[ViewVC] Diff of: cvs/Guard/Guard.pm

Comparing Guard/Guard.pm (file contents):
Revision 1.3 by root, Sat Dec 13 17:50:29 2008 UTC vs.
Revision 1.23 by root, Fri Mar 12 17:25:58 2010 UTC

…		…
2		2
3	Guard - safe cleanup blocks	3	Guard - safe cleanup blocks
4		4
5	=head1 SYNOPSIS	5	=head1 SYNOPSIS
6		6
7	use Guard;	7	use Guard;
		8
		9	# temporarily chdir to "/etc" directory, but make sure
		10	# to go back to "/" no matter how myfun exits:
		11	sub myfun {
		12	scope_guard { chdir "/" };
		13	chdir "/etc";
		14
		15	code_that_might_die_or_does_other_fun_stuff;
		16	}
		17
		18	# create an object that, when the last reference to it is gone,
		19	# invokes the given codeblock:
		20	my $guard = guard { print "destroyed!\n" };
		21	undef $guard; # probably destroyed here
8		22
9	=head1 DESCRIPTION	23	=head1 DESCRIPTION
10		24
11	This module implements so-called "guards". A guard is something (usually	25	This module implements so-called "guards". A guard is something (usually
12	an object) that "guards" a resource, ensuring that it is cleaned up when	26	an object) that "guards" a resource, ensuring that it is cleaned up when
…		…
25		39
26	=cut	40	=cut
27		41
28	package Guard;	42	package Guard;
29		43
		44	no warnings;
		45
30	BEGIN {	46	BEGIN {
31	$VERSION = '0.1';	47	$VERSION = '1.021';
32	@ISA = qw(Exporter);	48	@ISA = qw(Exporter);
33	@EXPORT = qw(guard scope_guard);	49	@EXPORT = qw(guard scope_guard);
34		50
35	require Exporter;	51	require Exporter;
36		52
…		…
40		56
41	our $DIED = sub { warn "$@" };	57	our $DIED = sub { warn "$@" };
42		58
43	=item scope_guard BLOCK	59	=item scope_guard BLOCK
44		60
		61	=item scope_guard ($coderef)
		62
45	Registers a block that is executed when the current scope (block,	63	Registers a block that is executed when the current scope (block,
46	function, method, eval etc.) is exited.	64	function, method, eval etc.) is exited.
		65
		66	See the EXCEPTIONS section for an explanation of how exceptions
		67	(i.e. C<die>) are handled inside guard blocks.
47		68
48	The description below sounds a bit complicated, but that's just because	69	The description below sounds a bit complicated, but that's just because
49	C<scope_guard> tries to get even corner cases "right": the goal is to	70	C<scope_guard> tries to get even corner cases "right": the goal is to
50	provide you with a rock solid clean up tool.	71	provide you with a rock solid clean up tool.
51		72
52	This is similar to this code fragment:	73	The behaviour is similar to this code fragment:
53		74
54	eval ... code following scope_guard ...	75	eval ... code following scope_guard ...
55	{	76	{
56	local $@;	77	local $@;
57	eval BLOCK;	78	eval BLOCK;
…		…
60	die if $@;	81	die if $@;
61		82
62	Except it is much faster, and the whole thing gets executed even when the	83	Except it is much faster, and the whole thing gets executed even when the
63	BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means.	84	BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means.
64		85
65	See B<EXCEPTIONS>, below, for an explanation of exception handling	86	If multiple BLOCKs are registered to the same scope, they will be executed
66	(C<die>) within guard blocks.	87	in reverse order. Other scope-related things such as C<local> are managed
		88	via the same mechanism, so variables C<local>ised I<after> calling
		89	C<scope_guard> will be restored when the guard runs.
67		90
68	Example: Temporarily change the directory to F</etc> and make sure it's	91	Example: temporarily change the timezone for the current process,
69	set back to F</> when the function returns:	92	ensuring it will be reset when the C<if> scope is exited:
70		93
71	sub dosomething {	94	use Guard;
72	scope_guard { chdir "/" };	95	use POSIX ();
73	chdir "/etc";
74		96
75	...	97	if ($need_to_switch_tz) {
