Subject: [PATCH 1/2] diff: do not display hunk context under -W

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Mon, 15 Feb 2021 16:50:19 +0100

Fix what I believe to be a long-standing bug in how "-W" interacts
with displaying the hunk context on the @@ line: It should not be
displayed at all under -W.

The long-standing semantics of how -W works and interacts with -U<n>
are rather easy to reason about:

 * -W extends the context line up to the start of the function. With
    userdiff this means the language-aware regex rules in userdiff.c,
    or user-supplied rules.

 * -U<n>, which defaults to -U3 shows at least <n> lines of context,
    if that's greater than what we'd extend the context to under -W
    then -U<n> wins.

 * When showing the hunk context we look up from the first line we
   show of the diff, and find whatever looks like useful context above
   that line.

Thus in e.g. the xdiff/xemit.c change being made in this commit we'll
correctly show "xdl_emit_diff()" in the hunk context under default
diff settings.

But if we viewed it with the -W option we'd show "is_empty_rec()",
because we'd first find the "xdl_emit_diff()" context line, extend the
diff to that, and then would go look for context to show again.

I don't think this behavior makes any sense, our context in this case
is what we're guaranteed to show as part of the diff itself.

The user already asked us to find that context line and show it, we
don't need to then start showing the context above that line, which
they didn't ask for.

This new behavior does give us the edge case that if we e.g. view the
diff here with "-U150 -W" we'd previously extend the context to the
middle of the "is_func_rec()" function, and show that function in the
hunk context. Now we'll show nothing.

I think that change also makes sense. We're showing a change in the
"xdl_emit_diff()" function. That's our context for the change. It
doesn't make sense with -W to start fishing around for other
context.

Arguably in that case we could save away the context we found in the
"XDL_EMIT_FUNCCONTEXT" in "xdl_emit_diff()" and show that if we end up
extending the diff past the function, either because of a high -U<n>
value, or because our change was right at the start.

I wouldn't really mind if we did that, perhaps it would be a useful
marker with high -U<n> values to remind the user of what they're
looking at, but I also don't see the usefulness in practice, so let's
punt that for now.

Signed-off-by: Ævar Arnfjörð Bjarmason <[email-masked]>
---
 Documentation/diff-options.txt | 4 ++++
 t/t4015-diff-whitespace.sh     | 2 +-
 t/t4018-diff-funcname.sh       | 7 +++++++
 xdiff/xemit.c                  | 4 +++-
 4 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
index e5733ccb2d..8ca59effa7 100644
--- a/Documentation/diff-options.txt
+++ b/Documentation/diff-options.txt
@@ -759,6 +759,10 @@ endif::git-format-patch[]
 	The function names are determined in the same way as
 	`git diff` works out patch hunk headers (see 'Defining a
 	custom hunk-header' in linkgit:gitattributes[5]).
++
+When showing the whole function for context the "@@" context line
+itself will always be empty, since the context that would otherwise be
+shown there will be the first line of the hunk being shown.
 
 ifndef::git-format-patch[]
 ifndef::git-log[]
diff --git a/t/t4015-diff-whitespace.sh b/t/t4015-diff-whitespace.sh
index 8c574221b2..0ffc845cdd 100755
--- a/t/t4015-diff-whitespace.sh
+++ b/t/t4015-diff-whitespace.sh
@@ -2133,7 +2133,7 @@ test_expect_success 'combine --ignore-blank-lines with --function-context 2' '
 		--ignore-blank-lines --function-context a b >actual.raw &&
 	sed -n "/@@/,\$p" <actual.raw >actual &&
 	cat <<-\EOF >expect &&
-	@@ -5,11 +6,9 @@ c
+	@@ -5,11 +6,9 @@
 	 function
 	 1
 	 2
diff --git a/t/t4018-diff-funcname.sh b/t/t4018-diff-funcname.sh
index 80f35c5e16..f3374abd98 100755
--- a/t/t4018-diff-funcname.sh
+++ b/t/t4018-diff-funcname.sh
@@ -91,6 +91,13 @@ test_diff_funcname () {
 		fi
 	' &&
 
+	test_expect_success "$desc -W" '
+		git diff -U0 -W "$what" >W-U0-diff &&
+		echo >W-U0-expected &&
+		last_diff_context_line W-U0-diff >W-U0-actual &&
+		test_cmp W-U0-expected W-U0-actual
+	' &&
+
 	test_expect_success "$desc (accumulated)" '
 		git diff -U1 "$what".acc >diff &&
 		last_diff_context_line diff >actual.lines &&
diff --git a/xdiff/xemit.c b/xdiff/xemit.c
index 9d7d6c5087..02b5dbcc70 100644
--- a/xdiff/xemit.c
+++ b/xdiff/xemit.c
@@ -274,7 +274,9 @@ int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
 		 */
 
 		if (xecfg->flags & XDL_EMIT_FUNCNAMES) {
-			get_func_line(xe, xecfg, &func_line,
+			get_func_line(xe, xecfg,
+				      xecfg->flags & XDL_EMIT_FUNCCONTEXT
+				      ? NULL : &func_line,
 				      s1 - 1, funclineprev);
 			funclineprev = s1 - 1;
 		}
-- 
2.30.0.284.gd98b1dd5eaa7