… | |
… | |
3 | Guard - safe cleanup blocks |
3 | Guard - safe cleanup blocks |
4 | |
4 | |
5 | =head1 SYNOPSIS |
5 | =head1 SYNOPSIS |
6 | |
6 | |
7 | use Guard; |
7 | 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 | } |
8 | |
17 | |
9 | =head1 DESCRIPTION |
18 | =head1 DESCRIPTION |
10 | |
19 | |
11 | This module implements so-called "guards". A guard is something (usually |
20 | 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 |
21 | an object) that "guards" a resource, ensuring that it is cleaned up when |
… | |
… | |
14 | |
23 | |
15 | Specifically, this module supports two different types of guards: guard |
24 | Specifically, this module supports two different types of guards: guard |
16 | objects, which execute a given code block when destroyed, and scoped |
25 | objects, which execute a given code block when destroyed, and scoped |
17 | guards, which are tied to the scope exit. |
26 | guards, which are tied to the scope exit. |
18 | |
27 | |
|
|
28 | =head1 FUNCTIONS |
|
|
29 | |
|
|
30 | This module currently exports the C<scope_guard> and C<guard> functions by |
|
|
31 | default. |
|
|
32 | |
19 | =over 4 |
33 | =over 4 |
20 | |
34 | |
21 | =cut |
35 | =cut |
22 | |
36 | |
23 | package Guard; |
37 | package Guard; |
24 | |
38 | |
25 | BEGIN { |
39 | BEGIN { |
26 | $VERSION = '0.01'; |
40 | $VERSION = '0.1'; |
27 | @ISA = qw(Exporter); |
41 | @ISA = qw(Exporter); |
28 | @EXPORT = qw(guard scope_guard cancel); |
42 | @EXPORT = qw(guard scope_guard); |
29 | |
43 | |
30 | require Exporter; |
44 | require Exporter; |
31 | |
45 | |
32 | require XSLoader; |
46 | require XSLoader; |
33 | XSLoader::load Guard, $VERSION; |
47 | XSLoader::load Guard, $VERSION; |
… | |
… | |
38 | =item scope_guard BLOCK |
52 | =item scope_guard BLOCK |
39 | |
53 | |
40 | Registers a block that is executed when the current scope (block, |
54 | Registers a block that is executed when the current scope (block, |
41 | function, method, eval etc.) is exited. |
55 | function, method, eval etc.) is exited. |
42 | |
56 | |
|
|
57 | The description below sounds a bit complicated, but that's just because |
|
|
58 | C<scope_guard> tries to get even corner cases "right": the goal is to |
|
|
59 | provide you with a rock solid clean up tool. |
|
|
60 | |
43 | This is similar to this code fragment: |
61 | This is similar to this code fragment: |
44 | |
62 | |
45 | eval ... code following scope_guard ... |
63 | eval ... code following scope_guard ... |
46 | { |
64 | { |
47 | local $@; |
65 | local $@; |
48 | eval BLOCK; |
66 | eval BLOCK; |
49 | eval { $Guard::DIED->() } if $@; |
67 | eval { $Guard::DIED->() } if $@; |
50 | } |
68 | } |
|
|
69 | die if $@; |
51 | |
70 | |
52 | Except it is much faster, and the whole thing gets executed even when the |
71 | Except it is much faster, and the whole thing gets executed even when the |
53 | BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means. |
72 | BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means. |
54 | |
73 | |
|
|
74 | If multiple BLOCKs are registered to the same scope, they will be executed |
|
|
75 | in reverse order. Stuff like C<local> is managed via the same mechanism, |
|
|
76 | so variables C<local>ised after calling C<scope_guard> will be restored |
|
|
77 | when the guard runs. |
|
|
78 | |
55 | See B<EXCEPTIONS>, below, for an explanation of exception handling |
79 | See B<EXCEPTIONS>, below, for an explanation of exception handling |
56 | (C<die>) within guard blocks. |
80 | (C<die>) within guard blocks. |
57 | |
81 | |
58 | Example: Temporarily change the directory to F</etc> and make sure it's |
82 | Example: temporarily change the timezone for the current process, |
59 | set back to F</> when the function returns: |
83 | ensuring it will be reset when the C<if> scope is exited: |
60 | |
84 | |
61 | sub dosomething { |
85 | use Guard; |
62 | scope_guard { chdir "/" }; |
86 | use POSIX (); |
63 | chdir "/etc"; |
|
|
64 | |
87 | |
65 | ... |
88 | if ($need_to_switch_tz) { |
|
|
89 | # make sure we call tzset after $ENV{TZ} has been restored |
|
|
90 | scope_guard { POSIX::tzset }; |
|
|
91 | |
|
|
92 | # localise after the scope_guard, so it gets undone in time |
|
|
93 | local $ENV{TZ} = "Europe/London"; |
|
|
94 | POSIX::tzset; |
|
|
95 | |
|
|
96 | # do something with the new timezone |
66 | } |
97 | } |
67 | |
98 | |
68 | =item my $guard = guard BLOCK |
99 | =item my $guard = guard BLOCK |
69 | |
100 | |
70 | Behaves the same as C<scope_guard>, except that instead of executing |
101 | Behaves the same as C<scope_guard>, except that instead of executing |
… | |
… | |
118 | |
149 | |
119 | =back |
150 | =back |
120 | |
151 | |
121 | =head1 EXCEPTIONS |
152 | =head1 EXCEPTIONS |
122 | |
153 | |
123 | Guard blocks should not normally throw exceptions (e.g. C<die>), after |
154 | Guard blocks should not normally throw exceptions (that is, C<die>). After |
124 | all, they are usually used to clean up after such exceptions. However, if |
155 | all, they are usually used to clean up after such exceptions. However, if |
125 | something truly exceptional is happening, a guard block should be allowed |
156 | something truly exceptional is happening, a guard block should be allowed |
126 | to die. Also, programming errors are a large source of exceptions, and the |
157 | to die. Also, programming errors are a large source of exceptions, and the |
127 | programmer certainly wants to know about those. |
158 | programmer certainly wants to know about those. |
128 | |
159 | |
… | |
… | |
145 | Marc Lehmann <schmorp@schmorp.de> |
176 | Marc Lehmann <schmorp@schmorp.de> |
146 | http://home.schmorp.de/ |
177 | http://home.schmorp.de/ |
147 | |
178 | |
148 | =head1 THANKS |
179 | =head1 THANKS |
149 | |
180 | |
150 | To Marco Maisenhelder, who reminded me of the C<$Guard::DIED> solution to |
181 | Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED> |
151 | the problem of exceptions. |
182 | solution to the problem of exceptions. |
152 | |
183 | |
153 | =cut |
184 | =cut |
154 | |
185 | |