		98	# make sure we call tzset after $ENV{TZ} has been restored
		99	scope_guard { POSIX::tzset };
		100
		101	# localise after the scope_guard, so it gets undone in time
		102	local $ENV{TZ} = "Europe/London";
		103	POSIX::tzset;
		104
		105	# do something with the new timezone
76	}	106	}
77		107
78	=item my $guard = guard BLOCK	108	=item my $guard = guard BLOCK
		109
		110	=item my $guard = guard ($coderef)
79		111
80	Behaves the same as C<scope_guard>, except that instead of executing	112	Behaves the same as C<scope_guard>, except that instead of executing
81	the block on scope exit, it returns an object whose lifetime determines	113	the block on scope exit, it returns an object whose lifetime determines
82	when the BLOCK gets executed: when the last reference to the object gets	114	when the BLOCK gets executed: when the last reference to the object gets
83	destroyed, the BLOCK gets executed as with C<scope_guard>.	115	destroyed, the BLOCK gets executed as with C<scope_guard>.
84		116
85	The returned object can be copied as many times as you want.	117	See the EXCEPTIONS section for an explanation of how exceptions
86		118	(i.e. C<die>) are handled inside guard blocks.
87	See B<EXCEPTIONS>, below, for an explanation of exception handling
88	(C<die>) within guard blocks.
89		119
90	Example: acquire a Coro::Semaphore for a second by registering a	120	Example: acquire a Coro::Semaphore for a second by registering a
91	timer. The timer callback references the guard used to unlock it again.	121	timer. The timer callback references the guard used to unlock it
		122	again. (Please ignore the fact that C<Coro::Semaphore> has a C<guard>
		123	method that does this already):
92		124
		125	use Guard;
93	use AnyEvent;	126	use Coro::AnyEvent;
94	use Coro::Semaphore;	127	use Coro::Semaphore;
95		128
96	my $sem = new Coro::Semaphore;	129	my $sem = new Coro::Semaphore;
97		130
98	sub lock_1s {	131	sub lock_for_a_second {
99	$sem->down;	132	$sem->down;
100	my $guard = guard { $sem->up };	133	my $guard = guard { $sem->up };
101		134
102	my $timer;	135	Coro::AnyEvent::sleep 1;
103	$timer = AnyEvent->timer (after => 1, sub {	136
104	# do something	137	# $sem->up gets executed when returning
105	undef $sem;
106	undef $timer;
107	});
108	}	138	}
109		139
110	The advantage of doing this with a guard instead of simply calling C<<	140	The advantage of doing this with a guard instead of simply calling C<<
111	$sem->down >> in the callback is that you can opt not to create the timer,	141	$sem->down >> in the callback is that you can opt not to create the timer,
112	or your code can throw an exception before it can create the timer, or you	142	or your code can throw an exception before it can create the timer (or
113	can create multiple timers or other event watchers and only when the last	143	the thread gets canceled), or you can create multiple timers or other
114	one gets executed will the lock be unlocked.	144	event watchers and only when the last one gets executed will the lock be
		145	unlocked. Using the C<guard>, you do not have to worry about catching all
		146	the places where you have to unlock the semaphore.
115		147
116	=item Guard::cancel $guard	148	=item $guard->cancel
117		149
118	Calling this function will "disable" the guard object returned by the	150	Calling this function will "disable" the guard object returned by the
119	C<guard> function, i.e. it will free the BLOCK originally passed to	151	C<guard> function, i.e. it will free the BLOCK originally passed to
120	C<guard >and will arrange for the BLOCK not to be executed.	152	C<guard >and will arrange for the BLOCK not to be executed.
121		153
122	This can be useful when you use C<guard> to create a fatal cleanup handler	154	This can be useful when you use C<guard> to create a cleanup handler to be
