Apache ignoring some C++-generated SSI output?

Question

Executive summary: Apache silently fails an SSI include, where the include'd HTML is generated by a C++ program which is linked with SQLite and/or pthreads. The SSI include works when the C++ doesn't use SQLite.

Longer version: I have two very similar C++ programs which produce the output required for an SSI virtual include. Both produce a list of about 100 options for an HTML select. One of the includes works as expected, but the other one is doing nothing.

1. Both programs (when run stand-alone) produce exactly the same output on stdout, and nothing on stderr. So, in principle, they should behave identically, as far as Apache is concerned, but they don't.
2. The program which works (version A) is straightforward stand-alone C++ code, which uses the STL to produce a string, which is written to cout; no problem.
3. The program which fails (version B) is slightly more complicated. It's a mix of C and C++, and includes an embedded SQLite database, talking to it through the C API. SQLite also uses pthreads.
4. Version B fails both when it is linked to the static, and the dynamic, SQLite code. Both the static and dynamic versions depend on the pthreads shared library.
5. Apache produces nothing in the error log when it runs version B, but it also substitutes nothing into the HTML select. I can easily get Apache to produce an error message (by specifying a wrong executable name in the HTML #include, for example) so I'm pretty sure that Apache does actually at least attempt to execute the program.
6. Both A and B are valgrind-clean.
7. Apache 2.2.16, Ubuntu 10.10, prefork MPM
8. I can't get any output at all out of ScriptLog, so I may have set it up incorrectly. I've set LogLevel to debug, but it still doesn't tell me anything about script execution.

Any ideas? Thanks.

Answer 1

Well, that was irritating. The database was a symlink in the cgi-bin directory, which pointed somewhere else. The apache user didn't have search permission on the last directory in the pointed-to location. If I su'ed to the Apache user, and then ran the CGI program manually, it returned a 400 error message. However, this didn't get to the Apache error.log, even with LogLevel set to debug, and I can't get any ScriptLog output, and firebug doesn't show anything in the net tab. Everything worked when I changed the directory permission.