[PATCH v2] commit: accept tag objects in HEAD/MERGE_HEAD

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

HEAD, MERGE_HEAD (or any other branches) should only have SHA-1 of a
commit object. However broken tools can put a tag object there. While
it's wrong, it'd be better to tolerate the situation and move on
("commit" is an often used operation, unable to commit could be bad).

In worse cases, where we cannot even resolve HEAD to a commit, die()
instead of segfault.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 builtin/commit.c |   15 +++++++++------
 commit.c         |   12 ++++++++++++
 commit.h         |    1 +
 3 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/builtin/commit.c b/builtin/commit.c
index 2088b6b..f327595 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1387,6 +1387,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 	unsigned char commit_sha1[20];
 	struct ref_lock *ref_lock;
 	struct commit_list *parents = NULL, **pptr = &parents;
+	struct commit *commit;
 	struct stat statbuf;
 	int allow_fast_forward = 1;
 	struct wt_status s;
@@ -1423,12 +1424,11 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 			reflog_msg = "commit (initial)";
 	} else if (amend) {
 		struct commit_list *c;
-		struct commit *commit;
 
 		if (!reflog_msg)
 			reflog_msg = "commit (amend)";
-		commit = lookup_commit(head_sha1);
-		if (!commit || parse_commit(commit))
+		commit = lookup_expect_commit(head_sha1, "HEAD");
+		if (parse_commit(commit))
 			die(_("could not parse HEAD commit"));
 
 		for (c = commit->parents; c; c = c->next)
@@ -1439,7 +1439,8 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 
 		if (!reflog_msg)
 			reflog_msg = "commit (merge)";
-		pptr = &commit_list_insert(lookup_commit(head_sha1), pptr)->next;
+		commit = lookup_expect_commit(head_sha1, "HEAD");
+		pptr = &commit_list_insert(commit, pptr)->next;
 		fp = fopen(git_path("MERGE_HEAD"), "r");
 		if (fp == NULL)
 			die_errno(_("could not open '%s' for reading"),
@@ -1448,7 +1449,8 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 			unsigned char sha1[20];
 			if (get_sha1_hex(m.buf, sha1) < 0)
 				die(_("Corrupt MERGE_HEAD file (%s)"), m.buf);
-			pptr = &commit_list_insert(lookup_commit(sha1), pptr)->next;
+			commit = lookup_expect_commit(sha1, "MERGE_HEAD");
+			pptr = &commit_list_insert(commit, pptr)->next;
 		}
 		fclose(fp);
 		strbuf_release(&m);
@@ -1465,7 +1467,8 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 			reflog_msg = (whence == FROM_CHERRY_PICK)
 					? "commit (cherry-pick)"
 					: "commit";
-		pptr = &commit_list_insert(lookup_commit(head_sha1), pptr)->next;
+		commit = lookup_expect_commit(head_sha1, "HEAD");
+		pptr = &commit_list_insert(commit, pptr)->next;
 	}
 
 	/* Finally, get the commit message */
diff --git a/commit.c b/commit.c
index ac337c7..dc22695 100644
--- a/commit.c
+++ b/commit.c
@@ -39,6 +39,18 @@ struct commit *lookup_commit_reference(const unsigned char *sha1)
 	return lookup_commit_reference_gently(sha1, 0);
 }
 
+struct commit *lookup_expect_commit(const unsigned char *sha1,
+				    const char *ref_name)
+{
+	struct commit *c = lookup_commit_reference(sha1);
+	if (!c)
+		die(_("could not parse %s"), ref_name);
+	if (hashcmp(sha1, c->object.sha1))
+		warning(_("%s %s is not a commit!"),
+			ref_name, sha1_to_hex(sha1));
+	return c;
+}
+
 struct commit *lookup_commit(const unsigned char *sha1)
 {
 	struct object *obj = lookup_object(sha1);
diff --git a/commit.h b/commit.h
index feb809f..0e36fd0 100644
--- a/commit.h
+++ b/commit.h
@@ -37,6 +37,7 @@ struct commit *lookup_commit_reference(const unsigned char *sha1);
 struct commit *lookup_commit_reference_gently(const unsigned char *sha1,
 					      int quiet);
 struct commit *lookup_commit_reference_by_name(const char *name);
+struct commit *lookup_expect_commit(const unsigned char *sha1, const char *ref_name);
 
 int parse_commit_buffer(struct commit *item, const void *buffer, unsigned long size);
 int parse_commit(struct commit *item);
-- 
1.7.4.74.g639db