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;

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.

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 $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:	1.11
Committed:	Sat Dec 13 18:51:19 2008 UTC (15 years, 5 months ago) by root
Branch:	MAIN
Changes since 1.10:	+3 -1 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			BEGIN {
40	root	1.3	$VERSION = '0.1';
41	root	1.1	@ISA = qw(Exporter);
42	root	1.3	@EXPORT = qw(guard scope_guard);
43	root	1.1
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	root	1.8	See the EXCEPTIONS section for an explanation of how exceptions
58			(i.e. C<die>) are handled inside guard blocks.
59
60	root	1.2	The description below sounds a bit complicated, but that's just because
61			C<scope_guard> tries to get even corner cases "right": the goal is to
62			provide you with a rock solid clean up tool.
63
64	root	1.8	The behaviour is similar to this code fragment:
65	root	1.1
66			eval ... code following scope_guard ...
67			{
68			local $@;
69			eval BLOCK;
70			eval { $Guard::DIED->() } if $@;
71			}
72	root	1.2	die if $@;
73	root	1.1
74			Except it is much faster, and the whole thing gets executed even when the
75			BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means.
76
77	root	1.4	If multiple BLOCKs are registered to the same scope, they will be executed
78	root	1.8	in reverse order. Other scope-related things such as C<local> are managed
79			via the same mechanism, so variables C<local>ised I<after> calling
80			C<scope_guard> will be restored when the guard runs.
81	root	1.1
82	root	1.4	Example: temporarily change the timezone for the current process,
83			ensuring it will be reset when the C<if> scope is exited:
84
85			use Guard;
86			use POSIX ();
87
88			if ($need_to_switch_tz) {
89			# make sure we call tzset after $ENV{TZ} has been restored
90			scope_guard { POSIX::tzset };
91	root	1.1
92	root	1.4	# localise after the scope_guard, so it gets undone in time
93			local $ENV{TZ} = "Europe/London";
94			POSIX::tzset;
95	root	1.1
96	root	1.4	# do something with the new timezone
97	root	1.1	}
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	root	1.8	See the EXCEPTIONS section for an explanation of how exceptions
109			(i.e. C<die>) are handled inside guard blocks.
110	root	1.1
111			Example: acquire a Coro::Semaphore for a second by registering a
112	root	1.10	timer. The timer callback references the guard used to unlock it
113			again. (Please ignore the fact that C<Coro::Semaphore> has a C<guard>
114			method that does this already):
115	root	1.1
116	root	1.9	use Guard;
117	root	1.1	use AnyEvent;
118			use Coro::Semaphore;
119
120			my $sem = new Coro::Semaphore;
121
122	root	1.9	sub lock_for_a_second {
123	root	1.1	$sem->down;
124			my $guard = guard { $sem->up };
125
126			my $timer;
127			$timer = AnyEvent->timer (after => 1, sub {
128			# do something
129			undef $sem;
130			undef $timer;
131			});
132			}
133
134			The advantage of doing this with a guard instead of simply calling C<<
135			$sem->down >> in the callback is that you can opt not to create the timer,
136			or your code can throw an exception before it can create the timer, or you
137			can create multiple timers or other event watchers and only when the last
138	root	1.11	one gets executed will the lock be unlocked. Using the C<guard>, you do
139			not have to worry about catching all the places where you have to unlock
140			the semaphore.
141	root	1.1
142			=item Guard::cancel $guard
143
144			Calling this function will "disable" the guard object returned by the
145			C<guard> function, i.e. it will free the BLOCK originally passed to
146			C<guard >and will arrange for the BLOCK not to be executed.
147
148			This can be useful when you use C<guard> to create a fatal cleanup handler
149			and later decide it is no longer needed.
150
151			=cut
152
153			1;
154
155			=back
156
157			=head1 EXCEPTIONS
158
159	root	1.5	Guard blocks should not normally throw exceptions (that is, C<die>). After
160	root	1.1	all, they are usually used to clean up after such exceptions. However, if
161			something truly exceptional is happening, a guard block should be allowed
162			to die. Also, programming errors are a large source of exceptions, and the
163			programmer certainly wants to know about those.
164
165			Since in most cases, the block executing when the guard gets executes does
166			not know or does not care about the guard blocks, it makes little sense to
167			let containing code handle the exception.
168
169			Therefore, whenever a guard block throws an exception, it will be caught,
170			and this module will call the code reference stored in C<$Guard::DIED>
171			(with C<$@> set to the actual exception), which is similar to how most
172			event loops handle this case.
173
174			The code reference stored in C<$Guard::DIED> should not die (behaviour is
175			not guaranteed, but right now, the exception will simply be ignored).
176
177			The default for C<$Guard::DIED> is to call C<warn "$@">.
178
179			=head1 AUTHOR
180
181			Marc Lehmann <schmorp@schmorp.de>
182			http://home.schmorp.de/
183
184			=head1 THANKS
185
186	root	1.6	Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED>
187			solution to the problem of exceptions.
188	root	1.1
189			=cut
190