[PATCH v3 4/4] history: re-edit a squash with every message

["Harald Nordgren via GitGitGadget" <gitgitgadget@gmail.com>]

From: Harald Nordgren <haraldnordgren@gmail.com>

By default "git history squash" reuses the oldest commit's message.
When --reedit-message is given it only reopened that one message, so the
messages of the folded-in commits were lost.

Gather the messages of every commit in the range, oldest first, and use
them as the editor template when re-editing, mirroring how "git rebase
-i" presents a squash. The combined message is built before the
descendant walk so it is not disturbed by the flags that walk leaves on
the commits.

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>
---
 Documentation/git-history.adoc |  5 +--
 builtin/history.c              | 61 +++++++++++++++++++++++++++++++++-
 t/t3454-history-squash.sh      | 37 +++++++++++++++++++++
 3 files changed, 100 insertions(+), 3 deletions(-)

diff --git a/Documentation/git-history.adoc b/Documentation/git-history.adoc
index d3a5ad28a3..dd3544832d 100644
--- a/Documentation/git-history.adoc
+++ b/Documentation/git-history.adoc
@@ -111,8 +111,9 @@ history squash @~3..` folds the three most recent commits into one, and
 `git history squash @~5..@~2` squashes an interior range while leaving
 the two newest commits in place.
 +
-The oldest commit's message and authorship are preserved by default,
-unless you specify `--reedit-message`. A merge commit inside the range is
+The oldest commit's message and authorship are preserved by default. With
+`--reedit-message`, an editor opens pre-filled with the messages of all the
+folded commits so you can combine them. A merge commit inside the range is
 folded like any other, but the range must have a single base, so a range
 that reaches more than one entry point (for example a side branch that
 forked before the range and was later merged into it) is rejected.
diff --git a/builtin/history.c b/builtin/history.c
index 9d9416870f..eb12a5d7e8 100644
--- a/builtin/history.c
+++ b/builtin/history.c
@@ -1047,6 +1047,56 @@ out:
 	return ret;
 }
 
+static int build_squash_message(struct repository *repo,
+				struct commit *base,
+				struct commit *tip,
+				struct strbuf *out)
+{
+	struct rev_info revs;
+	struct commit *commit;
+	struct strvec args = STRVEC_INIT;
+	int n = 0, ret;
+
+	repo_init_revisions(repo, &revs, NULL);
+	strvec_push(&args, "ignored");
+	strvec_push(&args, "--reverse");
+	strvec_push(&args, "--topo-order");
+	strvec_pushf(&args, "%s..%s", oid_to_hex(&base->object.oid),
+		     oid_to_hex(&tip->object.oid));
+	setup_revisions_from_strvec(&args, &revs, NULL);
+
+	if (prepare_revision_walk(&revs) < 0) {
+		ret = error(_("error preparing revisions"));
+		goto out;
+	}
+
+	while ((commit = get_revision(&revs))) {
+		const char *message, *body;
+		struct strbuf one = STRBUF_INIT;
+
+		message = repo_logmsg_reencode(repo, commit, NULL, NULL);
+		find_commit_subject(message, &body);
+		strbuf_addstr(&one, body);
+		strbuf_trim_trailing_newline(&one);
+
+		if (n++)
+			strbuf_addch(out, '\n');
+		strbuf_addbuf(out, &one);
+		strbuf_addch(out, '\n');
+
+		strbuf_release(&one);
+		repo_unuse_commit_buffer(repo, commit, message);
+	}
+
+	ret = 0;
+
+out:
+	reset_revision_walk();
+	release_revisions(&revs);
+	strvec_clear(&args);
+	return ret;
+}
+
 static int cmd_history_squash(int argc,
 			      const char **argv,
 			      const char *prefix,
@@ -1071,6 +1121,7 @@ static int cmd_history_squash(int argc,
 		OPT_END(),
 	};
 	struct strbuf reflog_msg = STRBUF_INIT;
+	struct strbuf message = STRBUF_INIT;
 	struct commit *base, *oldest, *tip, *rewritten;
 	const struct object_id *base_tree_oid, *tip_tree_oid;
 	struct commit_list *parents = NULL;
@@ -1091,6 +1142,12 @@ static int cmd_history_squash(int argc,
 	if (ret < 0)
 		goto out;
 
+	if (flags & COMMIT_TREE_EDIT_MESSAGE) {
+		ret = build_squash_message(repo, base, tip, &message);
+		if (ret < 0)
+			goto out;
+	}
+
 	ret = setup_revwalk(repo, action, tip, &revs);
 	if (ret < 0)
 		goto out;
@@ -1099,7 +1156,8 @@ static int cmd_history_squash(int argc,
 	tip_tree_oid = &repo_get_commit_tree(repo, tip)->object.oid;
 	commit_list_append(base, &parents);
 
-	ret = commit_tree_ext(repo, "squash", oldest, NULL, parents,
+	ret = commit_tree_ext(repo, "squash", oldest,
+			      message.len ? message.buf : NULL, parents,
 			      base_tree_oid, tip_tree_oid, &rewritten, flags);
 	if (ret < 0) {
 		ret = error(_("failed writing squashed commit"));
@@ -1120,6 +1178,7 @@ static int cmd_history_squash(int argc,
 
 out:
 	strbuf_release(&reflog_msg);
+	strbuf_release(&message);
 	commit_list_free(parents);
 	release_revisions(&revs);
 	return ret;
diff --git a/t/t3454-history-squash.sh b/t/t3454-history-squash.sh
index 6c6a75bf00..1edd148295 100755
--- a/t/t3454-history-squash.sh
+++ b/t/t3454-history-squash.sh
@@ -135,6 +135,43 @@ test_expect_success 'preserves authorship of the oldest commit' '
 	test_cmp expect actual
 '
 
+test_expect_success '--reedit-message offers every folded-in message' '
+	git reset --hard start &&
+	echo b >file &&
+	git add file &&
+	git commit -m "re-one subject" -m "re-one body line" &&
+	test_commit re-two file c &&
+	test_commit re-three file d &&
+
+	write_script editor <<-\EOF &&
+	cp "$1" buffer &&
+	echo combined >"$1"
+	EOF
+	test_set_editor "$(pwd)/editor" &&
+	git history squash --reedit-message start.. &&
+
+	grep "re-one subject" buffer &&
+	grep "re-one body line" buffer &&
+	grep re-two buffer &&
+	grep re-three buffer &&
+	git log --format="%s" -1 >actual &&
+	echo combined >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '--reedit-message aborts on an empty message' '
+	git reset --hard three &&
+	head_before=$(git rev-parse HEAD) &&
+
+	write_script editor <<-\EOF &&
+	>"$1"
+	EOF
+	test_set_editor "$(pwd)/editor" &&
+	test_must_fail git history squash --reedit-message start.. &&
+
+	test_cmp_rev "$head_before" HEAD
+'
+
 test_expect_success '--dry-run predicts the rewrite without performing it' '
 	git reset --hard three &&
 	head_before=$(git rev-parse HEAD) &&
-- 
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