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";
   
      code_that_might_die_or_does_other_fun_stuff;
   }

   # create an object that, when the last reference to it is gone,
   # invokes the given codeblock:
   my $guard = guard { print "destroyed!\n" };
   undef $guard; # probably destroyed here

=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 = '1.02';
   @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.

=head1 SEE ALSO

L<Scope::Guard> and L<Sub::ScopeFinalizer>, which actually implement
dynamic, not scoped guards, and have a lot higher CPU, memory and typing
overhead.

L<Hook::Scope>, which has apparently never been finished and corrupts
memory when used.

=cut

Revision:	1.20
Committed:	Mon Jun 1 10:33:16 2009 UTC (14 years, 11 months ago) by root
Branch:	MAIN
Changes since 1.19:	+5 -0 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	root	1.15	code_that_might_die_or_does_other_fun_stuff;
16	root	1.9	}
17	root	1.4
18	root	1.20	# 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
22
23	root	1.1	=head1 DESCRIPTION
24
25			This module implements so-called "guards". A guard is something (usually
26			an object) that "guards" a resource, ensuring that it is cleaned up when
27			expected.
28
29			Specifically, this module supports two different types of guards: guard
30			objects, which execute a given code block when destroyed, and scoped
31			guards, which are tied to the scope exit.
32
33	root	1.2	=head1 FUNCTIONS
34
35			This module currently exports the C<scope_guard> and C<guard> functions by
36			default.
37
38	root	1.1	=over 4
39
40			=cut
41
42			package Guard;
43
44	root	1.13	no warnings;
45
46	root	1.1	BEGIN {
47	root	1.19	$VERSION = '1.02';
48	root	1.1	@ISA = qw(Exporter);
49	root	1.3	@EXPORT = qw(guard scope_guard);
50	root	1.1
51			require Exporter;
52
53			require XSLoader;
54			XSLoader::load Guard, $VERSION;
55			}
56
57			our $DIED = sub { warn "$@" };
58
59			=item scope_guard BLOCK
60
61			Registers a block that is executed when the current scope (block,
62			function, method, eval etc.) is exited.
63
64	root	1.8	See the EXCEPTIONS section for an explanation of how exceptions
65			(i.e. C<die>) are handled inside guard blocks.
66
67	root	1.2	The description below sounds a bit complicated, but that's just because
68			C<scope_guard> tries to get even corner cases "right": the goal is to
69			provide you with a rock solid clean up tool.
70
71	root	1.8	The behaviour is similar to this code fragment:
72	root	1.1
73			eval ... code following scope_guard ...
74			{
75			local $@;
76			eval BLOCK;
77			eval { $Guard::DIED->() } if $@;
78			}
79	root	1.2	die if $@;
80	root	1.1
81			Except it is much faster, and the whole thing gets executed even when the
82			BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means.
83
84	root	1.4	If multiple BLOCKs are registered to the same scope, they will be executed
85	root	1.8	in reverse order. Other scope-related things such as C<local> are managed
86			via the same mechanism, so variables C<local>ised I<after> calling
87			C<scope_guard> will be restored when the guard runs.
88	root	1.1
89	root	1.4	Example: temporarily change the timezone for the current process,
90			ensuring it will be reset when the C<if> scope is exited:
91
92			use Guard;
93			use POSIX ();
94
95			if ($need_to_switch_tz) {
96			# make sure we call tzset after $ENV{TZ} has been restored
97			scope_guard { POSIX::tzset };
98	root	1.1
99	root	1.4	# localise after the scope_guard, so it gets undone in time
100			local $ENV{TZ} = "Europe/London";
101			POSIX::tzset;
102	root	1.1
103	root	1.4	# do something with the new timezone
104	root	1.1	}
105
106			=item my $guard = guard BLOCK
107
108			Behaves the same as C<scope_guard>, except that instead of executing
109			the block on scope exit, it returns an object whose lifetime determines
110			when the BLOCK gets executed: when the last reference to the object gets
111			destroyed, the BLOCK gets executed as with C<scope_guard>.
112
113			The returned object can be copied as many times as you want.
114
115	root	1.8	See the EXCEPTIONS section for an explanation of how exceptions
116			(i.e. C<die>) are handled inside guard blocks.
117	root	1.1
118			Example: acquire a Coro::Semaphore for a second by registering a
119	root	1.10	timer. The timer callback references the guard used to unlock it
120			again. (Please ignore the fact that C<Coro::Semaphore> has a C<guard>
121			method that does this already):
122	root	1.1
123	root	1.9	use Guard;
124	root	1.1	use AnyEvent;
125			use Coro::Semaphore;
126
127			my $sem = new Coro::Semaphore;
128
129	root	1.9	sub lock_for_a_second {
130	root	1.1	$sem->down;
131			my $guard = guard { $sem->up };
132
133			my $timer;
134			$timer = AnyEvent->timer (after => 1, sub {
135			# do something
136			undef $sem;
137			undef $timer;
138			});
139			}
140
141			The advantage of doing this with a guard instead of simply calling C<<
142			$sem->down >> in the callback is that you can opt not to create the timer,
143			or your code can throw an exception before it can create the timer, or you
144			can create multiple timers or other event watchers and only when the last
145	root	1.11	one gets executed will the lock be unlocked. Using the C<guard>, you do
146			not have to worry about catching all the places where you have to unlock
147			the semaphore.
148	root	1.1
149	root	1.13	=item $guard->cancel
150	root	1.1
151			Calling this function will "disable" the guard object returned by the
152			C<guard> function, i.e. it will free the BLOCK originally passed to
153			C<guard >and will arrange for the BLOCK not to be executed.
154
155			This can be useful when you use C<guard> to create a fatal cleanup handler
156			and later decide it is no longer needed.
157
158			=cut
159
160			1;
161
162			=back
163
164			=head1 EXCEPTIONS
165
166	root	1.5	Guard blocks should not normally throw exceptions (that is, C<die>). After
167	root	1.1	all, they are usually used to clean up after such exceptions. However, if
168			something truly exceptional is happening, a guard block should be allowed
169			to die. Also, programming errors are a large source of exceptions, and the
170			programmer certainly wants to know about those.
171
172	root	1.14	Since in most cases, the block executing when the guard gets executed does
173	root	1.1	not know or does not care about the guard blocks, it makes little sense to
174			let containing code handle the exception.
175
176			Therefore, whenever a guard block throws an exception, it will be caught,
177	root	1.14	followed by calling the code reference stored in C<$Guard::DIED> (with
178			C<$@> set to the actual exception), which is similar to how most event
179			loops handle this case.
180	root	1.1
181	root	1.12	The default for C<$Guard::DIED> is to call C<warn "$@">.
182
183			The C<$@> variable will be restored to its value before the guard call in
184			all cases, so guards will not disturb C<$@> in any way.
185
186	root	1.1	The code reference stored in C<$Guard::DIED> should not die (behaviour is
187			not guaranteed, but right now, the exception will simply be ignored).
188
189			=head1 AUTHOR
190
191			Marc Lehmann <schmorp@schmorp.de>
192			http://home.schmorp.de/
193
194			=head1 THANKS
195
196	root	1.6	Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED>
197			solution to the problem of exceptions.
198	root	1.1
199	root	1.16	=head1 SEE ALSO
200
201			L<Scope::Guard> and L<Sub::ScopeFinalizer>, which actually implement
202			dynamic, not scoped guards, and have a lot higher CPU, memory and typing
203			overhead.
204
205			L<Hook::Scope>, which has apparently never been finished and corrupts
206			memory when used.
207
208	root	1.1	=cut
209