Re: [PATCH] diff: fix lstat() error handling in diff_populate_filespec()

From: Junio C Hamano <gitster@pobox.com>
Date: Oct 27, 2017, 16:58 UTC
Message-ID: <xmqqy3nwbk9x.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <xmqq3764czv9.fsf@gitster.mtv.corp.google.com>

Junio C Hamano <gitster@pobox.com> writes:

Show 15 quoted lines

> Andrey Okoshkin <a.okoshkin@samsung.com> writes:
>
>> I'm not sure why only ENOENT error of lstat() is considered as an
>> error but passing by other errno values leads to reading of
>> uninitialized 'struct stat st' variable.  It means that the
>> populated 'diff_filespec' structure may be incorrectly filled.
>
> Entirely correct.  There is no fundamental reason to try special
> casing ENOENT, unless we are clearing the "this is an error" bit
> when the errno is ENOENT---but this code does not even do so.  All
> errors are errors---we wanted to know the result of lstat() to carry
> on, and we couldn't figure out the status.  We do not want to die
> immediately (instead we want to show diffs for other paths), so
> substituting the result with an empty string is the least bad thing
> we can do at this point in the code.

By the way, if the reason why we are hitting this lstat(2) (not the reason why lstat(2) is failing) is not because !s->oid.valid (i.e. we are reading the working tree side because that is what is on one side of the comparison) but because reuse_worktree_file() told us to (i.e. we actually are trying to fill the filespec with the blob contents, but found that we have already the same content in a working tree file and found it more efficient to read from there, rather than doing the read_sha1_file() on the s->oid.hash), it probably is better to fall back to the other side of the if/else when we see an error from this lstat(2) and pretend as if we got false from reuse_worktree_file().

That would allow us continue with the correct information we originally wanted to use; after all, reusing is merely an optimization.