APR::Error(3)
NAME
APR::Error - Perl API for APR/Apache/mod_perl exceptions
Synopsis
eval { $obj->mp_method() };
if ($@ && $ref $@ eq 'APR::Error' && $@ == $some_code) {
# handle the exception
}
else {
die $@; # rethrow it
}
Description
"APR::Error" handles APR/Apache/mod_perl exceptions for you, while
leaving you in control.
Apache and APR API return a status code for almost all methods, so if
you didn't check the return code and handled any possible problems, you
may have silent failures which may cause all kind of obscure problems.
On the other hand checking the status code after each call is just too
much of a kludge and makes quick prototyping/development almost
impossible, not talking about the code readability. Having methods
return status codes, also complicates the API if you need to return
other values.
Therefore to keep things nice and make the API readable we decided to
not return status codes, but instead throw exceptions with "APR::Error"
objects for each method that fails. If you don't catch those
exceptions, everything works transparently - perl will intercept the
exception object and "die()" with a proper error message. So you get
all the errors logged without doing any work.
Now, in certain cases you don't want to just die, but instead the error
needs to be trapped and handled. For example if some IO operation times
out, may be it is OK to trap that and try again. If we were to die with
an error message, you would have had to match the error message, which
is ugly, inefficient and may not work at all if locale error strings
are involved. Therefore you need to be able to get the original status
code that Apache or APR has generated. And the exception objects give
you that if you want to. Moreover the objects contain additional
information, such as the function name (in case you were eval'ing
several commands in one block), file and line number where that
function was invoked from. More attributes could be added in the
future.
"APR::Error" uses Perl operator overloading, such that in boolean and
numerical contexts, the object returns the status code; in the string
context the full error message is returned.
When intercepting exceptions you need to check whether $@ is an object
(reference). If your application uses other exception objects you
additionally need to check whether this is a an "APR::Error" object.
Therefore most of the time this is enough:
eval { $obj->mp_method() };
if ($@ && $ref $@ && $@ == $some_code)
warn "handled exception: $@";
}
But with other, non-mod_perl, exception objects you need to do:
eval { $obj->mp_method() };
if ($@ && $ref $@ eq 'APR::Error' && $@ == $some_code)
warn "handled exception: $@";
}
In theory you could even do:
eval { $obj->mp_method() };
if ($@ && $@ == $some_code)
warn "handled exception: $@";
}
but it's possible that the method will die with a plain string and not
an object, in which case "$@ == $some_code" won't quite work. Remember
that mod_perl throws exception objects only when Apache and APR fail,
and in a few other special cases of its own (like "exit").
warn "handled exception: $@" if $@ && $ref $@;
There are two ways to figure out whether an error fits your case. In
most cases you just compare $@ with an the error constant. For example
if a socket has a timeout set and the data wasn't read within the
timeout limit a "APR::Const::TIMEUP")
use APR::Const -compile => qw(TIMEUP);
$sock->timeout_set(1_000_000); # 1 sec
my $buff;
eval { $sock->recv($buff, BUFF_LEN) };
if ($@ && ref $@ && $@ == APR::Const::TIMEUP) {
}
However there are situations, where on different Operating Systems a
different error code will be returned. In which case to simplify the
code you should use the special subroutines provided by the
"APR::Status" class. One such condition is socket "recv()" timeout,
which on Unix throws the "EAGAIN" error, but on other system it throws
a different error. In this case "APR::Status::is_EAGAIN" should be
used.
Let's look at a complete example. Here is a code that performs a socket
read:
my $rlen = $sock->recv(my $buff, 1024);
warn "read $rlen bytes\n";
and in certain cases it times out. The code will die and log the reason
for the failure, which is fine, but later on you may decide that you
want to have another attempt to read before dying and add some fine
grained sleep time between attempts, which can be achieved with
"select". Which gives us:
use APR::Status ();
# ....
my $tries = 0;
my $buffer;
RETRY: my $rlen = eval { $sock->recv($buffer, SIZE) };
if ($@)
die $@ unless ref $@ && APR::Status::is_EAGAIN($@);
if ($tries++ < 3) {
# sleep 250msec
select undef, undef, undef, 0.25;
goto RETRY;
}
else {
# do something else
}
}
warn "read $rlen bytes\n"
Notice that we handle non-object and non-"APR::Error" exceptions as
well, by simply re-throwing them.
Finally, the class is called "APR::Error" because it needs to be used
outside mod_perl as well, when called from "APR" applications written
in Perl.
API
"cluck"
"cluck" is an equivalent of "Carp::cluck" that works with "APR::Error"
exception objects.
"confess"
"confess" is an equivalent of "Carp::confess" that works with
"APR::Error" exception objects.
"strerror"
Convert APR error code to its string representation.
$error_str = APR::Error::strerror($rc);
ret: $rc ( "APR::Const status constant" )
The numerical value for the return (error) code
ret: $error_str ( string )
The string error message corresponding to the numerical value
inside $rc. (Similar to the C function strerror(3))
since: 2.0.00
Example:
Try to retrieve the bucket brigade, and if the return value doesn't
indicate success or end of file (usually in protocol handlers) die, but
give the user the human-readable version of the error and not just the
code.
my $rc = $c->input_filters->get_brigade($bb_in,
Apache2::Const::MODE_GETLINE);
if ($rc != APR::Const::SUCCESS && $rc != APR::Const::EOF) {
my $error = APR::Error::strerror($rc);
die "get_brigade error: $rc: $error\n";
}
It's probably a good idea not to omit the numerical value in the error
message, in case the error string is generated with non-English locale.
See Also
mod_perl(3) 2.0 documentation.
Copyright
mod_perl 2.0 and its core modules are copyrighted under The Apache
Software License, Version 2.0.
Authors
The mod_perl development team and numerous contributors.
perl v5.10.0 APR::Error(3)
Mac OS X 10.6 - Generated Thu Sep 17 20:10:25 CDT 2009