Lighttpd

Setup:¶

OS: Debian 6.0.4
Lighttpd: lighttpd/1.4.28 (ssl) Build-Date: Mar 15 2013 20:08:27
Hardware: VPS OpenVZ, w/ abundant free RAM (1GB +), free disk space (90GB), and low CPU load (in light of eventual race conditions)
lighttpd.conf: file attached; also at http://pastebin.mozilla.org/4701923
User agent: Iceweasel 8.0, Debian

Description of problem:¶

I get intermittent HTTP 500 responses when requesting a CGI (no FastCGI) script.
About 80% of the requests return HTTP 200, the rest HTTP 500, for no particular reason.
This is a shell script, named cloudGenerator.js.sh, with ending .sh extension. As per the attached lighttpd.conf, lighttpd is instructed to handle .sh files via the default system shell - in this case Bash.

The script, cloudGenerator.js.sh, is a "self-extracting" javascript file: at line 1, its sha-bang points to the extractor program; by line 2, it becomes regular JavaScript:

#!/usr/local/bin/jsMinCacher.sh
var genericJsFunction = function() { return []; };

The extractor program, /usr/local/bin/jsMinCacher.sh, is a small (20 lines) regular Bash script (#!/bin/bash) which outputs to stdout a shrunk version of the original file. Its execution is consistent in that it's not influenced by volatile runtime factors, like environment variables or date; it always executed the same algorithms and produces the same output:

#!/bin/bash

source=${1##*/}
cache_dir=/tmp/jsMinCacher/
compressor=/usr/bin/yui-compressor

To summarize, executing "bash cloudGenerator.js.sh" launches "bash /usr/local/bin/jsMinCacher.sh", which in turn produces the stdout to feed thru CGI.

Running "bash cloudGenerator.js.sh" directly via command line never fails. This is tested with 2000 iterations and with PWD environment variable set to the path of cloudGenerator.js.sh.
Instead, when "bash cloudGenerator.js.sh" is called by CGI, 20% of the times I get HTTP 500. When this happens, the script still doesn't encounter any error by its own, and correctly completes its execution from start to end, producing the expected stdout, with stderr empty, and returning the exit status 0.

Apparently, I'm getting HTTP 500 in vain.
How can I debug this further?