rebase --skip: fix commit message clean up when skipping squash

During a series of "fixup" and/or "squash" commands, the interactive
rebase accumulates a commit message from all the commits that are being
squashed together. If one of the commits has conflicts when it is picked
and the user chooses to skip that commit then we need to remove that
commit's message from accumulated messages.  To do this 15ef69314d
(rebase --skip: clean up commit message after a failed fixup/squash,
2018-04-27) updated commit_staged_changes() to reset the accumulated
message to the commit message of HEAD (which does not contain the
message from the skipped commit) when the last command was "fixup" or
"squash" and there are no staged changes. Unfortunately the code to do
this contains two bugs.

(1) If parse_head() fails we pass an invalid pointer to
    unuse_commit_buffer().

(2) The reconstructed message uses the entire commit buffer from HEAD
    including the headers, rather than just the commit message.

The first issue is fixed by splitting up the "if" condition into several
statements each with its own error handling. The second issue is fixed
by finding the start of the commit message within the commit buffer
using find_commit_subject().

The existing test added by 15ef69314d is modified to show the effect of
this bug.  The bug is triggered when skipping the first command in the
chain (as the test does before this commit) but the effect is hidden
because opts->current_fixup_count is set to zero which leads
update_squash_messages() to recreate the squash message file from
scratch overwriting the bad message created by
commit_staged_changes(). The test is also updated to explicitly check
the commit messages rather than relying on grep to ensure they do not
contain any stray commit headers.

To check the commit message the function test_commit_message() is moved
from t3437-rebase-fixup-options.sh to test-lib.sh. As the function is
now publicly available it is updated to provide better error detection
and avoid overwriting the commonly used files "actual" and "expect".
Support for reading the expected commit message from stdin is also
added.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -5038,19 +5038,31 @@ static int commit_staged_changes(struct repository *r,
 				 * We need to update the squash message to skip
 				 * the latest commit message.
 				 */
+				int res = 0;
 				struct commit *commit;
+				const char *msg;
 				const char *path = rebase_path_squash_msg();
 				const char *encoding = get_commit_output_encoding();
 
-				if (parse_head(r, &commit) ||
-				    !(p = repo_logmsg_reencode(r, commit, NULL, encoding)) ||
-				    write_message(p, strlen(p), path, 0)) {
-					repo_unuse_commit_buffer(r, commit, p);
-					return error(_("could not write file: "
-						       "'%s'"), path);
+				if (parse_head(r, &commit))
+					return error(_("could not parse HEAD"));
+
+				p = repo_logmsg_reencode(r, commit, NULL, encoding);
+				if (!p)  {
+					res = error(_("could not parse commit %s"),
+						    oid_to_hex(&commit->object.oid));
+					goto unuse_commit_buffer;
 				}
-				repo_unuse_commit_buffer(r,
-							 commit, p);
+				find_commit_subject(p, &msg);
+				if (write_message(msg, strlen(msg), path, 0)) {
+					res = error(_("could not write file: "
+						       "'%s'"), path);
+					goto unuse_commit_buffer;
+				}
+			unuse_commit_buffer:
+				repo_unuse_commit_buffer(r, commit, p);
+				if (res)
+					return res;
 			}
 		}
 

t/t3418-rebase-continue.sh

@@ -115,15 +115,23 @@ test_expect_success '--skip after failed fixup cleans commit message' '
 	test_when_finished "test_might_fail git rebase --abort" &&
 	git checkout -b with-conflicting-fixup &&
 	test_commit wants-fixup &&
-	test_commit "fixup! wants-fixup" wants-fixup.t 1 wants-fixup-1 &&
-	test_commit "fixup! wants-fixup" wants-fixup.t 2 wants-fixup-2 &&
-	test_commit "fixup! wants-fixup" wants-fixup.t 3 wants-fixup-3 &&
+	test_commit "fixup 1" wants-fixup.t 1 wants-fixup-1 &&
+	test_commit "fixup 2" wants-fixup.t 2 wants-fixup-2 &&
+	test_commit "fixup 3" wants-fixup.t 3 wants-fixup-3 &&
 	test_must_fail env FAKE_LINES="1 fixup 2 squash 4" \
 		git rebase -i HEAD~4 &&
 
 	: now there is a conflict, and comments in the commit message &&
-	git show HEAD >out &&
-	grep "fixup! wants-fixup" out &&
+	test_commit_message HEAD <<-\EOF &&
+	# This is a combination of 2 commits.
+	# This is the 1st commit message:
+
+	wants-fixup
+
+	# The commit message #2 will be skipped:
+
+	# fixup 1
+	EOF
 
