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 myfun {
      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;

no warnings;

BEGIN {
   $VERSION = '0.5';
   @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.

See the EXCEPTIONS section for an explanation of how exceptions
(i.e. C<die>) are handled inside guard blocks.

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.

The behaviour 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. Other scope-related things such as C<local> are managed
via the same mechanism, so variables C<local>ised I<after> calling
C<scope_guard> will be restored when the guard runs.

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 the EXCEPTIONS section for an explanation of how exceptions
(i.e. C<die>) are handled inside 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. (Please ignore the fact that C<Coro::Semaphore> has a C<guard>
method that does this already):

   use Guard;
   use AnyEvent;
   use Coro::Semaphore;

   my $sem = new Coro::Semaphore;

   sub lock_for_a_second {
      $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. Using the C<guard>, you do
not have to worry about catching all the places where you have to unlock
the semaphore.

=item $guard->cancel

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 executed 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,
followed by calling 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 default for C<$Guard::DIED> is to call C<warn "$@">.

The C<$@> variable will be restored to its value before the guard call in
all cases, so guards will not disturb C<$@> in any way.

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

=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:	1.14
Committed:	Sat Dec 13 22:05:20 2008 UTC (17 years, 9 months ago) by root
Branch:	MAIN
Changes since 1.13:	+4 -4 lines
Log Message:	* empty log message *
#	User	Rev	Content
1	root	1.1	=head1 NAME
2
3			Guard - safe cleanup blocks
4
5			=head1 SYNOPSIS
6
7	root	1.9	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			call_function_that_might_die_or_other_fun_stuff;
16			}
17	root	1.4
18	root	1.1	=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	root	1.2	=head1 FUNCTIONS
29
30			This module currently exports the C<scope_guard> and C<guard> functions by
31			default.
32
33	root	1.1	=over 4
34
35			=cut
36
37			package Guard;
38
39	root	1.13	no warnings;
40
41	root	1.1	BEGIN {
42	root	1.13	$VERSION = '0.5';
43	root	1.1	@ISA = qw(Exporter);
44	root	1.3	@EXPORT = qw(guard scope_guard);
45	root	1.1
46			require Exporter;
47
48			require XSLoader;
49			XSLoader::load Guard, $VERSION;
50			}
51
52			our $DIED = sub { warn "$@" };
53
54			=item scope_guard BLOCK
55
56			Registers a block that is executed when the current scope (block,
57			function, method, eval etc.) is exited.
58
59	root	1.8	See the EXCEPTIONS section for an explanation of how exceptions
60			(i.e. C<die>) are handled inside guard blocks.
61
62	root	1.2	The description below sounds a bit complicated, but that's just because
63			C<scope_guard> tries to get even corner cases "right": the goal is to
64			provide you with a rock solid clean up tool.
65
66	root	1.8	The behaviour is similar to this code fragment:
67	root	1.1
68			eval ... code following scope_guard ...
69			{
70			local $@;
71			eval BLOCK;
72			eval { $Guard::DIED->() } if $@;
73			}
74	root	1.2	die if $@;
75	root	1.1
76			Except it is much faster, and the whole thing gets executed even when the
77			BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means.
78
79	root	1.4	If multiple BLOCKs are registered to the same scope, they will be executed
80	root	1.8	in reverse order. Other scope-related things such as C<local> are managed
81			via the same mechanism, so variables C<local>ised I<after> calling
82			C<scope_guard> will be restored when the guard runs.
83	root	1.1
84	root	1.4	Example: temporarily change the timezone for the current process,
85			ensuring it will be reset when the C<if> scope is exited:
86
87			use Guard;
88			use POSIX ();
89
90			if ($need_to_switch_tz) {
91			# make sure we call tzset after $ENV{TZ} has been restored
92			scope_guard { POSIX::tzset };
93	root	1.1
94	root	1.4	# localise after the scope_guard, so it gets undone in time
95			local $ENV{TZ} = "Europe/London";
96			POSIX::tzset;
97	root	1.1
98	root	1.4	# do something with the new timezone
99	root	1.1	}
100
101			=item my $guard = guard BLOCK
102
103			Behaves the same as C<scope_guard>, except that instead of executing
104			the block on scope exit, it returns an object whose lifetime determines
105			when the BLOCK gets executed: when the last reference to the object gets
106			destroyed, the BLOCK gets executed as with C<scope_guard>.
107
108			The returned object can be copied as many times as you want.
109
110	root	1.8	See the EXCEPTIONS section for an explanation of how exceptions
111			(i.e. C<die>) are handled inside guard blocks.
112	root	1.1
113			Example: acquire a Coro::Semaphore for a second by registering a
114	root	1.10	timer. The timer callback references the guard used to unlock it
115			again. (Please ignore the fact that C<Coro::Semaphore> has a C<guard>
116			method that does this already):
117	root	1.1
118	root	1.9	use Guard;
119	root	1.1	use AnyEvent;
120			use Coro::Semaphore;
121
122			my $sem = new Coro::Semaphore;
123
124	root	1.9	sub lock_for_a_second {
125	root	1.1	$sem->down;
126			my $guard = guard { $sem->up };
127
128			my $timer;
129			$timer = AnyEvent->timer (after => 1, sub {
130			# do something
131			undef $sem;
132			undef $timer;
133			});
134			}
135
136			The advantage of doing this with a guard instead of simply calling C<<
137			$sem->down >> in the callback is that you can opt not to create the timer,
138			or your code can throw an exception before it can create the timer, or you
139			can create multiple timers or other event watchers and only when the last
140	root	1.11	one gets executed will the lock be unlocked. Using the C<guard>, you do
141			not have to worry about catching all the places where you have to unlock
142			the semaphore.
143	root	1.1
144	root	1.13	=item $guard->cancel
145	root	1.1
146			Calling this function will "disable" the guard object returned by the
147			C<guard> function, i.e. it will free the BLOCK originally passed to
148			C<guard >and will arrange for the BLOCK not to be executed.
149
150			This can be useful when you use C<guard> to create a fatal cleanup handler
151			and later decide it is no longer needed.
152
153			=cut
154
155			1;
156
157			=back
158
159			=head1 EXCEPTIONS
160
161	root	1.5	Guard blocks should not normally throw exceptions (that is, C<die>). After
162	root	1.1	all, they are usually used to clean up after such exceptions. However, if
163			something truly exceptional is happening, a guard block should be allowed
164			to die. Also, programming errors are a large source of exceptions, and the
165			programmer certainly wants to know about those.
166
167	root	1.14	Since in most cases, the block executing when the guard gets executed does
168	root	1.1	not know or does not care about the guard blocks, it makes little sense to
169			let containing code handle the exception.
170
171			Therefore, whenever a guard block throws an exception, it will be caught,
172	root	1.14	followed by calling the code reference stored in C<$Guard::DIED> (with
173			C<$@> set to the actual exception), which is similar to how most event
174			loops handle this case.
175	root	1.1
176	root	1.12	The default for C<$Guard::DIED> is to call C<warn "$@">.
177
178			The C<$@> variable will be restored to its value before the guard call in
179			all cases, so guards will not disturb C<$@> in any way.
180
181	root	1.1	The code reference stored in C<$Guard::DIED> should not die (behaviour is
182			not guaranteed, but right now, the exception will simply be ignored).
183
184			=head1 AUTHOR
185
186			Marc Lehmann <schmorp@schmorp.de>
187			http://home.schmorp.de/
188
189			=head1 THANKS
190
191	root	1.6	Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED>
192			solution to the problem of exceptions.
193	root	1.1
194			=cut
195