Subject: Re: [RFC/PATCH] add-patch: handle splitting hunks with diff.suppressBlankEmpty

From: Phillip Wood <[email-masked]>
Date: Wed, 10 Jul 2024 14:46:30 +0100

Hi Peff

On 10/07/2024 10:36, Jeff King wrote:
> Subject: add-patch: handle splitting hunks with diff.suppressBlankEmpty

> When "add -p" parses diffs, it looks for context lines starting with a
> single space. But when diff.suppressBlankEmpty is in effect, an empty
> context line will omit the space, giving us a true empty line. This
> confuses the parser, which is unable to split based on such a line.

> It's tempting to say that we should just make sure that we generate a
> diff without that option. But we may parse diffs not only generated by
> Git, but ones that users have manually edited. And POSIX calls the
> decision of whether to print the space here "implementation-defined".

Do we ever parse an edited hunk with this code? I'm not sure there is a
way of splitting a hunk after it has been edited as I don't think we
ever display it again.

> So let's handle both cases: a context line either starts with a space or
> consists of a totally empty line.
> 
> Reported-by: Ilya Tumaykin <[email-masked]>
> Signed-off-by: Jeff King <[email-masked]>
> ---
> I'm a little worried that this creates ambiguities, since I don't think
> we are careful about following the hunk header's line counts. Imagine
> you had an input like this:
> 
>    @@ -1,2 +1,2 @@> 
>    -old
>    +new
>     stuff
> 
>    some garbage
> 
> We obviously know that "some garbage" is not a context line and is just
> trailing junk, because it does not begin with "-", "+" or space. But
> what about the blank line in between? It looks like an empty context
> line, but we can only know that it is not by respecting the counts in
> the hunk header.
> 
> I don't think we'd ever generate this ourselves, but could somebody
> manually edit a hunk into this shape? When I tried it in practice, it
> looks like we fail to apply the result even before my patch, though. I'm> not sure why that is. If I put "some garbage" without the blank line, we
> correctly realize it should be discarded. It's possible I'm just holding
> it wrong.

When we recount the hunk after it has been edited we ignore lines that
don't begin with '+', '-', ' ', or '\n' so if you add some garbage at
the end of the hunk the recounted hunk header excludes it when it gets
applied.

I think your patch looks good. I did wonder if we wanted to fix this
by normalizing context lines instead as shown in the diff below. That
might make it less likely to miss adding "|| '\n'" in future code that
is looking for a context line but I don't have a strong preference
either way.

Best Wishes

Phillip

---- >8 ----
diff --git a/add-patch.c b/add-patch.c
index d8ea05ff108..795aa772b7a 100644
--- a/add-patch.c
+++ b/add-patch.c
@@ -400,6 +400,12 @@ static void complete_file(char marker, struct hunk *hunk)
  		hunk->splittable_into++;
  }
  
+/* Empty context lines may omit the leading ' ' */
+static int normalize_marker(char marker)
+{
+	return marker == '\n' ? ' ' : marker;
+}
+
  static int parse_diff(struct add_p_state *s, const struct pathspec *ps)
  {
  	struct strvec args = STRVEC_INIT;
@@ -485,6 +491,7 @@ static int parse_diff(struct add_p_state *s, const struct pathspec *ps)
  	while (p != pend) {
  		char *eol = memchr(p, '\n', pend - p);
  		const char *deleted = NULL, *mode_change = NULL;
+		char ch = normalize_marker(*p);
  
  		if (!eol)
  			eol = pend;
@@ -532,7 +539,7 @@ static int parse_diff(struct add_p_state *s, const struct pathspec *ps)
  			 * Start counting into how many hunks this one can be
  			 * split
  			 */
-			marker = *p;
+			marker = ch;
  		} else if (hunk == &file_diff->head &&
  			   starts_with(p, "new file")) {
  			file_diff->added = 1;
@@ -586,10 +593,10 @@ static int parse_diff(struct add_p_state *s, const struct pathspec *ps)
  			    (int)(eol - (plain->buf + file_diff->head.start)),
  			    plain->buf + file_diff->head.start);
  
-		if ((marker == '-' || marker == '+') && *p == ' ')
+		if ((marker == '-' || marker == '+') && ch == ' ')
  			hunk->splittable_into++;
-		if (marker && *p != '\\')
-			marker = *p;
+		if (marker && ch != '\\')
+			marker = ch;
  
  		p = eol == pend ? pend : eol + 1;
  		hunk->end = p - plain->buf;
@@ -953,7 +960,7 @@ static int split_hunk(struct add_p_state *s, struct file_diff *file_diff,
  	context_line_count = 0;
  
  	while (splittable_into > 1) {
-		ch = s->plain.buf[current];
+		ch = normalize_marker(s->plain.buf[current]);
  
  		if (!ch)
  			BUG("buffer overrun while splitting hunks");
diff --git a/t/t3701-add-interactive.sh b/t/t3701-add-interactive.sh
index 5d78868ac16..385c246baf0 100755
--- a/t/t3701-add-interactive.sh
+++ b/t/t3701-add-interactive.sh
@@ -1164,4 +1164,31 @@ test_expect_success 'reset -p with unmerged files' '
  	test_must_be_empty staged
  '
  
+test_expect_success 'hunk splitting works with diff.suppressBlankEmpty' '
+	cat >expect <<-\EOF &&
+	diff --git a/file b/file
+	index 777b174..ebc9684 100755
+	--- a/file
+	+++ b/file
+	@@ -2,6 +2,6 @@ p
+	 q
+	 r
+
+	-d
+	-e
+	-f
+	+s
+	+t
+	+u
+	EOF
+
+	test_config diff.suppressBlankEmpty true &&
+	test_write_lines a b c "" d e f >file &&
+	git add file &&
+	test_write_lines p q r "" s t u >file &&
+	test_write_lines s y n q | git add -p &&
+	git diff >actual &&
+	diff_cmp expect actual
+'
+
  test_done