Re: [PATCH] git gui: show diffs with a minimum of 1 context line

Clemens Buchacher [off-list ref] writes:

git apply does not handle diffs without context correctly.

NAK on this part of the proposed commit log message.

"git-apply" is more anal than other "patch" implementations in that it
tries to make sure that a hunk that touches the trailing end actually
applies to the trailing end of the file.  If a patch is generated with
non-zero context, we can detect by presense of trailing context lines that
a patch is _not_ about modifying the trailing end, but with a -U0 patch,
every hunk will come without trailing context, so you need to disable that
safety by asking for --unidiff-zero option.

... Configuring git
gui to show zero context lines therefore breaks staging.

So another option might be to pass --unidiff-zero iff/when it is feeding
such a patch to fix this particular "user error" of "git-apply" program.

Having said that,

In reply to this patch I will send a first attempt at fixing this problem
instead of avoiding it.

I suspect there are some things "git-apply" should be able to _figure out_
that the user is giving it a -U0 patch and automatically flip unidiff_zero
option on.  For example, if the _first_ hunk of a patch does not begin
with "@@ -0,0 +N,M @@" nor with "@@ -1,L +N,M @@" (i.e. the hunk claims to
apply not at the beginning) and the hunk does not have leading context
lines, _and_ if that first hunk does not have trailing context lines, then
it is clearly a -U0 patch (or it could be a corrupt patch, but let's
discount that possibility for now).

Even if the hunk does claim to apply at the beginning, in which case we
cannot determine if it is a -U0 patch by looking at the lack of leading
context, if it has any context lines, we can tell it is _not_ a -U0 patch.
When the first hunk that applies to the beginning lacks any context, we
cannot really tell if it is -U0 or not (the other possibility is a total
rewrite of the file from the beginning to the end).  Even in that case,
you could look at the next hunk --- if you have a hunk that applies to
the same path after looking at such a "first" hunk without context, then
it clearly is a -U0 patch.