… | |
… | |
2 | |
2 | |
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 | |
8 | |
9 | # temporarily chdir to "/etc" directory, but make sure |
9 | # temporarily chdir to "/etc" directory, but make sure |
10 | # to go back to "/" no matter how myfun exits: |
10 | # to go back to "/" no matter how myfun exits: |
11 | sub myfun { |
11 | sub myfun { |
12 | scope_guard { chdir "/" }; |
12 | scope_guard { chdir "/" }; |
13 | chdir "/etc"; |
13 | chdir "/etc"; |
14 | |
14 | |
15 | call_function_that_might_die_or_other_fun_stuff; |
15 | call_function_that_might_die_or_other_fun_stuff; |
16 | } |
16 | } |
17 | |
17 | |
18 | =head1 DESCRIPTION |
18 | =head1 DESCRIPTION |
19 | |
19 | |
20 | This module implements so-called "guards". A guard is something (usually |
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 |
21 | an object) that "guards" a resource, ensuring that it is cleaned up when |
… | |
… | |
52 | =item scope_guard BLOCK |
52 | =item scope_guard BLOCK |
53 | |
53 | |
54 | Registers a block that is executed when the current scope (block, |
54 | Registers a block that is executed when the current scope (block, |
55 | function, method, eval etc.) is exited. |
55 | function, method, eval etc.) is exited. |
56 | |
56 | |
|
|
57 | See the EXCEPTIONS section for an explanation of how exceptions |
|
|
58 | (i.e. C<die>) are handled inside guard blocks. |
|
|
59 | |
57 | The description below sounds a bit complicated, but that's just because |
60 | 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 |
61 | C<scope_guard> tries to get even corner cases "right": the goal is to |
59 | provide you with a rock solid clean up tool. |
62 | provide you with a rock solid clean up tool. |
60 | |
63 | |
61 | This is similar to this code fragment: |
64 | The behaviour is similar to this code fragment: |
62 | |
65 | |
63 | eval ... code following scope_guard ... |
66 | eval ... code following scope_guard ... |
64 | { |
67 | { |
65 | local $@; |
68 | local $@; |
66 | eval BLOCK; |
69 | eval BLOCK; |
… | |
… | |
70 | |
73 | |
71 | Except it is much faster, and the whole thing gets executed even when the |
74 | Except it is much faster, and the whole thing gets executed even when the |
72 | BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means. |
75 | BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means. |
73 | |
76 | |
74 | If multiple BLOCKs are registered to the same scope, they will be executed |
77 | 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, |
78 | in reverse order. Other scope-related things such as C<local> are managed |
76 | so variables C<local>ised after calling C<scope_guard> will be restored |
79 | via the same mechanism, so variables C<local>ised I<after> calling |
77 | when the guard runs. |
80 | C<scope_guard> will be restored when the guard runs. |
78 | |
|
|
79 | See B<EXCEPTIONS>, below, for an explanation of exception handling |
|
|
80 | (C<die>) within guard blocks. |
|
|
81 | |
81 | |
82 | Example: temporarily change the timezone for the current process, |
82 | Example: temporarily change the timezone for the current process, |
83 | ensuring it will be reset when the C<if> scope is exited: |
83 | ensuring it will be reset when the C<if> scope is exited: |
84 | |
84 | |
85 | use Guard; |
85 | use Guard; |
… | |
… | |
103 | when the BLOCK gets executed: when the last reference to the object gets |
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>. |
104 | destroyed, the BLOCK gets executed as with C<scope_guard>. |
105 | |
105 | |
106 | The returned object can be copied as many times as you want. |
106 | The returned object can be copied as many times as you want. |
107 | |
107 | |
108 | See B<EXCEPTIONS>, below, for an explanation of exception handling |
108 | See the EXCEPTIONS section for an explanation of how exceptions |
109 | (C<die>) within guard blocks. |
109 | (i.e. C<die>) are handled inside guard blocks. |
110 | |
110 | |
111 | Example: acquire a Coro::Semaphore for a second by registering a |
111 | Example: acquire a Coro::Semaphore for a second by registering a |
112 | timer. The timer callback references the guard used to unlock it again. |
112 | 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): |
113 | |
115 | |
|
|
116 | use Guard; |
114 | use AnyEvent; |
117 | use AnyEvent; |
115 | use Coro::Semaphore; |
118 | use Coro::Semaphore; |
116 | |
119 | |
117 | my $sem = new Coro::Semaphore; |
120 | my $sem = new Coro::Semaphore; |
118 | |
121 | |
119 | sub lock_1s { |
122 | sub lock_for_a_second { |
120 | $sem->down; |
123 | $sem->down; |
121 | my $guard = guard { $sem->up }; |
124 | my $guard = guard { $sem->up }; |
122 | |
125 | |
123 | my $timer; |
126 | my $timer; |
124 | $timer = AnyEvent->timer (after => 1, sub { |
127 | $timer = AnyEvent->timer (after => 1, sub { |