 	: skip and continue &&
 	echo "cp \"\$1\" .git/copy.txt" | write_script copy-editor.sh &&
@@ -133,33 +141,49 @@ test_expect_success '--skip after failed fixup cleans commit message' '
 	test_path_is_missing .git/copy.txt &&
 
 	: now the comments in the commit message should have been cleaned up &&
-	git show HEAD >out &&
-	! grep "fixup! wants-fixup" out &&
+	test_commit_message HEAD -m wants-fixup &&
 
 	: now, let us ensure that "squash" is handled correctly &&
 	git reset --hard wants-fixup-3 &&
-	test_must_fail env FAKE_LINES="1 squash 4 squash 2 squash 4" \
+	test_must_fail env FAKE_LINES="1 squash 2 squash 1 squash 3 squash 1" \
 		git rebase -i HEAD~4 &&
 
-	: the first squash failed, but there are two more in the chain &&
+	: the second squash failed, but there are two more in the chain &&
 	(test_set_editor "$PWD/copy-editor.sh" &&
 	 test_must_fail git rebase --skip) &&
 
 	: not the final squash, no need to edit the commit message &&
 	test_path_is_missing .git/copy.txt &&
 
-	: The first squash was skipped, therefore: &&
-	git show HEAD >out &&
-	test_i18ngrep "# This is a combination of 2 commits" out &&
-	test_i18ngrep "# This is the commit message #2:" out &&
+	: The first and third squashes succeeded, therefore: &&
+	test_commit_message HEAD <<-\EOF &&
+	# This is a combination of 3 commits.
+	# This is the 1st commit message:
+
+	wants-fixup
+
+	# This is the commit message #2:
+
+	fixup 1
+
+	# This is the commit message #3:
+
+	fixup 2
+	EOF
 
 	(test_set_editor "$PWD/copy-editor.sh" && git rebase --skip) &&
-	git show HEAD >out &&
-	test_i18ngrep ! "# This is a combination" out &&
+	test_commit_message HEAD <<-\EOF &&
+	wants-fixup
+
+	fixup 1
+
+	fixup 2
+	EOF
 
 	: Final squash failed, but there was still a squash &&
-	test_i18ngrep "# This is a combination of 2 commits" .git/copy.txt &&
-	test_i18ngrep "# This is the commit message #2:" .git/copy.txt
+	head -n1 .git/copy.txt >first-line &&
+	test_i18ngrep "# This is a combination of 3 commits" first-line &&
+	test_i18ngrep "# This is the commit message #3:" .git/copy.txt
 '
 
 test_expect_success 'setup rerere database' '

t/t3437-rebase-fixup-options.sh

@@ -21,21 +21,6 @@ TEST_PASSES_SANITIZE_LEAK=true
 
 EMPTY=""
 
-# test_commit_message <rev> -m <msg>
-# test_commit_message <rev> <path>
-# Verify that the commit message of <rev> matches
-# <msg> or the content of <path>.
-test_commit_message () {
-	git show --no-patch --pretty=format:%B "$1" >actual &&
-	case "$2" in
-	-m)
-		echo "$3" >expect &&
-		test_cmp expect actual ;;
-	*)
-		test_cmp "$2" actual ;;
-	esac
-}
-
 get_author () {
 	rev="$1" &&
 	git log -1 --pretty=format:"%an %ae %at" "$rev"

t/test-lib-functions.sh

@@ -1273,6 +1273,39 @@ test_cmp_rev () {
 	fi
 }
 
+# Tests that a commit message matches the expected text
+#
+# Usage: test_commit_message <rev> [-m <msg> | <file>]
+#
+# When using "-m" <msg> will have a line feed appended. If the second
+# argument is omitted then the expected message is read from stdin.
+
+test_commit_message () {
+	local msg_file=expect.msg
+
+	case $# in
+	3)
+		if test "$2" = "-m"
+		then
+			printf "%s\n" "$3" >"$msg_file"
+		else
+			BUG "Usage: test_commit_message <rev> [-m <message> | <file>]"
+		fi
+		;;
+	2)
+		msg_file="$2"
+		;;
+	1)
+		cat >"$msg_file"
+		;;
+	*)
+		BUG "Usage: test_commit_message <rev> [-m <message> | <file>]"
+		;;
+	esac
+	git show --no-patch --pretty=format:%B "$1" -- >actual.msg &&
+	test_cmp "$msg_file" actual.msg
+}
+
 # Compare paths respecting core.ignoreCase
 test_cmp_fspath () {
 	if test "x$1" = "x$2"