Re: [Bug?] log -p -W showing the whole file for a patch that adds to the end?

From: Junio C Hamano <gitster@pobox.com>
Date: May 10, 2016, 22:51 UTC
Message-ID: <xmqq4ma5msrd.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <xmqqh9e5mvjs.fsf@gitster.mtv.corp.google.com>

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

Show 12 quoted lines

> Just noticed a curiosity:
>
>     $ git show -W 3e3ceaa58 quote.c
>
> shows the entire file.  The commit in question adds a whole new
> function at the end of the file.  If I move that addition to just
> before the last function the file already had before the change and
> amend the commit, "show -W" would work as expected, i.e. addition of
> the function, with three lines before and three lines after context.
>
> The -W feature was added by 14937c2c (diff: add option to show whole
> functions as context, 2011-10-09).

A workaround (atEnd) seems to give me a slightly better result, but I am not sure if this breaks other corner cases.

The real problem is that get_func_line() is called with (start, limit) but a hunk that adds to the end does not even enter the loop that goes back (i.e. step = -1) to find the funcline:

	for (l = start; l != limit && 0 <= l && l < xe->xdf1.nrec; l += step) {
		const char *rec;
		long reclen = xdl_get_rec(&xe->xdf1, l, &rec);
		long len = ff(rec, reclen, buf, size, xecfg->find_func_priv);

because start == xe->xdf1.nrec in that case. We could instead force it to enter the loop by decrementing start when it is xe->xdf1.nrec and we are going down, but that would show two functions in the resulting hunk (one that is the existing function at the end of the file, the other that is added by the patch), which is not better than showing the patch unmodified.

-- >8 --
Subject: diff -W: do not show the whole file when punting

The -W option to "diff" family of commands allows the pre- and post- context of hunks extended to cover the previous and the next "function header line", so that the whole function that is patched can be seen in the hunk.

The helper function get_func_line() however gets confused when a hunk adds a new function at the very end, and returns -1 to signal that it did not find a suitable "function header line", i.e. the beginning of previous function. The caller then takes this signal and shows from the very beginning of the file. We end up showing the entire file, starting from the very beginning to the end of the newly added lines.

We can avoid this by using the original hunk boundary when the helper gives up.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 xdiff/xemit.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xdiff/xemit.c b/xdiff/xemit.c
index 993724b..4e58482 100644
--- a/xdiff/xemit.c
+++ b/xdiff/xemit.c
@@ -166,7 +166,7 @@ int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
 		if (xecfg->flags & XDL_EMIT_FUNCCONTEXT) {
 			long fs1 = get_func_line(xe, xecfg, NULL, xch->i1, -1);
 			if (fs1 < 0)
-				fs1 = 0;
+				fs1 = xch->i1;
 			if (fs1 < s1) {
 				s2 -= s1 - fs1;
 				s1 = fs1;