From ce48a6bbac600732af0cb3411b5ac672ea6a32e6 Mon Sep 17 00:00:00 2001
From: Michael J Gruber <git@drmicha.warpmail.net>
Date: Tue, 14 Dec 2010 21:39:28 +0100
Subject: [PATCH] log and friends: --second-parent

Introduce an option '--second-parent' to the revision walker which walks
the second parent if present, or else the first parent.

This is useful for walking a side branch from the merge point downwards.
---
 builtin/log.c | 2 +-
 graph.c       | 3 +++
 log-tree.c    | 2 +-
 revision.c    | 7 +++++++
 revision.h    | 2 +-
 5 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/builtin/log.c b/builtin/log.c
index 59de484bc2..e1e0847bbd 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -449,7 +449,7 @@ static void show_rev_tweak_rev(struct rev_info *rev, struct setup_revision_opt *
 		/* There was no "-m" on the command line */
 		rev->ignore_merges = 0;
 		if (!rev->first_parent_only && !rev->combine_merges) {
-			/* No "--first-parent", "-c", nor "--cc" */
+			/* No "--first/second-parent", "-c", nor "--cc" */
 			rev->combine_merges = 1;
 			rev->dense_combined_merges = 1;
 		}
diff --git a/graph.c b/graph.c
index b24d04c406..d38ee9b17f 100644
--- a/graph.c
+++ b/graph.c
@@ -335,6 +335,9 @@ static struct commit_list *first_interesting_parent(struct git_graph *graph)
 	if (!parents)
 		return NULL;
 
+	if ((graph->revs->first_parent_only == 2) && parents->next &&
+		graph_is_interesting(graph, parents->next->item))
+		return parents->next;
 	/*
 	 * If the first parent is interesting, return it
 	 */
diff --git a/log-tree.c b/log-tree.c
index 92bb2bf48e..010f715b99 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -736,7 +736,7 @@ static int log_tree_diff(struct rev_info *opt, struct commit *commit, struct log
 			 * parent, showing summary diff of the others
 			 * we merged _in_.
 			 */
-			diff_tree_sha1(parents->item->object.sha1, sha1, "", &opt->diffopt);
+			diff_tree_sha1(((opt->first_parent_only == 2) ? parents->next : parents)->item->object.sha1, sha1, "", &opt->diffopt);
 			log_tree_diff_flush(opt);
 			return !opt->loginfo;
 		}
diff --git a/revision.c b/revision.c
index 78397d6486..5e863e19c8 100644
--- a/revision.c
+++ b/revision.c
@@ -593,6 +593,10 @@ static int add_parents_to_list(struct rev_info *revs, struct commit *commit,
 
 		if (parse_commit(p) < 0)
 			return -1;
+		/* skip 1st prnt if there is a 2nd one for --second-parent */
+		if ((revs->first_parent_only == 2) && (parent->next) &&
+		    (parent == commit->parents))
+			continue;
 		if (revs->show_source && !p->util)
 			p->util = commit->util;
 		p->object.flags |= left_flag;
@@ -600,6 +604,7 @@ static int add_parents_to_list(struct rev_info *revs, struct commit *commit,
 			p->object.flags |= SEEN;
 			commit_list_insert_by_date_cached(p, list, cached_base, cache_ptr);
 		}
+		/* only use 1 parent for --first/second-parent */
 		if (revs->first_parent_only)
 			break;
 	}
@@ -1359,6 +1364,8 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 		return argcount;
 	} else if (!strcmp(arg, "--first-parent")) {
 		revs->first_parent_only = 1;
+	} else if (!strcmp(arg, "--second-parent")) {
+		revs->first_parent_only = 2;
 	} else if (!strcmp(arg, "--ancestry-path")) {
 		revs->ancestry_path = 1;
 		revs->simplify_history = 0;
diff --git a/revision.h b/revision.h
index 01bd2b7c07..b4174434e2 100644
--- a/revision.h
+++ b/revision.h
@@ -96,7 +96,7 @@ struct rev_info {
 			cherry_mark:1,
 			bisect:1,
 			ancestry_path:1,
-			first_parent_only:1;
+			first_parent_only:2;
 
 	/* Diff flags */
 	unsigned int	diff:1,
-- 
2.11.4.GIT