refs_resolve_ref_unsafe: handle d/f conflicts for writes

refs_resolve_ref_unsafe: handle d/f conflicts for writes

If our call to refs_read_raw_ref() fails, we check errno to
see if the ref is simply missing, or if we encountered a
more serious error. If it's just missing, then in "write"
mode (i.e., when RESOLVE_REFS_READING is not set), this is
perfectly fine.

However, checking for ENOENT isn't sufficient to catch all
missing-ref cases. In the filesystem backend, we may also
see EISDIR when we try to resolve "a" and "a/b" exists.
Likewise, we may see ENOTDIR if we try to resolve "a/b" and
"a" exists. In both of those cases, we know that our
resolved ref doesn't exist, but we return an error (rather
than reporting the refname and returning a null sha1).

This has been broken for a long time, but nobody really
noticed because the next step after resolving without the
READING flag is usually to lock the ref and write it. But in
both of those cases, the write will fail with the same
errno due to the directory/file conflict.

There are two cases where we can notice this, though:

  1. If we try to write "a" and there's a leftover directory
     already at "a", even though there is no ref "a/b". The
     actual write is smart enough to move the empty "a" out
     of the way.

     This is reasonably rare, if only because the writing
     code has to do an independent resolution before trying
     its write (because the actual update_ref() code handles
     this case fine). The notes-merge code does this, and
     before the fix in the prior commit t3308 erroneously
     expected this case to fail.

  2. When resolving symbolic refs, we typically do not use
     the READING flag because we want to resolve even
     symrefs that point to unborn refs. Even if those unborn
     refs could not actually be written because of d/f
     conflicts with existing refs.

     You can see this by asking "git symbolic-ref" to report
     the target of a symref pointing past a d/f conflict.

We can fix the problem by recognizing the other "missing"
errnos and treating them like ENOENT. This should be safe to
do even for callers who are then going to actually write the
ref, because the actual writing process will fail if the d/f
conflict is a real one (and t1404 checks these cases).

Arguably this should be the responsibility of the
files-backend to normalize all "missing ref" errors into
ENOENT (since something like EISDIR may not be meaningful at
all to a database backend). However other callers of
refs_read_raw_ref() may actually care about the distinction;
putting this into resolve_ref() is the minimal fix for now.

The new tests in t1401 use git-symbolic-ref, which is the
most direct way to check the resolution by itself.
Interestingly we actually had a test that setup this case
already, but we only used it to verify that the funny state
could be overwritten, not that it could be resolved.

We also add a new test in t3200, as "branch -m" was the
original motivation for looking into this. What happens is
this:

  0. HEAD is pointing to branch "a"

  1. The user asks to rename "a" to "a/b".

  2. We create "a/b" and delete "a".

  3. We then try to update any worktree HEADs that point to
     the renamed ref (including the main repo HEAD). To do
     that, we have to resolve each HEAD. But now our HEAD is
     pointing at "a", and we get EISDIR due to the loose
     "a/b". As a result, we think there is no HEAD, and we
     do not update it. It now points to the bogus "a".

Interestingly this case used to work, but only accidentally.
Before 31824d180d (branch: fix branch renaming not updating
HEADs correctly, 2017-08-24), we'd update any HEAD which we
couldn't resolve. That was wrong, but it papered over the
fact that we were incorrectly failing to resolve HEAD.

So while the bug demonstrated by the git-symbolic-ref is
quite old, the regression to "branch -m" is recent.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>