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

Comparing Guard/Guard.pm (file contents):
Revision 1.5 by root, Sat Dec 13 18:42:45 2008 UTC vs.
Revision 1.26 by root, Tue Mar 21 11:39:27 2017 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 = 1.023;
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
…		…
49		56
50	our $DIED = sub { warn "$@" };	57	our $DIED = sub { warn "$@" };
51		58
52	=item scope_guard BLOCK	59	=item scope_guard BLOCK
53		60
		61	=item scope_guard ($coderef)
		62
54	Registers a block that is executed when the current scope (block,	63	Registers a block that is executed when the current scope (block,
55	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.
56		68
57	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
58	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
59	provide you with a rock solid clean up tool.	71	provide you with a rock solid clean up tool.
60		72
61	This is similar to this code fragment:	73	The behaviour is similar to this code fragment:
62		74
63	eval ... code following scope_guard ...	75	eval ... code following scope_guard ...
64	{	76	{
65	local $@;	77	local $@;
66	eval BLOCK;	78	eval BLOCK;
…		…
70		82
71	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
72	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.
73		85
74	If multiple BLOCKs are registered to the same scope, they will be executed	86	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,	87	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	88	via the same mechanism, so variables C<local>ised I<after> calling
77	when the guard runs.	89	C<scope_guard> will be restored I<before> the guard runs.
78
79	See B<EXCEPTIONS>, below, for an explanation of exception handling
80	(C<die>) within guard blocks.
81		90
82	Example: temporarily change the timezone for the current process,	91	Example: temporarily change the timezone for the current process,
83	ensuring it will be reset when the C<if> scope is exited:	92	ensuring it will be reset when the C<if> scope is exited:
84		93
85	use Guard;	94	use Guard;
…		…
95		104
96	# do something with the new timezone	105	# do something with the new timezone
97	}	106	}
98		107
99	=item my $guard = guard BLOCK	108	=item my $guard = guard BLOCK
		109
		110	=item my $guard = guard ($coderef)
100		111
101	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
102	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
103	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
104	destroyed, the BLOCK gets executed as with C<scope_guard>.	115	destroyed, the BLOCK gets executed as with C<scope_guard>.
105		116
106	The returned object can be copied as many times as you want.	117	See the EXCEPTIONS section for an explanation of how exceptions
107		118	(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		119
111	Example: acquire a Coro::Semaphore for a second by registering a	120	Example: acquire a Coro::Semaphore for a second by registering a
112	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):
113		124
		125	use Guard;
114	use AnyEvent;	126	use Coro::AnyEvent;
115	use Coro::Semaphore;	127	use Coro::Semaphore;
116		128
117	my $sem = new Coro::Semaphore;	129	my $sem = new Coro::Semaphore;
118		130
119	sub lock_1s {	131	sub lock_for_a_second {
120	$sem->down;	132	$sem->down;
121	my $guard = guard { $sem->up };	133	my $guard = guard { $sem->up };
122		134
123	my $timer;	135	Coro::AnyEvent::sleep 1;
124	$timer = AnyEvent->timer (after => 1, sub {	136
125	# do something	137	# $sem->up gets executed when returning
126	undef $sem;
127	undef $timer;
128	});
129	}	138	}
130		139
131	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<<
132	$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,
133	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
134	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
135	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.
136		147
137	=item Guard::cancel $guard	148	=item $guard->cancel
138		149
139	Calling this function will "disable" the guard object returned by the	150	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	151	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.	152	C<guard >and will arrange for the BLOCK not to be executed.
142		153
143	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
144	and later decide it is no longer needed.	155	called under fatal conditions and later decide it is no longer needed.
145		156
146	=cut	157	=cut
147		158
148	1;	159	1;
149		160
150	=back	161	=back
151		162
152	=head1 EXCEPTIONS	163	=head1 EXCEPTIONS
153		164
154	Guard blocks should not normally throw exceptions (that is, C<die>). After	165	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	166	all, they are usually used to clean up after such exceptions. However,
156	something truly exceptional is happening, a guard block should be allowed	167	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	168	course be allowed to die. Also, programming errors are a large source of
158	programmer certainly wants to know about those.	169	exceptions, and the programmer certainly wants to know about those.
159		170
160	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
161	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
162	let containing code handle the exception.	173	let containing code handle the exception.
163		174
164	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
165	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<$Guard::DIED>
166	(with C<$@> set to the actual exception), which is similar to how most	177	(with C<$@> set to the actual exception), which is similar to how most
167	event loops handle this case.	178	event loops handle this case.
168		179
		180	The default for C<$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<$@> in any way.
		185
169	The code reference stored in C<$Guard::DIED> should not die (behaviour is	186	The code reference stored in C<$Guard::DIED> should not die (behaviour is
170	not guaranteed, but right now, the exception will simply be ignored).	187	not guaranteed, but right now, the exception will simply be ignored).
171		188
172	The default for C<$Guard::DIED> is to call C<warn "$@">.
173
174	=head1 AUTHOR	189	=head1 AUTHOR
175		190
176	Marc Lehmann <schmorp@schmorp.de>	191	Marc Lehmann <schmorp@schmorp.de>
177	http://home.schmorp.de/	192	http://home.schmorp.de/
178		193
179	=head1 THANKS	194	=head1 THANKS
180		195
181	To Marco Maisenhelder, who reminded me of the C<$Guard::DIED> solution to	196	Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED>
182	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	dynamically scoped guards only, not the lexically scoped guards that their
		203	documentation promises, and have a lot higher CPU, memory and typing
		204	overhead.
		205
		206	L<Hook::Scope>, which has apparently never been finished and can corrupt
		207	memory when used.
		208
		209	L<Scope::Guard> seems to have a big SEE ALSO section for even more
		210	modules like it.
183		211
184	=cut	212	=cut
185		213

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Comparing Guard/Guard.pm (file contents): Revision 1.5 by root, Sat Dec 13 18:42:45 2008 UTC vs. Revision 1.26 by root, Tue Mar 21 11:39:27 2017 UTC

Diff Legend

Comparing Guard/Guard.pm (file contents):
Revision 1.5 by root, Sat Dec 13 18:42:45 2008 UTC vs.
Revision 1.26 by root, Tue Mar 21 11:39:27 2017 UTC