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

Comparing Guard/Guard.pm (file contents):
Revision 1.6 by root, Sat Dec 13 18:43:01 2008 UTC vs.
Revision 1.22 by root, Sun Jul 26 08:07:11 2009 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		8
9	# temporarily chdir to "/etc" directory, but make sure	9	# temporarily chdir to "/etc" directory, but make sure
10	# to go back to "/" no matter how myfun exits:	10	# to go back to "/" no matter how myfun exits:
11	sub dosomething {	11	sub myfun {
12	scope_guard { chdir "/" };	12	scope_guard { chdir "/" };
13	chdir "/etc";	13	chdir "/etc";
14		14
15	call_function_that_might_die_or_other_fun_stuff;	15	code_that_might_die_or_does_other_fun_stuff;
16	}	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
17		22
18	=head1 DESCRIPTION	23	=head1 DESCRIPTION
19		24
20	This module implements so-called "guards". A guard is something (usually	25	This module implements so-called "guards". A guard is something (usually
21	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
…		…
34		39
35	=cut	40	=cut
36		41
37	package Guard;	42	package Guard;
38		43
		44	no warnings;
		45
39	BEGIN {	46	BEGIN {
40	$VERSION = '0.1';	47	$VERSION = '1.021';
41	@ISA = qw(Exporter);	48	@ISA = qw(Exporter);
42	@EXPORT = qw(guard scope_guard);	49	@EXPORT = qw(guard scope_guard);
43		50
44	require Exporter;	51	require Exporter;
45		52
…		…
52	=item scope_guard BLOCK	59	=item scope_guard BLOCK
53		60
54	Registers a block that is executed when the current scope (block,	61	Registers a block that is executed when the current scope (block,
55	function, method, eval etc.) is exited.	62	function, method, eval etc.) is exited.
56		63
		64	See the EXCEPTIONS section for an explanation of how exceptions
		65	(i.e. C<die>) are handled inside guard blocks.
		66
57	The description below sounds a bit complicated, but that's just because	67	The description below sounds a bit complicated, but that's just because
58	C<scope_guard> tries to get even corner cases "right": the goal is to	68	C<scope_guard> tries to get even corner cases "right": the goal is to
59	provide you with a rock solid clean up tool.	69	provide you with a rock solid clean up tool.
60		70
61	This is similar to this code fragment:	71	The behaviour is similar to this code fragment:
62		72
63	eval ... code following scope_guard ...	73	eval ... code following scope_guard ...
64	{	74	{
65	local $@;	75	local $@;
66	eval BLOCK;	76	eval BLOCK;
…		…
70		80
71	Except it is much faster, and the whole thing gets executed even when the	81	Except it is much faster, and the whole thing gets executed even when the
72	BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means.	82	BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means.
73		83
74	If multiple BLOCKs are registered to the same scope, they will be executed	84	If multiple BLOCKs are registered to the same scope, they will be executed
75	in reverse order. Stuff like C<local> is managed via the same mechanism,	85	in reverse order. Other scope-related things such as C<local> are managed
76	so variables C<local>ised after calling C<scope_guard> will be restored	86	via the same mechanism, so variables C<local>ised I<after> calling
77	when the guard runs.	87	C<scope_guard> will be restored when the guard runs.
78
79	See B<EXCEPTIONS>, below, for an explanation of exception handling
80	(C<die>) within guard blocks.
81		88
82	Example: temporarily change the timezone for the current process,	89	Example: temporarily change the timezone for the current process,
83	ensuring it will be reset when the C<if> scope is exited:	90	ensuring it will be reset when the C<if> scope is exited:
84		91
85	use Guard;	92	use Guard;
…		…
101	Behaves the same as C<scope_guard>, except that instead of executing	108	Behaves the same as C<scope_guard>, except that instead of executing
102	the block on scope exit, it returns an object whose lifetime determines	109	the block on scope exit, it returns an object whose lifetime determines
103	when the BLOCK gets executed: when the last reference to the object gets	110	when the BLOCK gets executed: when the last reference to the object gets
104	destroyed, the BLOCK gets executed as with C<scope_guard>.	111	destroyed, the BLOCK gets executed as with C<scope_guard>.
105		112
106	The returned object can be copied as many times as you want.	113	See the EXCEPTIONS section for an explanation of how exceptions
107		114	(i.e. C<die>) are handled inside guard blocks.
108	See B<EXCEPTIONS>, below, for an explanation of exception handling
109	(C<die>) within guard blocks.
110		115
111	Example: acquire a Coro::Semaphore for a second by registering a	116	Example: acquire a Coro::Semaphore for a second by registering a
112	timer. The timer callback references the guard used to unlock it again.	117	timer. The timer callback references the guard used to unlock it
		118	again. (Please ignore the fact that C<Coro::Semaphore> has a C<guard>
		119	method that does this already):
113		120
		121	use Guard;
114	use AnyEvent;	122	use Coro::AnyEvent;
115	use Coro::Semaphore;	123	use Coro::Semaphore;
116		124
117	my $sem = new Coro::Semaphore;	125	my $sem = new Coro::Semaphore;
118		126
119	sub lock_1s {	127	sub lock_for_a_second {
120	$sem->down;	128	$sem->down;
121	my $guard = guard { $sem->up };	129	my $guard = guard { $sem->up };
122		130
123	my $timer;	131	Coro::AnyEvent::sleep 1;
124	$timer = AnyEvent->timer (after => 1, sub {	132
