… | |
… | |
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 | code_that_might_die_or_does_other_fun_stuff; |
16 | } |
16 | } |
|
|
17 | |
|
|
18 | # 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 |
17 | |
22 | |
18 | =head1 DESCRIPTION |
23 | =head1 DESCRIPTION |
19 | |
24 | |
20 | This module implements so-called "guards". A guard is something (usually |
25 | 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 |
26 | an object) that "guards" a resource, ensuring that it is cleaned up when |
… | |
… | |
34 | |
39 | |
35 | =cut |
40 | =cut |
36 | |
41 | |
37 | package Guard; |
42 | package Guard; |
38 | |
43 | |
|
|
44 | no warnings; |
|
|
45 | |
39 | BEGIN { |
46 | BEGIN { |
40 | $VERSION = '0.1'; |
47 | $VERSION = 1.023; |
41 | @ISA = qw(Exporter); |
48 | @ISA = qw(Exporter); |
42 | @EXPORT = qw(guard scope_guard); |
49 | @EXPORT = qw(guard scope_guard); |
43 | |
50 | |
44 | require Exporter; |
51 | require Exporter; |
45 | |
52 | |
… | |
… | |
48 | } |
55 | } |
49 | |
56 | |
50 | our $DIED = sub { warn "$@" }; |
57 | our $DIED = sub { warn "$@" }; |
51 | |
58 | |
52 | =item scope_guard BLOCK |
59 | =item scope_guard BLOCK |
|
|
60 | |
|
|
61 | =item scope_guard ($coderef) |
53 | |
62 | |
54 | Registers a block that is executed when the current scope (block, |
63 | Registers a block that is executed when the current scope (block, |
55 | function, method, eval etc.) is exited. |
64 | function, method, eval etc.) is exited. |
56 | |
65 | |
57 | See the EXCEPTIONS section for an explanation of how exceptions |
66 | See the EXCEPTIONS section for an explanation of how exceptions |
… | |
… | |
75 | BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means. |
84 | BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means. |
76 | |
85 | |
77 | If multiple BLOCKs are registered to the same scope, they will be executed |
86 | If multiple BLOCKs are registered to the same scope, they will be executed |
78 | in reverse order. Other scope-related things such as C<local> are managed |
87 | 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 |
88 | via the same mechanism, so variables C<local>ised I<after> calling |
80 | C<scope_guard> will be restored when the guard runs. |
89 | C<scope_guard> will be restored I<before> the guard runs. |
81 | |
90 | |
82 | Example: temporarily change the timezone for the current process, |
91 | Example: temporarily change the timezone for the current process, |
83 | ensuring it will be reset when the C<if> scope is exited: |
92 | ensuring it will be reset when the C<if> scope is exited: |
84 | |
93 | |
85 | use Guard; |
94 | use Guard; |
… | |
… | |
95 | |
104 | |
96 | # do something with the new timezone |
105 | # do something with the new timezone |
97 | } |
106 | } |
98 | |
107 | |
99 | =item my $guard = guard BLOCK |
108 | =item my $guard = guard BLOCK |
|
|
109 | |
|
|
110 | =item my $guard = guard ($coderef) |
100 | |
111 | |
101 | Behaves the same as C<scope_guard>, except that instead of executing |
112 | 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 |
113 | 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 |
114 | when the BLOCK gets executed: when the last reference to the object gets |
104 | destroyed, the BLOCK gets executed as with C<scope_guard>. |
115 | destroyed, the BLOCK gets executed as with C<scope_guard>. |
105 | |
116 | |
106 | The returned object can be copied as many times as you want. |
|
|
107 | |
|
|
108 | See the EXCEPTIONS section for an explanation of how exceptions |
117 | See the EXCEPTIONS section for an explanation of how exceptions |
109 | (i.e. C<die>) are handled inside guard blocks. |
118 | (i.e. C<die>) are handled inside guard blocks. |
110 | |
119 | |
111 | Example: acquire a Coro::Semaphore for a second by registering a |
120 | Example: acquire a Coro::Semaphore for a second by registering a |
112 | timer. The timer callback references the guard used to unlock it |
121 | 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> |
122 | again. (Please ignore the fact that C<Coro::Semaphore> has a C<guard> |
114 | method that does this already): |
123 | method that does this already): |
115 | |
124 | |
116 | use Guard; |
125 | use Guard; |
117 | use AnyEvent; |
126 | use Coro::AnyEvent; |
118 | use Coro::Semaphore; |
127 | use Coro::Semaphore; |
119 | |
128 | |
120 | my $sem = new Coro::Semaphore; |
129 | my $sem = new Coro::Semaphore; |
121 | |
130 | |
122 | sub lock_for_a_second { |
131 | sub lock_for_a_second { |
123 | $sem->down; |
132 | $sem->down; |
124 | my $guard = guard { $sem->up }; |
133 | my $guard = guard { $sem->up }; |
125 | |
134 | |
126 | my $timer; |
135 | Coro::AnyEvent::sleep 1; |
127 | $timer = AnyEvent->timer (after => 1, sub { |
136 | |
128 | # do something |
137 | # $sem->up gets executed when returning |
129 | undef $sem; |
|
|
130 | undef $timer; |
|
|
131 | }); |
|
|
132 | } |
138 | } |
133 | |
139 | |
134 | The advantage of doing this with a guard instead of simply calling C<< |
140 | 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, |
141 | $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 |
142 | or your code can throw an exception before it can create the timer (or |
