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Section 13.14.  Unpacking Exceptions

 
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13.14. Unpacking Exceptions

Unpack the exception variable in extended exception handlers.

If an exception handler becomes long or complex, you may need to refactor parts of it. For example, consider the X::EOF handler inside TRy_next_line( ) from the "OO Exceptions" guideline:


    sub try_next_line {
        
# Give get_next_line(  ) two chances...
for my $already_retried (0..1) {
# Return immediately on success, but catch any failure...
eval { return get_next_line( ) };
# Rethrow the caught exception if it isn't an EOF problem...
croak $EVAL_ERROR if !X::EOF->caught( );
# Also rethrow the caught exception
            # if we've already tried rewinding the filehandle...
croak $EVAL_ERROR if $already_retried;
# Otherwise, try rewinding the filehandle...
seek $EVAL_ERROR->handle( ), 0, 0; } }

This code would seem to be cleaner and easier to extend if the separate rethrows were refactored like this:

    sub try_next_line {
        # Give get_next_line(  ) two chances...
        for my $already_retried (0..1) {

            # Return immediately on success, but catch any failure...
            eval {
                return get_next_line(  )
            };

            # If we can handle this exception...
            if (X::EOF->caught(  ) ) {
                # Fail on irremedially bad cases...
                fail_if_incorrigible($EVAL_ERROR, $already_retried);

                # Otherwise, try rewinding the filehandle...
                seek $EVAL_ERROR->handle(  ), 0, 0;
            }
            # Otherwise, let some caller handle it...
            else {
                $EVAL_ERROR->rethrow(  );
            }
        }
    }

Refactoring in this way is usually a highly recommended practice, but in this case it has the potential to introduce a subtle bug. The problem is that the $EVAL_ERROR exception variable (a.k.a. $@) is a global variable. So if fail_if_incorrigible( ) happens to throwand then internally catchsome other exception during its execution, that nested exception will overwrite $EVAL_ERROR.

So, after the call to fail_if_incorrigible( ), the exception variable might still have the original X::EOF exception, or it might contain some other totally unrelated exception, left over from the internal machinations of fail_if_incorrigible( ). That uncertainty makes the seek statement after the call to fail_if_incorrigible( ) problematic. It tries to seek the handle that was sent back inside the original exception, but there's now no guarantee that $EVAL_ERROR still contains that exception.

Fortunately, it's comparatively simple to solve this problem: just copy the exception into a lexical variable before attempting to handle it:


    if (X::EOF->caught(  ) ) {
        my $exception = $EVAL_ERROR;

        
# Fail on irremedially bad cases...
fail_if_incorrigible($exception, $already_retried);
# Otherwise, try rewinding the filehandle...
seek $exception->handle( ), 0, 0; }

Using the Exception::Class module makes this practice even easier to follow, because its version of the caught( ) method always returns a copy of $EVAL_ERROR on success. So you can just write:


    if (my $exception = X::EOF->caught(  ) ) {
        
# Fail on irremedially bad cases...
fail_if_incorrigible($exception, $already_retried);
# Otherwise, try rewinding the filehandle...
seek $exception->fh( ), 0, 0; }

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