Guard/Guard.pm

=head1 NAME

Guard - safe cleanup blocks

=head1 SYNOPSIS

 use Guard;

 # temporarily chdir to "/etc" directory, but make sure
 # to go back to "/" no matter how myfun exits:
 sub dosomething {
    scope_guard { chdir "/" };
    chdir "/etc";

    call_function_that_might_die_or_other_fun_stuff;
 }

=head1 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.

=head1 FUNCTIONS

This module currently exports the C<scope_guard> and C<guard> functions by
default.

=over 4

=cut

package Guard;

BEGIN {
   $VERSION = '0.1';
   @ISA = qw(Exporter);
   @EXPORT = qw(guard scope_guard);

   require Exporter;

   require XSLoader;
   XSLoader::load Guard, $VERSION;
}

our $DIED = sub { warn "$@" };

=item scope_guard BLOCK

Registers a block that is executed when the current scope (block,
function, method, eval etc.) is exited.

The description below sounds a bit complicated, but that's just because
C<scope_guard> tries to get even corner cases "right": the goal is to
provide you with a rock solid clean up tool.

This 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 C<exit>, C<goto>, C<last> or escapes via other means.

If multiple BLOCKs are registered to the same scope, they will be executed
in reverse order. Stuff like C<local> is managed via the same mechanism,
so variables C<local>ised after calling C<scope_guard> will be restored
when the guard runs.

See B<EXCEPTIONS>, below, for an explanation of exception handling
(C<die>) within guard blocks.

Example: temporarily change the timezone for the current process,
ensuring it will be reset when the C<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
   }

=item my $guard = guard BLOCK

Behaves the same as C<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 C<scope_guard>.

The returned object can be copied as many times as you want.

See B<EXCEPTIONS>, below, for an explanation of exception handling
(C<die>) within 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.

   use AnyEvent;
   use Coro::Semaphore;

   my $sem = new Coro::Semaphore;

   sub lock_1s {
      $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 C<<
$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.

=item Guard::cancel $guard

Calling this function will "disable" the guard object returned by the
C<guard> function, i.e. it will free the BLOCK originally passed to
C<guard >and will arrange for the BLOCK not to be executed.

This can be useful when you use C<guard> to create a fatal cleanup handler
and later decide it is no longer needed.

=cut

1;

=back

=head1 EXCEPTIONS

Guard blocks should not normally throw exceptions (that is, C<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 C<$Guard::DIED>
(with C<$@> set to the actual exception), which is similar to how most
event loops handle this case.

The code reference stored in C<$Guard::DIED> should not die (behaviour is
not guaranteed, but right now, the exception will simply be ignored).

The default for C<$Guard::DIED> is to call C<warn "$@">.

=head1 AUTHOR

 Marc Lehmann <schmorp@schmorp.de>
 http://home.schmorp.de/

=head1 THANKS

Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED>
solution to the problem of exceptions.