123	and later decide it is no longer needed.	155	called under fatal conditions and later decide it is no longer needed.
124		156
125	=cut	157	=cut
126		158
127	1;	159	1;
128		160
129	=back	161	=back
130		162
131	=head1 EXCEPTIONS	163	=head1 EXCEPTIONS
132		164
133	Guard blocks should not normally throw exceptions (e.g. C<die>), after	165	Guard blocks should not normally throw exceptions (that is, C<die>). After
134	all, they are usually used to clean up after such exceptions. However, if	166	all, they are usually used to clean up after such exceptions. However,
135	something truly exceptional is happening, a guard block should be allowed	167	if something truly exceptional is happening, a guard block should of
136	to die. Also, programming errors are a large source of exceptions, and the	168	course be allowed to die. Also, programming errors are a large source of
137	programmer certainly wants to know about those.	169	exceptions, and the programmer certainly wants to know about those.
138		170
139	Since in most cases, the block executing when the guard gets executes does	171	Since in most cases, the block executing when the guard gets executed does
140	not know or does not care about the guard blocks, it makes little sense to	172	not know or does not care about the guard blocks, it makes little sense to
141	let containing code handle the exception.	173	let containing code handle the exception.
142		174
143	Therefore, whenever a guard block throws an exception, it will be caught,	175	Therefore, whenever a guard block throws an exception, it will be caught
144	and this module will call the code reference stored in C<$Guard::DIED>	176	by Guard, followed by calling the code reference stored in C<$Guard::DIED>
145	(with C<$@> set to the actual exception), which is similar to how most	177	(with C<$@> set to the actual exception), which is similar to how most
146	event loops handle this case.	178	event loops handle this case.
147		179
		180	The default for C<$Guard::DIED> is to call C<warn "$@">, i.e. the error is
		181	printed as a warning and the program continues.
		182
		183	The C<$@> variable will be restored to its value before the guard call in
		184	all cases, so guards will not disturb C<$@> in any way.
		185
148	The code reference stored in C<$Guard::DIED> should not die (behaviour is	186	The code reference stored in C<$Guard::DIED> should not die (behaviour is
149	not guaranteed, but right now, the exception will simply be ignored).	187	not guaranteed, but right now, the exception will simply be ignored).
150		188
151	The default for C<$Guard::DIED> is to call C<warn "$@">.
152
153	=head1 AUTHOR	189	=head1 AUTHOR
154		190
155	Marc Lehmann <schmorp@schmorp.de>	191	Marc Lehmann <schmorp@schmorp.de>
156	http://home.schmorp.de/	192	http://home.schmorp.de/
157		193
158	=head1 THANKS	194	=head1 THANKS
159		195
160	To Marco Maisenhelder, who reminded me of the C<$Guard::DIED> solution to	196	Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED>
161	the problem of exceptions.	197	solution to the problem of exceptions.
		198
		199	=head1 SEE ALSO
		200
		201	L<Scope::Guard> and L<Sub::ScopeFinalizer>, which actually implement
		202	dynamic guards only, not scoped guards, and have a lot higher CPU, memory
		203	and typing overhead.
		204
		205	L<Hook::Scope>, which has apparently never been finished and can corrupt
		206	memory when used.
162		207
163	=cut	208	=cut
164		209

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Comparing Guard/Guard.pm (file contents): Revision 1.3 by root, Sat Dec 13 17:50:29 2008 UTC vs. Revision 1.23 by root, Fri Mar 12 17:25:58 2010 UTC

Diff Legend

Comparing Guard/Guard.pm (file contents):
Revision 1.3 by root, Sat Dec 13 17:50:29 2008 UTC vs.
Revision 1.23 by root, Fri Mar 12 17:25:58 2010 UTC