=head1 NAME Guard - safe cleanup blocks =head1 SYNOPSIS use Guard; # temporarily chdir to "/etc" directory, but make sure # to go back to "/" no matter how myfun exits: sub dosomething { scope_guard { chdir "/" }; chdir "/etc"; call_function_that_might_die_or_other_fun_stuff; } =head1 DESCRIPTION This module implements so-called "guards". A guard is something (usually an object) that "guards" a resource, ensuring that it is cleaned up when expected. Specifically, this module supports two different types of guards: guard objects, which execute a given code block when destroyed, and scoped guards, which are tied to the scope exit. =head1 FUNCTIONS This module currently exports the C and C functions by default. =over 4 =cut package Guard; BEGIN { $VERSION = '0.1'; @ISA = qw(Exporter); @EXPORT = qw(guard scope_guard); require Exporter; require XSLoader; XSLoader::load Guard, $VERSION; } our $DIED = sub { warn "$@" }; =item scope_guard BLOCK Registers a block that is executed when the current scope (block, function, method, eval etc.) is exited. The description below sounds a bit complicated, but that's just because C tries to get even corner cases "right": the goal is to provide you with a rock solid clean up tool. This is similar to this code fragment: eval ... code following scope_guard ... { local $@; eval BLOCK; eval { $Guard::DIED->() } if $@; } die if $@; Except it is much faster, and the whole thing gets executed even when the BLOCK calls C, C, C or escapes via other means. If multiple BLOCKs are registered to the same scope, they will be executed in reverse order. Stuff like C is managed via the same mechanism, so variables Cised after calling C will be restored when the guard runs. See B, below, for an explanation of exception handling (C) within guard blocks. Example: temporarily change the timezone for the current process, ensuring it will be reset when the C scope is exited: use Guard; use POSIX (); if ($need_to_switch_tz) { # make sure we call tzset after $ENV{TZ} has been restored scope_guard { POSIX::tzset }; # localise after the scope_guard, so it gets undone in time local $ENV{TZ} = "Europe/London"; POSIX::tzset; # do something with the new timezone } =item my $guard = guard BLOCK Behaves the same as C, except that instead of executing the block on scope exit, it returns an object whose lifetime determines when the BLOCK gets executed: when the last reference to the object gets destroyed, the BLOCK gets executed as with C. The returned object can be copied as many times as you want. See B, below, for an explanation of exception handling (C) within guard blocks. Example: acquire a Coro::Semaphore for a second by registering a timer. The timer callback references the guard used to unlock it again. use AnyEvent; use Coro::Semaphore; my $sem = new Coro::Semaphore; sub lock_1s { $sem->down; my $guard = guard { $sem->up }; my $timer; $timer = AnyEvent->timer (after => 1, sub { # do something undef $sem; undef $timer; }); } The advantage of doing this with a guard instead of simply calling C<< $sem->down >> in the callback is that you can opt not to create the timer, or your code can throw an exception before it can create the timer, or you can create multiple timers or other event watchers and only when the last one gets executed will the lock be unlocked. =item Guard::cancel $guard Calling this function will "disable" the guard object returned by the C function, i.e. it will free the BLOCK originally passed to Cand will arrange for the BLOCK not to be executed. This can be useful when you use C to create a fatal cleanup handler and later decide it is no longer needed. =cut 1; =back =head1 EXCEPTIONS Guard blocks should not normally throw exceptions (e.g. C), after all, they are usually used to clean up after such exceptions. However, if something truly exceptional is happening, a guard block should be allowed to die. Also, programming errors are a large source of exceptions, and the programmer certainly wants to know about those. Since in most cases, the block executing when the guard gets executes does not know or does not care about the guard blocks, it makes little sense to let containing code handle the exception. Therefore, whenever a guard block throws an exception, it will be caught, and this module will call the code reference stored in C<$Guard::DIED> (with C<$@> set to the actual exception), which is similar to how most event loops handle this case. The code reference stored in C<$Guard::DIED> should not die (behaviour is not guaranteed, but right now, the exception will simply be ignored). The default for C<$Guard::DIED> is to call C. =head1 AUTHOR Marc Lehmann http://home.schmorp.de/ =head1 THANKS To Marco Maisenhelder, who reminded me of the C<$Guard::DIED> solution to the problem of exceptions. =cut