This is implemented in Python as `gdb.find_source_hook`. If that property exists
and is callable it will be called with the path to the missing source file,
and if the callable returns a string that will be used as the new path
to the source file.
This can be used to fetch missing source files from a VCS or over HTTP,
for example.
---
Thanks to Tom Tromey for pointing me to where to start with this. I'm interested
in using this to provide on-demand source fetching from Mozilla's public
Mercurial server over HTTP for debugging Nightly or Release builds, as well
as post-mortem debugging of crashes from our user population. Microsoft's
debuggers have this functionality built in and it's really useful for those
scenarios.