Re: [PATCH 0/2] Fix a refname trimming problem in `log --bisect`

Michael Haggerty [off-list ref] writes:

The code for `git log --bisect` was calling
`for_each_ref_in_submodule()` with prefix set to "refs/bisect/bad",
which is the actual name of the reference that it wants. This resulted
in the refname being trimmed completely away and the empty string
being passed to the callback. That became impermissible after

    b9c8e7f2fb prefix_ref_iterator: don't trim too much, 2017-05-22

, so the command was failing.

Fix the problem in two orthogonal ways:

1. Add a new function, `for_each_fullref_in_submodule()`, that doesn't
   trim the refnames that it passes to callbacks, and us that instead.
   I *think* that this is a strict improvement, though I don't know
   the `git log` code well enough to be sure that it won't have bad
   side-effects.

2. Relax the "trimming too many characters" check to allow the full
   length of the refname to be trimmed away (though not more than
   that).

In an ideal world the second patch shouldn't be necessary, because
this calling pattern is questionable and it might be better that we
learn about any other offenders. But if we'd rather be conservative
and not break any other code that might rely on the old behavior,
patch 2 is my suggestion for how to do it.

Thanks for a nice summary.  

I agree that 2. is a nice safety to have, especially if the code
before b9c8e7f2 ("prefix_ref_iterator: don't trim too much",
2017-05-22) has been seeing the completely trimmed result (i.e. an
empty string) in the callback function.

And I agree that 1. is also a good interface to have.