Re: [PATCH 2/8] refs/files-backend: stop setting errno from lock_ref_oid_basic

From: Han-Wen Nienhuys <redacted>

Errno is a global variable written by almost all system calls, and therefore it
is hard to reason about its state. It's also useless for user-visible errors, as
it leaves no place to report the offending file and/or syscall.

I don't think this paragraph is useful.

For the copy/rename support, calls to lock_ref_oid_basic() in this file are
followed by:

* lock_ref_oid_basic (copy/rename rollback error path)

* write_ref_to_lockfile (both in the rollback path and the success path of
  copy/rename)

These calls do not inspect the incoming errno. As they perform I/O, they can
clobber errno. For this reason, callers cannot reliably observe the errno that
lock_ref_oid_basic() generated, so it is unsound for programmatic use.

For files_create_symref() and files_reflog_expire(), grepping over callers
showed no callers inspecting errno.

This is probably written more clearly as follows:

 No call to the static function lock_ref_oid_basic() is immediately
 followed by an errno check, so stopping setting errno is safe. But as a
 sanity check, lock_ref_oid_basic() is used in these functions:
 - files_copy_or_rename_ref() - here, calls are followed by error() (which
   performs I/O) or write_ref_to_lockfile() (which calls parse_object() which
   may perform I/O)
 - files_create_symref() - here, calls are followed by error() or
   create_symref_locked() (which performs I/O and does not inspect
   errno)
 - files_reflog_expire() - here, calls are followed by error() or
   refs_reflog_exists() (which calls a function in a vtable that is not
   documented to use and/or preserve errno)

 So it is safe to stop setting errno in lock_ref_oid_basic().

The diff itself looks good.