Guard/Guard.pm

=head1 NAME

Guard - safe cleanup blocks

=head1 SYNOPSIS

 use Guard;

=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 = '0.01';
   @ISA = qw(Exporter);
   @EXPORT = qw(guard scope_guard cancel);

   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.

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

Example: Temporarily change the directory to F</etc> and make sure it's
set back to F</> when the function returns:

   sub dosomething {
      scope_guard { chdir "/" };
      chdir "/etc";

      ...
   }

=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 (e.g. 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

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

=cut

Revision:	1.2
Committed:	Sat Dec 13 17:49:12 2008 UTC (15 years, 5 months ago) by root
Branch:	MAIN
Changes since 1.1:	+10 -0 lines
Log Message:	fixes
#	User	Rev	Content
1	root	1.1	=head1 NAME
2
3			Guard - safe cleanup blocks
4
5			=head1 SYNOPSIS
6
7			use Guard;
8
9			=head1 DESCRIPTION
10
11			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
13			expected.
14
15			Specifically, this module supports two different types of guards: guard
16			objects, which execute a given code block when destroyed, and scoped
17			guards, which are tied to the scope exit.
18
19	root	1.2	=head1 FUNCTIONS
20
21			This module currently exports the C<scope_guard> and C<guard> functions by
22			default.
23
24	root	1.1	=over 4
25
26			=cut
27
28			package Guard;
29
30			BEGIN {
31			$VERSION = '0.01';
32			@ISA = qw(Exporter);
33			@EXPORT = qw(guard scope_guard cancel);
34
35			require Exporter;
36
37			require XSLoader;
38			XSLoader::load Guard, $VERSION;
39			}
40
41			our $DIED = sub { warn "$@" };
42
43			=item scope_guard BLOCK
44
45			Registers a block that is executed when the current scope (block,
46			function, method, eval etc.) is exited.
47
48	root	1.2	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
50			provide you with a rock solid clean up tool.
51
52	root	1.1	This is similar to this code fragment:
53
54			eval ... code following scope_guard ...
55			{
56			local $@;
57			eval BLOCK;
58			eval { $Guard::DIED->() } if $@;
59			}
60	root	1.2	die if $@;
61	root	1.1
62			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.
64
65			See B<EXCEPTIONS>, below, for an explanation of exception handling
66			(C<die>) within guard blocks.
67
68			Example: Temporarily change the directory to F</etc> and make sure it's
69			set back to F</> when the function returns:
70
71			sub dosomething {
72			scope_guard { chdir "/" };
73			chdir "/etc";
74
75			...
76			}
77
78			=item my $guard = guard BLOCK
79
80			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
82			when the BLOCK gets executed: when the last reference to the object gets
83			destroyed, the BLOCK gets executed as with C<scope_guard>.
84
85			The returned object can be copied as many times as you want.
86
87			See B<EXCEPTIONS>, below, for an explanation of exception handling
88			(C<die>) within guard blocks.
89
90			Example: acquire a Coro::Semaphore for a second by registering a
91			timer. The timer callback references the guard used to unlock it again.
92
93			use AnyEvent;
94			use Coro::Semaphore;
95
96			my $sem = new Coro::Semaphore;
97
98			sub lock_1s {
99			$sem->down;
100			my $guard = guard { $sem->up };
101
102			my $timer;
103			$timer = AnyEvent->timer (after => 1, sub {
104			# do something
105			undef $sem;
106			undef $timer;
107			});
108			}
109
110			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,
112			or your code can throw an exception before it can create the timer, or you
113			can create multiple timers or other event watchers and only when the last
114			one gets executed will the lock be unlocked.
115
116			=item Guard::cancel $guard
117
118			Calling this function will "disable" the guard object returned by the
119			C<guard> function, i.e. it will free the BLOCK originally passed to
120			C<guard >and will arrange for the BLOCK not to be executed.
121
122			This can be useful when you use C<guard> to create a fatal cleanup handler
123			and later decide it is no longer needed.
124
125			=cut
126
127			1;
128
129			=back
130
131			=head1 EXCEPTIONS
132
133			Guard blocks should not normally throw exceptions (e.g. C<die>), after
134			all, they are usually used to clean up after such exceptions. However, if
135			something truly exceptional is happening, a guard block should be allowed
136			to die. Also, programming errors are a large source of exceptions, and the
137			programmer certainly wants to know about those.
138
139			Since in most cases, the block executing when the guard gets executes does
140			not know or does not care about the guard blocks, it makes little sense to
141			let containing code handle the exception.
142
143			Therefore, whenever a guard block throws an exception, it will be caught,
144			and this module will call the code reference stored in C<$Guard::DIED>
145			(with C<$@> set to the actual exception), which is similar to how most
146			event loops handle this case.
147
148			The code reference stored in C<$Guard::DIED> should not die (behaviour is
149			not guaranteed, but right now, the exception will simply be ignored).
150
151			The default for C<$Guard::DIED> is to call C<warn "$@">.
152
153			=head1 AUTHOR
154
155			Marc Lehmann <schmorp@schmorp.de>
156			http://home.schmorp.de/
157
158			=head1 THANKS
159
160			To Marco Maisenhelder, who reminded me of the C<$Guard::DIED> solution to
161			the problem of exceptions.
162
163			=cut
164