git-cherry-pick: Add keep-redundant-commits option

The git-cherry-pick --allow-empty command by default only preserves empty
commits that were originally empty, i.e only those commits for which
<commit>^{tree} and <commit>^^{tree} are equal.  By default commits which are
non-empty, but were made empty by the inclusion of a prior commit on the current
history are filtered out.  This option allows us to override that behavior and
include redundant commits as empty commits in the change history.

Note that this patch changes the default behavior of git cherry-pick slightly.
Prior to this patch all commits in a cherry-pick sequence were applied and git
commit was run.  The implication here was that, if a commit was redundant, and
the commit did not trigger the fast forward logic, the git commit operation, and
therefore the git cherry-pick operation would fail, displaying the cherry pick
advice (i.e. run git commit --allow-empty).  With this patch however, such
redundant commits are automatically skipped without stopping, unless
--keep-redundant-commits is specified, in which case, they are automatically
applied as empty commits.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 83 insertions, 11 deletions

diff --git a/sequencer.c b/sequencer.c
index 71929ba..4e3af82 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -13,6 +13,7 @@
 #include "rerere.h"
 #include "merge-recursive.h"
 #include "refs.h"
+#include "argv-array.h"
 
 #define GIT_REFLOG_ACTION "GIT_REFLOG_ACTION"
 
@@ -251,6 +252,30 @@ static int do_recursive_merge(struct commit *base, struct commit *next,
 	return !clean;
 }
 
+static int is_index_unchanged(void)
+{
+	unsigned char head_sha1[20];
+	struct commit *head_commit;
+
+	if (!resolve_ref_unsafe("HEAD", head_sha1, 1, NULL))
+		return error(_("Could not resolve HEAD commit\n"));
+
+	head_commit = lookup_commit(head_sha1);
+	if (!head_commit || parse_commit(head_commit))
+		return error(_("could not parse commit %s\n"),
+			     sha1_to_hex(head_commit->object.sha1));
+
+	if (!active_cache_tree)
+		active_cache_tree = cache_tree();
+
+	if (!cache_tree_fully_valid(active_cache_tree))
+		if (cache_tree_update(active_cache_tree, active_cache,
+				  active_nr, 0))
+			return error(_("Unable to update cache tree\n"));
+
+	return !hashcmp(active_cache_tree->sha1, head_commit->tree->object.sha1);
+}
+
 /*
  * If we are cherry-pick, and if the merge did not result in
  * hand-editing, we will hit this commit and inherit the original
@@ -260,24 +285,46 @@ static int do_recursive_merge(struct commit *base, struct commit *next,
  */
 static int run_git_commit(const char *defmsg, struct replay_opts *opts)
 {
-	/* 7 is max possible length of our args array including NULL */
-	const char *args[7];
-	int i = 0;
+	struct argv_array array;
+	int rc;
+
+	argv_array_init(&array);
+	argv_array_push(&array, "commit");
+	argv_array_push(&array, "-n");
 
-	args[i++] = "commit";
-	args[i++] = "-n";
 	if (opts->signoff)
-		args[i++] = "-s";
+		argv_array_push(&array, "-s");
 	if (!opts->edit) {
-		args[i++] = "-F";
-		args[i++] = defmsg;
+		argv_array_push(&array, "-F");
+		argv_array_push(&array, defmsg);
 	}
+
 	if (opts->allow_empty)
-		args[i++] = "--allow-empty";
+		argv_array_push(&array, "--allow-empty");
 
-	args[i] = NULL;
+	rc = run_command_v_opt(array.argv, RUN_GIT_CMD);
+	argv_array_clear(&array);
+	return rc;
+}
+
+static int is_original_commit_empty(struct commit *commit)
+{
+	const unsigned char *ptree_sha1;
+
+	if (parse_commit(commit))
+		return error(_("Could not parse commit %s\n"),
+			     sha1_to_hex(commit->object.sha1));
+	if (commit->parents) {
+		struct commit *parent = commit->parents->item;
+		if (parse_commit(parent))
+			return error(_("Could not parse parent commit %s\n"),
+				sha1_to_hex(parent->object.sha1));
+		ptree_sha1 = parent->tree->object.sha1;
+	} else {
+		ptree_sha1 = EMPTY_TREE_SHA1_BIN; /* commit is root */
+	}
 
-	return run_command_v_opt(args, RUN_GIT_CMD);
+	return !hashcmp(ptree_sha1, commit->tree->object.sha1);
 }
 
 static int do_pick_commit(struct commit *commit, struct replay_opts *opts)
@@ -289,6 +336,8 @@ static int do_pick_commit(struct commit *commit, struct replay_opts *opts)
 	char *defmsg = NULL;
 	struct strbuf msgbuf = STRBUF_INIT;
 	int res;
+	int empty_commit;
+	int index_unchanged;
 
 	if (opts->no_commit) {
 		/*
@@ -414,6 +463,10 @@ static int do_pick_commit(struct commit *commit, struct replay_opts *opts)
 		free_commit_list(remotes);
 	}
 
+	empty_commit = is_original_commit_empty(commit);
+	if (empty_commit < 0)
+		return empty_commit;
+
 	/*
 	 * If the merge was clean or if it failed due to conflict, we write
 	 * CHERRY_PICK_HEAD for the subsequent invocation of commit to use.
@@ -434,6 +487,25 @@ static int do_pick_commit(struct commit *commit, struct replay_opts *opts)
 		print_advice(res == 1, opts);
 		rerere(opts->allow_rerere_auto);
 	} else {
+		index_unchanged = is_index_unchanged();
+		/*
+		 * If index_unchanged is less than 0, that indicates we either
+		 * couldn't parse HEAD or the index, so error out here.
+		 */
+		if (index_unchanged < 0)
+			return index_unchanged;
+
+		if (!empty_commit && !opts->keep_redundant_commits && index_unchanged)
+			/*
+			 * The head tree and the index match
+			 * meaning the commit is empty.  Since it wasn't created
+			 * empty (based on the previous test), we can conclude
+			 * the commit has been made redundant.  Since we don't
+			 * want to keep redundant commits, we can just return
+			 * here, skipping this commit
+			 */
+			return 0;
+
 		if (!opts->no_commit)
 			res = run_git_commit(defmsg, opts);
 	}