Re: [PATCH] Fix overwriting of files when applying contextually independent diffs

Alex Riesen [off-list ref] writes:

Noticed by applying two diffs of different contexts to the same file.

The check for existence of a file was wrong: the test assumed it was
a directory and reset the errno (twice: directly and by calling
lstat). So if an entry existed and was _not_ a directory no attempt
was made to rename into it, because the errno (expected by renaming
code) was already reset to 0. This resulted in error:

    fatal: unable to write file file mode 100644

For Linux, removing "errno = 0" is enough, as lstat wont modify errno
if it was successful. The behavior should not be depended upon,
though, so modify the "if" as well.

The test simulates this situation.

Ok.  I briefly thought this might disable a more important
safety to refuse overwriting an untracked working tree file with
a creation patch, but that is caught much earlier in an
independent codepath already, so this should be safe to apply.

Thanks.