The Vegan Militia

... because we are all made out of meat!

Catch Me Now, I'm Trying


I first learned to program when the primary mechanism of error handling was to check the return value of each function call and react appropriately. This often led to awful code where functions are called with no checking, and things mysteriously fail several steps removed from the actual error. Therefore I learned to add in lots of error handling so that any error messages would be very specific and easy to fix, to the point that I was once told I had too much error handling. Theoretically more modern programming languages with their try/catch syntax would help, but I am not convinced of that. syntactic sugar is no replacement for programmer discipline. Here’s a case in point, what’s the difference between this:

open(F, "$confdir/config")
$conf = parseconfig(<F>);
dbconnect($conf->{db}{uri}) or die;

and this:

try {
    open(F, "$confdir/config")
    $conf = parseconfig(<F>);
catch {

I can think of at least 6 ways this could fail, all yielding the exact same error, in either version. Don’t tell me you’ve never seen code like either one of those!

But, moving on from hypotheticals, here’s a simplified version of my code:

use Search::Elasticsearch;

my $e = Search::Elasticsearch->new( nodes => [$esurl]);
die "Error: unable to connect to $esurl\n" unless $e->ping;
print "connected to $esurl\n";

Running it gets this error:

[NoNodes] ** No nodes are available: [], called from sub Search::Elasticsearch::Role::Client::Direct::__ANON__ at line 31.

Hmmm… neither my die or print generated any output; it is crashing the entire program while inside ping(). So this is my fault, I didn’t realize this module was written assuming try/catch blocks would be used, but doing that doesn’t really change much. The error message is the same; I still don’t know why it is failing. The server at the url works perfectly fine. So I dig in with the Perl debugger, and finally narrow down the crash to this line in HTTP::Tiny:

_croak(qq{$type URL must be in format http[s]://[auth@]<host>:<port>/\n});

That looks like a potentially helpful error message! It’s too bad it is getting lost somewhere along the way. Going up the call stack I come to this:

return HTTP::Tiny->new( %args, %{ $self->handle_args } );

No error handling, no try/catch, they just blindly create the HTTP::Tiny object and hope it all works. There are a number of try/catch blocks further up the stack, but the error message gets lost along the way, and by the time my catch happens, it is gone. I did discover that enabling detailed logging via “use Log::Any::Adapter qw(Stderr);” did yield that error message:

http_proxy URL must be in format http[s]://[auth@]<host>:<port>/
 at /usr/local/share/perl5/Search/Elasticsearch/Cxn/ line 86.

Proxy?! There shouldn’t be a proxy set! After some searching I finally find that someone kludged /etc/environment with the proxy. Obviously that was a bad idea. I never could figure out what was wrong with the proxy url, but since it should not have happened, and I had wasted at least an hour on this, I stopped digging.

So what went wrong here? First the error message says that the proxy url is invalid, but it doesn’t show me what that value was. Secondly, that real error message got lost somewhere along the way. This sort of sloppiness is what leads to many of the awful error messages I have documented here previously (like this), and trying to catch the exceptions doesn’t fix sloppy code.

Tags: error programming