[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
    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
#	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 $guard; # probably destroyed here
20
21	DESCRIPTION
22	This module implements so-called "guards". A guard is something (usually
23	an object) that "guards" a resource, ensuring that it is cleaned up when
24	expected.
25
26	Specifically, this module supports two different types of guards: guard
27	objects, which execute a given code block when destroyed, and scoped
28	guards, which are tied to the scope exit.
29
30	FUNCTIONS
31	This module currently exports the "scope_guard" and "guard" functions by
32	default.
33
34	scope_guard BLOCK
35	scope_guard ($coderef)
36	Registers a block that is executed when the current scope (block,
37	function, method, eval etc.) is exited.
38
39	See the EXCEPTIONS section for an explanation of how exceptions
40	(i.e. "die") are handled inside guard blocks.
41
42	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	The behaviour is similar to this code fragment:
47
48	eval ... code following scope_guard ...
49	{
50	local $@;
51	eval BLOCK;
52	eval { $Guard::DIED->() } if $@;
53	}
54	die if $@;
55
56	Except it is much faster, and the whole thing gets executed even
57	when the BLOCK calls "exit", "goto", "last" or escapes via other
58	means.
59
60	If multiple BLOCKs are registered to the same scope, they will be
61	executed in reverse order. Other scope-related things such as
62	"local" are managed via the same mechanism, so variables "local"ised
63	after calling "scope_guard" will be restored when the guard runs.
64
65	Example: temporarily change the timezone for the current process,
66	ensuring it will be reset when the "if" scope is exited:
67
68	use Guard;
69	use POSIX ();
70
71	if ($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
79	# do something with the new timezone
80	}
81
82	my $guard = guard BLOCK
83	my $guard = guard ($coderef)
84	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	See the EXCEPTIONS section for an explanation of how exceptions
91	(i.e. "die") are handled inside guard blocks.
92
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	again. (Please ignore the fact that "Coro::Semaphore" has a "guard"
96	method that does this already):
97
98	use Guard;
99	use Coro::AnyEvent;
100	use Coro::Semaphore;
101
102	my $sem = new Coro::Semaphore;
103
104	sub lock_for_a_second {
105	$sem->down;
106	my $guard = guard { $sem->up };
107
108	Coro::AnyEvent::sleep 1;
109
110	# $sem->up gets executed when returning
111	}
112
113	The advantage of doing this with a guard instead of simply calling
114	"$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	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
122	$guard->cancel
123	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	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
131	EXCEPTIONS
132	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	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
139	Since in most cases, the block executing when the guard gets executed
140	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 caught
144	by Guard, 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
148	The default for $Guard::DIED is to call "warn "$@"", i.e. the error is
149	printed as a warning and the program continues.
150
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	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	AUTHOR
158	Marc Lehmann <schmorp@schmorp.de>
159	http://home.schmorp.de/
160
161	THANKS
162	Thanks to Marco Maisenhelder, who reminded me of the $Guard::DIED
163	solution to the problem of exceptions.
164
165	SEE ALSO
166	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	typing overhead.
170
171	Hook::Scope, which has apparently never been finished and can corrupt
172	memory when used.
173
174	Scope::Guard seems to have a big SEE ALSO section for even more modules
175	like it.
176