NAME¶
Guard - safe cleanup blocks
SYNOPSIS¶
use Guard;
# temporarily chdir to "/etc" directory, but make sure
# to go back to "/" no matter how myfun exits:
sub myfun {
scope_guard { chdir "/" };
chdir "/etc";
code_that_might_die_or_does_other_fun_stuff;
}
# create an object that, when the last reference to it is gone,
# invokes the given codeblock:
my $guard = guard { print "destroyed!\n" };
undef $guard; # probably destroyed here
DESCRIPTION¶
This module implements so-called "guards". A guard is something
(usually an object) that "guards" a resource, ensuring that it is
cleaned up when expected.
Specifically, this module supports two different types of guards: guard objects,
which execute a given code block when destroyed, and scoped guards, which are
tied to the scope exit.
FUNCTIONS¶
This module currently exports the "scope_guard" and "guard"
functions by default.
- scope_guard BLOCK
- scope_guard ($coderef)
- Registers a block that is executed when the current scope (block,
function, method, eval etc.) is exited.
See the EXCEPTIONS section for an explanation of how exceptions (i.e.
"die") are handled inside guard blocks.
The description below sounds a bit complicated, but that's just because
"scope_guard" tries to get even corner cases "right":
the goal is to provide you with a rock solid clean up tool.
The behaviour is similar to this code fragment:
eval ... code following scope_guard ...
{
local $@;
eval BLOCK;
eval { $Guard::DIED->() } if $@;
}
die if $@;
Except it is much faster, and the whole thing gets executed even when the
BLOCK calls "exit", "goto", "last" or
escapes via other means.
If multiple BLOCKs are registered to the same scope, they will be executed
in reverse order. Other scope-related things such as "local" are
managed via the same mechanism, so variables "local"ised
after calling "scope_guard" will be restored when the
guard runs.
Example: temporarily change the timezone for the current process, ensuring
it will be reset when the "if" scope is exited:
use Guard;
use POSIX ();
if ($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
}
- my $guard = guard BLOCK
- my $guard = guard ($coderef)
- Behaves the same as "scope_guard", 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
"scope_guard".
See the EXCEPTIONS section for an explanation of how exceptions (i.e.
"die") are handled inside 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. (Please
ignore the fact that "Coro::Semaphore" has a "guard"
method that does this already):
use Guard;
use Coro::AnyEvent;
use Coro::Semaphore;
my $sem = new Coro::Semaphore;
sub lock_for_a_second {
$sem->down;
my $guard = guard { $sem->up };
Coro::AnyEvent::sleep 1;
# $sem->up gets executed when returning
}
The advantage of doing this with a guard instead of simply calling
"$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 the thread gets canceled), or you can create multiple timers
or other event watchers and only when the last one gets executed will the
lock be unlocked. Using the "guard", you do not have to worry
about catching all the places where you have to unlock the semaphore.
- $guard->cancel
- Calling this function will "disable" the guard object returned
by the "guard" function, i.e. it will free the BLOCK originally
passed to "guard "and will arrange for the BLOCK not to be
executed.
This can be useful when you use "guard" to create a cleanup
handler to be called under fatal conditions and later decide it is no
longer needed.
EXCEPTIONS¶
Guard blocks should not normally throw exceptions (that is, "die").
After all, they are usually used to clean up after such exceptions. However,
if something truly exceptional is happening, a guard block should of course 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 executed 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 by
Guard, followed by calling the code reference stored in $Guard::DIED (with $@
set to the actual exception), which is similar to how most event loops handle
this case.
The default for $Guard::DIED is to call "warn "$@"", i.e.
the error is printed as a warning and the program continues.
The $@ variable will be restored to its value before the guard call in all
cases, so guards will not disturb $@ in any way.
The code reference stored in $Guard::DIED should not die (behaviour is not
guaranteed, but right now, the exception will simply be ignored).
AUTHOR¶
Marc Lehmann <schmorp@schmorp.de>
http://home.schmorp.de/
THANKS¶
Thanks to Marco Maisenhelder, who reminded me of the $Guard::DIED solution to
the problem of exceptions.
SEE ALSO¶
Scope::Guard and Sub::ScopeFinalizer, which actually implement dynamically
scoped guards only, not the lexically scoped guards that their documentation
promises, and have a lot higher CPU, memory and typing overhead.
Hook::Scope, which has apparently never been finished and can corrupt memory
when used.
Scope::Guard seems to have a big SEE ALSO section for even more modules like
it.