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 |
=over 4 |
20 |
|
21 |
=cut |
22 |
|
23 |
package Guard; |
24 |
|
25 |
BEGIN { |
26 |
$VERSION = '0.01'; |
27 |
@ISA = qw(Exporter); |
28 |
@EXPORT = qw(guard scope_guard cancel); |
29 |
|
30 |
require Exporter; |
31 |
|
32 |
require XSLoader; |
33 |
XSLoader::load Guard, $VERSION; |
34 |
} |
35 |
|
36 |
our $DIED = sub { warn "$@" }; |
37 |
|
38 |
=item scope_guard BLOCK |
39 |
|
40 |
Registers a block that is executed when the current scope (block, |
41 |
function, method, eval etc.) is exited. |
42 |
|
43 |
This is similar to this code fragment: |
44 |
|
45 |
eval ... code following scope_guard ... |
46 |
{ |
47 |
local $@; |
48 |
eval BLOCK; |
49 |
eval { $Guard::DIED->() } if $@; |
50 |
} |
51 |
|
52 |
Except it is much faster, and the whole thing gets executed even when the |
53 |
BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means. |
54 |
|
55 |
See B<EXCEPTIONS>, below, for an explanation of exception handling |
56 |
(C<die>) within guard blocks. |
57 |
|
58 |
Example: Temporarily change the directory to F</etc> and make sure it's |
59 |
set back to F</> when the function returns: |
60 |
|
61 |
sub dosomething { |
62 |
scope_guard { chdir "/" }; |
63 |
chdir "/etc"; |
64 |
|
65 |
... |
66 |
} |
67 |
|
68 |
=item my $guard = guard BLOCK |
69 |
|
70 |
Behaves the same as C<scope_guard>, except that instead of executing |
71 |
the block on scope exit, it returns an object whose lifetime determines |
72 |
when the BLOCK gets executed: when the last reference to the object gets |
73 |
destroyed, the BLOCK gets executed as with C<scope_guard>. |
74 |
|
75 |
The returned object can be copied as many times as you want. |
76 |
|
77 |
See B<EXCEPTIONS>, below, for an explanation of exception handling |
78 |
(C<die>) within guard blocks. |
79 |
|
80 |
Example: acquire a Coro::Semaphore for a second by registering a |
81 |
timer. The timer callback references the guard used to unlock it again. |
82 |
|
83 |
use AnyEvent; |
84 |
use Coro::Semaphore; |
85 |
|
86 |
my $sem = new Coro::Semaphore; |
87 |
|
88 |
sub lock_1s { |
89 |
$sem->down; |
90 |
my $guard = guard { $sem->up }; |
91 |
|
92 |
my $timer; |
93 |
$timer = AnyEvent->timer (after => 1, sub { |
94 |
# do something |
95 |
undef $sem; |
96 |
undef $timer; |
97 |
}); |
98 |
} |
99 |
|
100 |
The advantage of doing this with a guard instead of simply calling C<< |
101 |
$sem->down >> in the callback is that you can opt not to create the timer, |
102 |
or your code can throw an exception before it can create the timer, or you |
103 |
can create multiple timers or other event watchers and only when the last |
104 |
one gets executed will the lock be unlocked. |
105 |
|
106 |
=item Guard::cancel $guard |
107 |
|
108 |
Calling this function will "disable" the guard object returned by the |
109 |
C<guard> function, i.e. it will free the BLOCK originally passed to |
110 |
C<guard >and will arrange for the BLOCK not to be executed. |
111 |
|
112 |
This can be useful when you use C<guard> to create a fatal cleanup handler |
113 |
and later decide it is no longer needed. |
114 |
|
115 |
=cut |
116 |
|
117 |
1; |
118 |
|
119 |
=back |
120 |
|
121 |
=head1 EXCEPTIONS |
122 |
|
123 |
Guard blocks should not normally throw exceptions (e.g. C<die>), after |
124 |
all, they are usually used to clean up after such exceptions. However, if |
125 |
something truly exceptional is happening, a guard block should be allowed |
126 |
to die. Also, programming errors are a large source of exceptions, and the |
127 |
programmer certainly wants to know about those. |
128 |
|
129 |
Since in most cases, the block executing when the guard gets executes does |
130 |
not know or does not care about the guard blocks, it makes little sense to |
131 |
let containing code handle the exception. |
132 |
|
133 |
Therefore, whenever a guard block throws an exception, it will be caught, |
134 |
and this module will call the code reference stored in C<$Guard::DIED> |
135 |
(with C<$@> set to the actual exception), which is similar to how most |
136 |
event loops handle this case. |
137 |
|
138 |
The code reference stored in C<$Guard::DIED> should not die (behaviour is |
139 |
not guaranteed, but right now, the exception will simply be ignored). |
140 |
|
141 |
The default for C<$Guard::DIED> is to call C<warn "$@">. |
142 |
|
143 |
=head1 AUTHOR |
144 |
|
145 |
Marc Lehmann <schmorp@schmorp.de> |
146 |
http://home.schmorp.de/ |
147 |
|
148 |
=head1 THANKS |
149 |
|
150 |
To Marco Maisenhelder, who reminded me of the C<$Guard::DIED> solution to |
151 |
the problem of exceptions. |
152 |
|
153 |
=cut |
154 |
|