add -p: mark split hunks as undecided

When a hunk is split each of the new hunks inherits whether it is
selected or not from the original hunk. If a selected hunk is split
all of the new hunks are selected and the user is only prompted with
the first of the split hunks, they are not asked whether or not they
want to select the rest of the new hunks. If they want to deselect
one of the other new hunks they have to navigate back to it to do
that. This is unfortunate as the user is presumably splitting the
original hunk because they only want to select some sub-set of it.

Instead mark all the new hunks as "undecided" so that we prompt the
user to decide whether to select them or not. In the case where the
user only wants to change the selection of the first of the split
hunks they will now have to do more work re-selecting the remaining
split hunks. However changing the selection of any of the other newly
created hunks is now much simpler as the user no-longer has to navigate
back to them before being able to change their selected state. Due
to concerns that users may be relying on the current behaviour [1]
this change is guarded by WITH_BREAKING_CHANGES.

[1] https://lore.kernel.org/git/[email protected]

Signed-off-by: Phillip Wood <[email protected]>

Author: phillipwood

Documentation/BreakingChanges.adoc

@@ -118,6 +118,11 @@ Cf. <[email protected]>,
 <[email protected]>,
 <CA+EOSBncr=4a4d8n9xS4FNehyebpmX8JiUwCsXD47EQDE+DiUQ@mail.gmail.com>.
 
+* The behavior of "git add -p" has been changed so that splitting a
+  hunk that has already been marked as selected or unselected will now
+  prompt the user to select each of the new hunks created by the
+  split instead of them inheriting their state from the original hunk.
+
 === Removals
 
 * Support for grafting commits has long been superseded by git-replace(1).

add-patch.c

@@ -953,6 +953,9 @@ static int split_hunk(struct add_p_state *s, struct file_diff *file_diff,
 			* sizeof(*hunk));
 	hunk = file_diff->hunk + hunk_index;
 	hunk->splittable_into = 1;
+#ifdef WITH_BREAKING_CHANGES
+	hunk->use = UNDECIDED_HUNK;
+#endif
 	memset(hunk + 1, 0, (splittable_into - 1) * sizeof(*hunk));
 
 	header = &hunk->header;
@@ -1054,7 +1057,11 @@ static int split_hunk(struct add_p_state *s, struct file_diff *file_diff,
 
 		hunk++;
 		hunk->splittable_into = 1;
+#ifdef WITH_BREAKING_CHANGES
+		hunk->use = UNDECIDED_HUNK;
+#else
 		hunk->use = hunk[-1].use;
+#endif
 		header = &hunk->header;
 
 		header->old_count = header->new_count = context_line_count;

t/t3701-add-interactive.sh

@@ -1230,4 +1230,14 @@ test_expect_success 'hunk splitting works with diff.suppressBlankEmpty' '
 	test_cmp expect actual
 '
 
+test_expect_success WITH_BREAKING_CHANGES 'splitting previous hunk marks split hunks as undecided' '
+	test_write_lines a " " b c d e f g h i j k >file &&
+	git add file &&
+	test_write_lines x " " b y d e f g h i j x >file &&
+	test_write_lines n K s n y q | git add -p file &&
+	git cat-file blob :file >actual &&
+	test_write_lines a " " b y d e f g h i j k >expect &&
+	test_cmp expect actual
+'
+
 test_done