[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";
       
      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
#	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	call_function_that_might_die_or_other_fun_stuff;
14	}
15
16	DESCRIPTION
17	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	See the EXCEPTIONS section for an explanation of how exceptions
34	(i.e. "die") are handled inside guard blocks.
35
36	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	The behaviour is similar to this code fragment:
41
42	eval ... code following scope_guard ...
43	{
44	local $@;
45	eval BLOCK;
46	eval { $Guard::DIED->() } if $@;
47	}
48	die if $@;
49
50	Except it is much faster, and the whole thing gets executed even
51	when the BLOCK calls "exit", "goto", "last" or escapes via other
52	means.
53
54	If multiple BLOCKs are registered to the same scope, they will be
55	executed in reverse order. Other scope-related things such as
56	"local" are managed via the same mechanism, so variables "local"ised
57	after calling "scope_guard" will be restored when the guard runs.
58
59	Example: temporarily change the timezone for the current process,
60	ensuring it will be reset when the "if" scope is exited:
61
62	use Guard;
63	use POSIX ();
64
65	if ($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
73	# do something with the new timezone
74	}
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	See the EXCEPTIONS section for an explanation of how exceptions
86	(i.e. "die") are handled inside guard blocks.
87
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	again. (Please ignore the fact that "Coro::Semaphore" has a "guard"
91	method that does this already):
92
93	use Guard;
94	use AnyEvent;
95	use Coro::Semaphore;
96
97	my $sem = new Coro::Semaphore;
98
99	sub lock_for_a_second {
100	$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	Using the "guard", you do not have to worry about catching all the
117	places where you have to unlock the semaphore.
118
119	$guard->cancel
120	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	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	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	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	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	AUTHOR
152	Marc Lehmann <schmorp@schmorp.de>
153	http://home.schmorp.de/
154
155	THANKS
156	Thanks to Marco Maisenhelder, who reminded me of the $Guard::DIED
157	solution to the problem of exceptions.
158