[PATCH 2/2] built-in rebase: reinstate `checkout -q` behavior where appropriate

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

When we converted a `git checkout -q $onto^0` call to use
`reset_head()`, we inadvertently incurred a change from a twoway_merge
to a oneway_merge, as if we wanted a `git reset --hard` instead.

This has performance ramifications under certain, though, as the
oneway_merge needs to lstat() every single index entry whereas
twoway_merge does not.

So let's go back to the old behavior.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 builtin/rebase.c | 45 ++++++++++++++++++++++++++-------------------
 1 file changed, 26 insertions(+), 19 deletions(-)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index 6f6d7de156..c1cc50f3f8 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -523,11 +523,12 @@ finished_rebase:
 #define GIT_REFLOG_ACTION_ENVIRONMENT "GIT_REFLOG_ACTION"
 
 static int reset_head(struct object_id *oid, const char *action,
-		      const char *switch_to_branch, int detach_head,
+		      const char *switch_to_branch,
+		      int detach_head, int reset_hard,
 		      const char *reflog_orig_head, const char *reflog_head)
 {
 	struct object_id head_oid;
-	struct tree_desc desc;
+	struct tree_desc desc[2];
 	struct lock_file lock = LOCK_INIT;
 	struct unpack_trees_options unpack_tree_opts;
 	struct tree *tree;
@@ -536,7 +537,7 @@ static int reset_head(struct object_id *oid, const char *action,
 	size_t prefix_len;
 	struct object_id *orig = NULL, oid_orig,
 		*old_orig = NULL, oid_old_orig;
-	int ret = 0;
+	int ret = 0, nr = 0;
 
 	if (switch_to_branch && !starts_with(switch_to_branch, "refs/"))
 		BUG("Not a fully qualified branch: '%s'", switch_to_branch);
@@ -544,20 +545,20 @@ static int reset_head(struct object_id *oid, const char *action,
 	if (hold_locked_index(&lock, LOCK_REPORT_ON_ERROR) < 0)
 		return -1;
 
-	if (!oid) {
-		if (get_oid("HEAD", &head_oid)) {
-			rollback_lock_file(&lock);
-			return error(_("could not determine HEAD revision"));
-		}
-		oid = &head_oid;
+	if (get_oid("HEAD", &head_oid)) {
+		rollback_lock_file(&lock);
+		return error(_("could not determine HEAD revision"));
 	}
 
+	if (!oid)
+		oid = &head_oid;
+
 	memset(&unpack_tree_opts, 0, sizeof(unpack_tree_opts));
 	setup_unpack_trees_porcelain(&unpack_tree_opts, action);
 	unpack_tree_opts.head_idx = 1;
 	unpack_tree_opts.src_index = the_repository->index;
 	unpack_tree_opts.dst_index = the_repository->index;
-	unpack_tree_opts.fn = oneway_merge;
+	unpack_tree_opts.fn = reset_hard ? oneway_merge : twoway_merge;
 	unpack_tree_opts.update = 1;
 	unpack_tree_opts.merge = 1;
 	if (!detach_head)
@@ -568,12 +569,17 @@ static int reset_head(struct object_id *oid, const char *action,
 		return error(_("could not read index"));
 	}
 
-	if (!fill_tree_descriptor(&desc, oid)) {
+	if (!reset_hard && !fill_tree_descriptor(&desc[nr++], &head_oid)) {
+		ret = error(_("failed to find tree of %s"), oid_to_hex(oid));
+		goto leave_reset_head;
+	}
+
+	if (!fill_tree_descriptor(&desc[nr++], oid)) {
 		ret = error(_("failed to find tree of %s"), oid_to_hex(oid));
 		goto leave_reset_head;
 	}
 
-	if (unpack_trees(1, &desc, &unpack_tree_opts)) {
+	if (unpack_trees(nr, desc, &unpack_tree_opts)) {
 		ret = -1;
 		goto leave_reset_head;
 	}
@@ -621,7 +627,8 @@ static int reset_head(struct object_id *oid, const char *action,
 leave_reset_head:
 	strbuf_release(&msg);
 	rollback_lock_file(&lock);
-	free((void *)desc.buffer);
+	while (nr)
+		free((void *)desc[--nr].buffer);
 	return ret;
 }
 
@@ -999,7 +1006,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		rerere_clear(&merge_rr);
 		string_list_clear(&merge_rr, 1);
 
-		if (reset_head(NULL, "reset", NULL, 0, NULL, NULL) < 0)
+		if (reset_head(NULL, "reset", NULL, 0, 1, NULL, NULL) < 0)
 			die(_("could not discard worktree changes"));
 		if (read_basic_state(&options))
 			exit(1);
@@ -1015,7 +1022,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		if (read_basic_state(&options))
 			exit(1);
 		if (reset_head(&options.orig_head, "reset",
-			       options.head_name, 0, NULL, NULL) < 0)
+			       options.head_name, 0, 1, NULL, NULL) < 0)
 			die(_("could not move back to %s"),
 			    oid_to_hex(&options.orig_head));
 		ret = finish_rebase(&options);
@@ -1379,7 +1386,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			write_file(autostash, "%s", oid_to_hex(&oid));
 			printf(_("Created autostash: %s\n"), buf.buf);
 			if (reset_head(&head->object.oid, "reset --hard",
-				       NULL, 0, NULL, NULL) < 0)
+				       NULL, 0, 1, NULL, NULL) < 0)
 				die(_("could not reset --hard"));
 			printf(_("HEAD is now at %s"),
 			       find_unique_abbrev(&head->object.oid,
@@ -1433,7 +1440,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 				strbuf_addf(&buf, "rebase: checkout %s",
 					    options.switch_to);
 				if (reset_head(&oid, "checkout",
-					       options.head_name, 0,
+					       options.head_name, 0, 0,
 					       NULL, NULL) < 0) {
 					ret = !!error(_("could not switch to "
 							"%s"),
@@ -1499,7 +1506,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			 "it...\n"));
 
 	strbuf_addf(&msg, "rebase: checkout %s", options.onto_name);
-	if (reset_head(&options.onto->object.oid, "checkout", NULL, 1,
+	if (reset_head(&options.onto->object.oid, "checkout", NULL, 1, 0,
 	    NULL, msg.buf))
 		die(_("Could not detach HEAD"));
 	strbuf_release(&msg);
@@ -1515,7 +1522,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		strbuf_addf(&msg, "rebase finished: %s onto %s",
 			options.head_name ? options.head_name : "detached HEAD",
 			oid_to_hex(&options.onto->object.oid));
-		reset_head(NULL, "Fast-forwarded", options.head_name, 0,
+		reset_head(NULL, "Fast-forwarded", options.head_name, 0, 0,
 			   "HEAD", msg.buf);
 		strbuf_release(&msg);
 		ret = !!finish_rebase(&options);
-- 
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