125	# do something	133	# $sem->up gets executed when returning
126	undef $sem;
127	undef $timer;
128	});
129	}	134	}
130		135
131	The advantage of doing this with a guard instead of simply calling C<<	136	The advantage of doing this with a guard instead of simply calling C<<
132	$sem->down >> in the callback is that you can opt not to create the timer,	137	$sem->down >> in the callback is that you can opt not to create the timer,
133	or your code can throw an exception before it can create the timer, or you	138	or your code can throw an exception before it can create the timer (or
134	can create multiple timers or other event watchers and only when the last	139	the thread gets canceled), or you can create multiple timers or other
135	one gets executed will the lock be unlocked.	140	event watchers and only when the last one gets executed will the lock be
		141	unlocked. Using the C<guard>, you do not have to worry about catching all
		142	the places where you have to unlock the semaphore.
136		143
137	=item Guard::cancel $guard	144	=item $guard->cancel
138		145
139	Calling this function will "disable" the guard object returned by the	146	Calling this function will "disable" the guard object returned by the
140	C<guard> function, i.e. it will free the BLOCK originally passed to	147	C<guard> function, i.e. it will free the BLOCK originally passed to
141	C<guard >and will arrange for the BLOCK not to be executed.	148	C<guard >and will arrange for the BLOCK not to be executed.
142		149
143	This can be useful when you use C<guard> to create a fatal cleanup handler	150	This can be useful when you use C<guard> to create a cleanup handler to be
144	and later decide it is no longer needed.	151	called under fatal conditions and later decide it is no longer needed.
145		152
146	=cut	153	=cut
147		154
148	1;	155	1;
149		156
150	=back	157	=back
151		158
152	=head1 EXCEPTIONS	159	=head1 EXCEPTIONS
153		160
154	Guard blocks should not normally throw exceptions (that is, C<die>). After	161	Guard blocks should not normally throw exceptions (that is, C<die>). After
155	all, they are usually used to clean up after such exceptions. However, if	162	all, they are usually used to clean up after such exceptions. However,
156	something truly exceptional is happening, a guard block should be allowed	163	if something truly exceptional is happening, a guard block should of
157	to die. Also, programming errors are a large source of exceptions, and the	164	course be allowed to die. Also, programming errors are a large source of
158	programmer certainly wants to know about those.	165	exceptions, and the programmer certainly wants to know about those.
159		166
160	Since in most cases, the block executing when the guard gets executes does	167	Since in most cases, the block executing when the guard gets executed does
161	not know or does not care about the guard blocks, it makes little sense to	168	not know or does not care about the guard blocks, it makes little sense to
162	let containing code handle the exception.	169	let containing code handle the exception.
163		170
164	Therefore, whenever a guard block throws an exception, it will be caught,	171	Therefore, whenever a guard block throws an exception, it will be caught
165	and this module will call the code reference stored in C<$Guard::DIED>	172	by Guard, followed by calling the code reference stored in C<$Guard::DIED>
166	(with C<$@> set to the actual exception), which is similar to how most	173	(with C<$@> set to the actual exception), which is similar to how most
167	event loops handle this case.	174	event loops handle this case.
168		175
		176	The default for C<$Guard::DIED> is to call C<warn "$@">, i.e. the error is
		177	printed as a warning and the program continues.
		178
		179	The C<$@> variable will be restored to its value before the guard call in
		180	all cases, so guards will not disturb C<$@> in any way.
		181
169	The code reference stored in C<$Guard::DIED> should not die (behaviour is	182	The code reference stored in C<$Guard::DIED> should not die (behaviour is
170	not guaranteed, but right now, the exception will simply be ignored).	183	not guaranteed, but right now, the exception will simply be ignored).
171		184
172	The default for C<$Guard::DIED> is to call C<warn "$@">.
173
174	=head1 AUTHOR	185	=head1 AUTHOR
175		186
176	Marc Lehmann <schmorp@schmorp.de>	187	Marc Lehmann <schmorp@schmorp.de>
177	http://home.schmorp.de/	188	http://home.schmorp.de/
178		189
179	=head1 THANKS	190	=head1 THANKS
180		191
181	Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED>	192	Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED>
182	solution to the problem of exceptions.	193	solution to the problem of exceptions.
183		194
		195	=head1 SEE ALSO
		196
		197	L<Scope::Guard> and L<Sub::ScopeFinalizer>, which actually implement
		198	dynamic guards only, not scoped guards, and have a lot higher CPU, memory
		199	and typing overhead.
		200
		201	L<Hook::Scope>, which has apparently never been finished and can corrupt
		202	memory when used.
		203
184	=cut	204	=cut
185		205

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Comparing Guard/Guard.pm (file contents): Revision 1.6 by root, Sat Dec 13 18:43:01 2008 UTC vs. Revision 1.22 by root, Sun Jul 26 08:07:11 2009 UTC

Diff Legend

Comparing Guard/Guard.pm (file contents):
Revision 1.6 by root, Sat Dec 13 18:43:01 2008 UTC vs.
Revision 1.22 by root, Sun Jul 26 08:07:11 2009 UTC