returned by
122	the "guard" function, i.e. it will free the BLOCK originally passed
123	to "guard "and will arrange for the BLOCK not to be executed.
124
125	This can be useful when you use "guard" to create a cleanup handler
126	to be called under fatal conditions and later decide it is no longer
127	needed.
128
129	EXCEPTIONS
130	Guard blocks should not normally throw exceptions (that is, "die").
131	After all, they are usually used to clean up after such exceptions.
132	However, if something truly exceptional is happening, a guard block
133	should of course be allowed to die. Also, programming errors are a large
134	source of exceptions, and the programmer certainly wants to know about
135	those.
136
137	Since in most cases, the block executing when the guard gets executed
138	does not know or does not care about the guard blocks, it makes little
139	sense to let containing code handle the exception.
140
141	Therefore, whenever a guard block throws an exception, it will be caught
142	by Guard, followed by calling the code reference stored in $Guard::DIED
143	(with $@ set to the actual exception), which is similar to how most
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.
151
152	The code reference stored in $Guard::DIED should not die (behaviour is
153	not guaranteed, but right now, the exception will simply be ignored).
154
155	AUTHOR
156	Marc Lehmann <schmorp@schmorp.de>
157	http://home.schmorp.de/
158
159	THANKS
160	Thanks to Marco Maisenhelder, who reminded me of the $Guard::DIED
161	solution to the problem of exceptions.
162
163	SEE 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