I have an occasion to revisit this:
http://sourceware.org/ml/gdb-patches/2007-11/msg00314.html

Or in a different form:
http://sourceware.org/ml/gdb-patches/2008-01/msg00103.html

When we last left our story, watching for a single path mismatch of
the main source file only seemed ok.  That's what this patch does.
It's just a minor revision of
http://sourceware.org/ml/gdb-patches/2008-01/msg00103.html