=cut

Revision:	1.6
Committed:	Sat Dec 13 18:43:01 2008 UTC (15 years, 5 months ago) by root
Branch:	MAIN
Changes since 1.5:	+2 -2 lines
Log Message:	* empty log message *
#	Content
1	=head1 NAME
2
3	Guard - safe cleanup blocks
4
5	=head1 SYNOPSIS
6
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 dosomething {
12	scope_guard { chdir "/" };
13	chdir "/etc";
14
15	call_function_that_might_die_or_other_fun_stuff;
16	}
17
18	=head1 DESCRIPTION
19
20	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
22	expected.
23
24	Specifically, this module supports two different types of guards: guard
25	objects, which execute a given code block when destroyed, and scoped
26	guards, which are tied to the scope exit.
27
28	=head1 FUNCTIONS
29
30	This module currently exports the C<scope_guard> and C<guard> functions by
31	default.
32
33	=over 4
34
35	=cut
36
37	package Guard;
38
39	BEGIN {
40	$VERSION = '0.1';
41	@ISA = qw(Exporter);
42	@EXPORT = qw(guard scope_guard);
43
44	require Exporter;
45
46	require XSLoader;
47	XSLoader::load Guard, $VERSION;
48	}
49
50	our $DIED = sub { warn "$@" };
51
52	=item scope_guard BLOCK
53
54	Registers a block that is executed when the current scope (block,
55	function, method, eval etc.) is exited.
56
57	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
59	provide you with a rock solid clean up tool.
60
61	This is similar to this code fragment:
62
63	eval ... code following scope_guard ...
64	{
65	local $@;
66	eval BLOCK;
67	eval { $Guard::DIED->() } if $@;
68	}
69	die if $@;
70
71	Except it is much faster, and the whole thing gets executed even when the
72	BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means.
73
74	If multiple BLOCKs are registered to the same scope, they will be executed
75	in reverse order. Stuff like C<local> is managed via the same mechanism,
76	so variables C<local>ised after calling C<scope_guard> will be restored
77	when the guard runs.
78
79	See B<EXCEPTIONS>, below, for an explanation of exception handling
80	(C<die>) within guard blocks.
81
82	Example: temporarily change the timezone for the current process,
83	ensuring it will be reset when the C<if> scope is exited:
84
85	use Guard;
86	use POSIX ();
87
88	if ($need_to_switch_tz) {
89	# make sure we call tzset after $ENV{TZ} has been restored
90	scope_guard { POSIX::tzset };
91
92	# localise after the scope_guard, so it gets undone in time
93	local $ENV{TZ} = "Europe/London";
94	POSIX::tzset;
95
96	# do something with the new timezone
97	}
98
99	=item my $guard = guard BLOCK
100
101	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
103	when the BLOCK gets executed: when the last reference to the object gets
104	destroyed, the BLOCK gets executed as with C<scope_guard>.
105
106	The returned object can be copied as many times as you want.
107
108	See B<EXCEPTIONS>, below, for an explanation of exception handling
109	(C<die>) within guard blocks.
110
111	Example: acquire a Coro::Semaphore for a second by registering a
112	timer. The timer callback references the guard used to unlock it again.
113
114	use AnyEvent;
115	use Coro::Semaphore;
116
117	my $sem = new Coro::Semaphore;
118
119	sub lock_1s {
120	$sem->down;
121	my $guard = guard { $sem->up };
122
123	my $timer;
124	$timer = AnyEvent->timer (after => 1, sub {
125	# do something
126	undef $sem;
127	undef $timer;
128	});
129	}
130
131	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,
133	or your code can throw an exception before it can create the timer, or you
134	can create multiple timers or other event watchers and only when the last
135	one gets executed will the lock be unlocked.
136
137	=item Guard::cancel $guard
138
139	Calling this function will "disable" the guard object returned by the
140	C<guard> function, i.e. it will free the BLOCK originally passed to
141	C<guard >and will arrange for the BLOCK not to be executed.
142
143	This can be useful when you use C<guard> to create a fatal cleanup handler
144	and later decide it is no longer needed.
145
146	=cut
147
148	1;
149
150	=back
151
152	=head1 EXCEPTIONS
153
154	Guard blocks should not normally throw exceptions (that is, C<die>). After
155	all, they are usually used to clean up after such exceptions. However, if
156	something truly exceptional is happening, a guard block should be allowed
157	to die. Also, programming errors are a large source of exceptions, and the
158	programmer certainly wants to know about those.
159
160	Since in most cases, the block executing when the guard gets executes does
161	not know or does not care about the guard blocks, it makes little sense to
162	let containing code handle the exception.
163
164	Therefore, whenever a guard block throws an exception, it will be caught,
165	and this module will call the code reference stored in C<$Guard::DIED>
166	(with C<$@> set to the actual exception), which is similar to how most
167	event loops handle this case.
168
169	The code reference stored in C<$Guard::DIED> should not die (behaviour is
170	not guaranteed, but right now, the exception will simply be ignored).
171
172	The default for C<$Guard::DIED> is to call C<warn "$@">.
173
174	=head1 AUTHOR
175
176	Marc Lehmann <schmorp@schmorp.de>
177	http://home.schmorp.de/
178
179	=head1 THANKS
180
181	Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED>
182	solution to the problem of exceptions.
183
184	=cut
185