[RFC] Thread safety in some low-level functions

[Matheus Tavares Bernardino <matheus.bernardino@usp.br>]

Hi,

I'm working with threads in unpack-trees and noticed that
[warning|error]_errno() uses strerror(), which is not thread-safe. We
could try to avoid calling these functions in threaded code, but they
are sometimes too deep in the call stack to be noticed... (or even
avoided). The same happens with oid_to_hex(), which writes to a static
buffer.

I don't think I've ever seen a bug report involving these functions
being called racily, but this possibility is not currently excluded in
our codebase. For example, see grep_source_load_file(), which is
called by multiple threads concurrently and might call the
thread-unsafe error_errno(). (Although, I admit, the chance of a race
here must be very low...)

I still haven't been able to come up with a simple / easy change that
could make these functions thread safe, but here are my thoughts so
far:

- For strerror(), there is a thread-safe variant: strerror_r().
However IIUC, this variant is not present on Windows (although there
is strerror_s() which *seems* to be somewhat similar). Also, there are
two versions of strerror_r() on Linux: one is XSI-compliant and the
other is GNU-specific. I don't know what the situation is in other
OSes...

- Regarding, oid_to_hex(), a patch from 2010 [1] proposed a solution
using thread-local storage and pthread_once(). But as Hannes pointed
out in this other thread [2] , implementing a Windows equivalence for
pthread_once() could be tricky and voluminous. Since this thread dates
from 7 years ago, I was wondering if we would be able to implement it
nowadays with InitOnceExecuteOnce() [3].

Finally, leaving these functions thread-unsafe is also a
possibility... As I mentioned earlier, they don't seem to be causing
problems out there for now (at least not reported). But if we can find
a feasible solution to introduce thread-safety, I think it would be
great. The codebase would be more robust and we would be able to work
with threads with much more confidence.

Any thoughts?

Thanks,
Matheus

[1] https://lore.kernel.org/git/20100323173130.GC4218@fredrik-laptop/
[2] https://lore.kernel.org/git/516D5CA4.7000500@viscovery.net/
[3]: https://docs.microsoft.com/en-us/windows/win32/sync/using-one-time-initialization