137 | can create multiple timers or other event watchers and only when the last |
143 | the thread gets canceled), or you can create multiple timers or other |
138 | one gets executed will the lock be unlocked. |
144 | event watchers and only when the last one gets executed will the lock be |
|
|
145 | unlocked. Using the C<guard>, you do not have to worry about catching all |
|
|
146 | the places where you have to unlock the semaphore. |
139 | |
147 | |
140 | =item Guard::cancel $guard |
148 | =item $guard->cancel |
141 | |
149 | |
142 | Calling this function will "disable" the guard object returned by the |
150 | Calling this function will "disable" the guard object returned by the |
143 | C<guard> function, i.e. it will free the BLOCK originally passed to |
151 | C<guard> function, i.e. it will free the BLOCK originally passed to |
144 | C<guard >and will arrange for the BLOCK not to be executed. |
152 | C<guard >and will arrange for the BLOCK not to be executed. |
145 | |
153 | |
146 | This can be useful when you use C<guard> to create a fatal cleanup handler |
154 | This can be useful when you use C<guard> to create a cleanup handler to be |
147 | and later decide it is no longer needed. |
155 | called under fatal conditions and later decide it is no longer needed. |
148 | |
156 | |
149 | =cut |
157 | =cut |
150 | |
158 | |
151 | 1; |
159 | 1; |
152 | |
160 | |
153 | =back |
161 | =back |
154 | |
162 | |
155 | =head1 EXCEPTIONS |
163 | =head1 EXCEPTIONS |
156 | |
164 | |
157 | Guard blocks should not normally throw exceptions (that is, C<die>). After |
165 | Guard blocks should not normally throw exceptions (that is, C<die>). After |
158 | all, they are usually used to clean up after such exceptions. However, if |
166 | all, they are usually used to clean up after such exceptions. However, |
159 | something truly exceptional is happening, a guard block should be allowed |
167 | if something truly exceptional is happening, a guard block should of |
160 | to die. Also, programming errors are a large source of exceptions, and the |
168 | course be allowed to die. Also, programming errors are a large source of |
161 | programmer certainly wants to know about those. |
169 | exceptions, and the programmer certainly wants to know about those. |
162 | |
170 | |
163 | Since in most cases, the block executing when the guard gets executes does |
171 | Since in most cases, the block executing when the guard gets executed does |
164 | not know or does not care about the guard blocks, it makes little sense to |
172 | not know or does not care about the guard blocks, it makes little sense to |
165 | let containing code handle the exception. |
173 | let containing code handle the exception. |
166 | |
174 | |
167 | Therefore, whenever a guard block throws an exception, it will be caught, |
175 | Therefore, whenever a guard block throws an exception, it will be caught |
168 | and this module will call the code reference stored in C<$Guard::DIED> |
176 | by Guard, followed by calling the code reference stored in C<$Guard::DIED> |
169 | (with C<$@> set to the actual exception), which is similar to how most |
177 | (with C<$@> set to the actual exception), which is similar to how most |
170 | event loops handle this case. |
178 | event loops handle this case. |
171 | |
179 | |
|
|
180 | The default for C<$Guard::DIED> is to call C<warn "$@">, i.e. the error is |
|
|
181 | printed as a warning and the program continues. |
|
|
182 | |
|
|
183 | The C<$@> variable will be restored to its value before the guard call in |
|
|
184 | all cases, so guards will not disturb C<$@> in any way. |
|
|
185 | |
172 | The code reference stored in C<$Guard::DIED> should not die (behaviour is |
186 | The code reference stored in C<$Guard::DIED> should not die (behaviour is |
173 | not guaranteed, but right now, the exception will simply be ignored). |
187 | not guaranteed, but right now, the exception will simply be ignored). |
174 | |
188 | |
175 | The default for C<$Guard::DIED> is to call C<warn "$@">. |
|
|
176 | |
|
|
177 | =head1 AUTHOR |
189 | =head1 AUTHOR |
178 | |
190 | |
179 | Marc Lehmann <schmorp@schmorp.de> |
191 | Marc Lehmann <schmorp@schmorp.de> |
180 | http://home.schmorp.de/ |
192 | http://home.schmorp.de/ |
181 | |
193 | |
182 | =head1 THANKS |
194 | =head1 THANKS |
183 | |
195 | |
184 | Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED> |
196 | Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED> |
185 | solution to the problem of exceptions. |
197 | solution to the problem of exceptions. |
186 | |
198 | |
|
|
199 | =head1 SEE ALSO |
|
|
200 | |
|
|
201 | L<Scope::Guard> and L<Sub::ScopeFinalizer>, which actually implement |
|
|
202 | dynamically scoped guards only, not the lexically scoped guards that their |
|
|
203 | documentation promises, and have a lot higher CPU, memory and typing |
|
|
204 | overhead. |
|
|
205 | |
|
|
206 | L<Hook::Scope>, which has apparently never been finished and can corrupt |
|
|
207 | memory when used. |
|
|
208 | |
|
|
209 | L<Scope::Guard> seems to have a big SEE ALSO section for even more |
|
|
210 | modules like it. |
|
|
211 | |
187 | =cut |
212 | =cut |
188 | |
213 | |