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=head1 NAME |
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Guard - safe cleanup blocks |
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=head1 SYNOPSIS |
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|
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use Guard; |
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|
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=head1 DESCRIPTION |
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|
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This module implements so-called "guards". A guard is something (usually |
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an object) that "guards" a resource, ensuring that it is cleaned up when |
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expected. |
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Specifically, this module supports two different types of guards: guard |
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objects, which execute a given code block when destroyed, and scoped |
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guards, which are tied to the scope exit. |
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|
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=head1 FUNCTIONS |
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|
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This module currently exports the C<scope_guard> and C<guard> functions by |
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default. |
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|
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=over 4 |
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|
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=cut |
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|
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package Guard; |
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|
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BEGIN { |
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$VERSION = '0.01'; |
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@ISA = qw(Exporter); |
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@EXPORT = qw(guard scope_guard cancel); |
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|
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require Exporter; |
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|
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require XSLoader; |
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XSLoader::load Guard, $VERSION; |
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} |
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our $DIED = sub { warn "$@" }; |
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|
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=item scope_guard BLOCK |
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Registers a block that is executed when the current scope (block, |
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function, method, eval etc.) is exited. |
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The description below sounds a bit complicated, but that's just because |
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C<scope_guard> tries to get even corner cases "right": the goal is to |
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provide you with a rock solid clean up tool. |
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|
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This is similar to this code fragment: |
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|
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eval ... code following scope_guard ... |
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{ |
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local $@; |
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eval BLOCK; |
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eval { $Guard::DIED->() } if $@; |
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} |
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die if $@; |
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|
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Except it is much faster, and the whole thing gets executed even when the |
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BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means. |
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See B<EXCEPTIONS>, below, for an explanation of exception handling |
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(C<die>) within guard blocks. |
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Example: Temporarily change the directory to F</etc> and make sure it's |
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set back to F</> when the function returns: |
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|
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sub dosomething { |
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scope_guard { chdir "/" }; |
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chdir "/etc"; |
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|
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... |
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} |
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|
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=item my $guard = guard BLOCK |
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Behaves the same as C<scope_guard>, except that instead of executing |
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the block on scope exit, it returns an object whose lifetime determines |
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when the BLOCK gets executed: when the last reference to the object gets |
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destroyed, the BLOCK gets executed as with C<scope_guard>. |
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|
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The returned object can be copied as many times as you want. |
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See B<EXCEPTIONS>, below, for an explanation of exception handling |
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(C<die>) within guard blocks. |
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Example: acquire a Coro::Semaphore for a second by registering a |
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timer. The timer callback references the guard used to unlock it again. |
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use AnyEvent; |
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use Coro::Semaphore; |
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my $sem = new Coro::Semaphore; |
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sub lock_1s { |
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$sem->down; |
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my $guard = guard { $sem->up }; |
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|
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my $timer; |
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$timer = AnyEvent->timer (after => 1, sub { |
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# do something |
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undef $sem; |
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undef $timer; |
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}); |
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} |
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The advantage of doing this with a guard instead of simply calling C<< |
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$sem->down >> in the callback is that you can opt not to create the timer, |
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or your code can throw an exception before it can create the timer, or you |
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can create multiple timers or other event watchers and only when the last |
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one gets executed will the lock be unlocked. |
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=item Guard::cancel $guard |
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Calling this function will "disable" the guard object returned by the |
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C<guard> function, i.e. it will free the BLOCK originally passed to |
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C<guard >and will arrange for the BLOCK not to be executed. |
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This can be useful when you use C<guard> to create a fatal cleanup handler |
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and later decide it is no longer needed. |
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=cut |
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1; |
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=back |
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=head1 EXCEPTIONS |
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Guard blocks should not normally throw exceptions (e.g. C<die>), after |
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all, they are usually used to clean up after such exceptions. However, if |
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something truly exceptional is happening, a guard block should be allowed |
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to die. Also, programming errors are a large source of exceptions, and the |
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programmer certainly wants to know about those. |
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Since in most cases, the block executing when the guard gets executes does |
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not know or does not care about the guard blocks, it makes little sense to |
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let containing code handle the exception. |
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Therefore, whenever a guard block throws an exception, it will be caught, |
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and this module will call the code reference stored in C<$Guard::DIED> |
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(with C<$@> set to the actual exception), which is similar to how most |
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event loops handle this case. |
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The code reference stored in C<$Guard::DIED> should not die (behaviour is |
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not guaranteed, but right now, the exception will simply be ignored). |
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The default for C<$Guard::DIED> is to call C<warn "$@">. |
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=head1 AUTHOR |
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|
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Marc Lehmann <schmorp@schmorp.de> |
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http://home.schmorp.de/ |
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=head1 THANKS |
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To Marco Maisenhelder, who reminded me of the C<$Guard::DIED> solution to |
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the problem of exceptions. |
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|
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=cut |
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