builtin-commit: use reduce_heads() only when appropriate

Since commit 6bb6b034 (builtin-commit: use commit_tree(), 2008-09-10),
builtin-commit performs a reduce_heads() unconditionally.  However,
it's not always needed, and in some cases even harmful.

reduce_heads() is not needed for the initial commit or for an
"ordinary" commit, because they don't have any or have only one
parent, respectively.

reduce_heads() must be avoided when 'git commit' is run after a 'git
merge --no-ff --no-commit', otherwise it will turn the
non-fast-forward merge into fast-forward.  For the same reason,
reduce_heads() must be avoided when amending such a merge commit.

To resolve this issue, 'git merge' will write info about whether
fast-forward is allowed or not to $GIT_DIR/MERGE_MODE.  Based on this
info, 'git commit' will only perform reduce_heads() when it's
committing a merge and fast-forward is enabled.

Also add test cases to ensure that non-fast-forward merges are
committed and amended properly.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

branch.c

@@ -168,5 +168,6 @@ void remove_branch_state(void)
 	unlink(git_path("MERGE_HEAD"));
 	unlink(git_path("MERGE_RR"));
 	unlink(git_path("MERGE_MSG"));
+	unlink(git_path("MERGE_MODE"));
 	unlink(git_path("SQUASH_MSG"));
 }

builtin-commit.c

@@ -937,6 +937,8 @@ 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 stat statbuf;
+	int allow_fast_forward = 1;
 
 	git_config(git_commit_config, NULL);
 
@@ -951,6 +953,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		return 1;
 	}
 
+	strbuf_init(&sb, 0);
 	/* Determine parents */
 	if (initial_commit) {
 		reflog_msg = "commit (initial)";
@@ -984,14 +987,22 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		}
 		fclose(fp);
 		strbuf_release(&m);
+		if (!stat(git_path("MERGE_MODE"), &statbuf)) {
+			if (strbuf_read_file(&sb, git_path("MERGE_MODE"), 0) < 0)
+				die("could not read MERGE_MODE: %s",
+						strerror(errno));
+			if (!strcmp(sb.buf, "no-ff"))
+				allow_fast_forward = 0;
+		}
+		if (allow_fast_forward)
+			parents = reduce_heads(parents);
 	} else {
 		reflog_msg = "commit";
 		pptr = &commit_list_insert(lookup_commit(head_sha1), pptr)->next;
 	}
-	parents = reduce_heads(parents);
 
 	/* Finally, get the commit message */
-	strbuf_init(&sb, 0);
+	strbuf_reset(&sb);
 	if (strbuf_read_file(&sb, git_path(commit_editmsg), 0) < 0) {
 		rollback_index_files();
 		die("could not read commit message");
@@ -1040,6 +1051,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 
 	unlink(git_path("MERGE_HEAD"));
 	unlink(git_path("MERGE_MSG"));
+	unlink(git_path("MERGE_MODE"));
 	unlink(git_path("SQUASH_MSG"));
 
 	if (commit_index_files())

builtin-merge.c

@@ -180,6 +180,7 @@ static void drop_save(void)
 {
 	unlink(git_path("MERGE_HEAD"));
 	unlink(git_path("MERGE_MSG"));
+	unlink(git_path("MERGE_MODE"));
 }
 
 static void save_state(void)
@@ -1210,6 +1211,15 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 			merge_msg.len)
 			die("Could not write to %s", git_path("MERGE_MSG"));
 		close(fd);
+		fd = open(git_path("MERGE_MODE"), O_WRONLY | O_CREAT | O_TRUNC, 0666);
+		if (fd < 0)
+			die("Could open %s for writing", git_path("MERGE_MODE"));
+		strbuf_reset(&buf);
+		if (!allow_fast_forward)
+			strbuf_addf(&buf, "no-ff");
+		if (write_in_full(fd, buf.buf, buf.len) != buf.len)
+			die("Could not write to %s", git_path("MERGE_MODE"));
+		close(fd);
 	}
 
 	if (merge_was_ok) {

t/t7600-merge.sh

@@ -511,4 +511,20 @@ test_expect_success 'in-index merge' '
 
 test_debug 'gitk --all'
 
+test_expect_success 'merge --no-ff --no-commit && commit' '
+	git reset --hard c0 &&
+	git merge --no-ff --no-commit c1 &&
+	EDITOR=: git commit &&
+	verify_parents $c0 $c1
+'
+
+test_debug 'gitk --all'
+
+test_expect_success 'amending no-ff merge commit' '
+	EDITOR=: git commit --amend &&
+	verify_parents $c0 $c1
+'
+
+test_debug 'gitk --all'
+
 test_done