log: add --show-linear-break to help see non-linear history

Option explanation is in rev-list-options.txt. The interaction with -z
is left undecided.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/rev-list-options.txt

@@ -750,6 +750,13 @@ This enables parent rewriting, see 'History Simplification' below.
 This implies the `--topo-order` option by default, but the
 `--date-order` option may also be specified.
 
+--show-linear-break[=<barrier>]::
+	When --graph is not used, all history branches are flattened
+	which can make it hard to see that the two consecutive commits
+	do not belong to a linear branch. This option puts a barrier
+	in between them in that case. If `<barrier>` is specified, it
+	is the string that will be shown instead of the default one.
+
 ifdef::git-rev-list[]
 --count::
 	Print a number stating how many commits would have been

log-tree.c

@@ -805,12 +805,16 @@ int log_tree_commit(struct rev_info *opt, struct commit *commit)
 	if (opt->line_level_traverse)
 		return line_log_print(opt, commit);
 
+	if (opt->track_linear && !opt->linear && !opt->reverse_output_stage)
+		printf("\n%s\n", opt->break_bar);
 	shown = log_tree_diff(opt, commit, &log);
 	if (!shown && opt->loginfo && opt->always_show_header) {
 		log.parent = NULL;
 		show_log(opt);
 		shown = 1;
 	}
+	if (opt->track_linear && !opt->linear && opt->reverse_output_stage)
+		printf("\n%s\n", opt->break_bar);
 	opt->loginfo = NULL;
 	maybe_flush_or_die(stdout, "stdout");
 	return shown;

object.h

@@ -28,7 +28,7 @@ struct object_array {
 #define TYPE_BITS   3
 /*
  * object flag allocation:
- * revision.h:      0---------10
+ * revision.h:      0---------10                                26
  * fetch-pack.c:    0---4
  * walker.c:        0-2
  * upload-pack.c:               11----------------19

revision.c

@@ -1832,6 +1832,14 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 		revs->notes_opt.use_default_notes = 1;
 	} else if (!strcmp(arg, "--show-signature")) {
 		revs->show_signature = 1;
+	} else if (!strcmp(arg, "--show-linear-break") ||
+		   starts_with(arg, "--show-linear-break=")) {
+		if (starts_with(arg, "--show-linear-break="))
+			revs->break_bar = xstrdup(arg + 20);
+		else
+			revs->break_bar = "                    ..........";
+		revs->track_linear = 1;
+		revs->track_first_time = 1;
 	} else if (starts_with(arg, "--show-notes=") ||
 		   starts_with(arg, "--notes=")) {
 		struct strbuf buf = STRBUF_INIT;
@@ -1955,6 +1963,8 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 			unkv[(*unkc)++] = arg;
 		return opts;
 	}
+	if (revs->graph && revs->track_linear)
+		die("--show-linear-break and --graph are incompatible");
 
 	return 1;
 }
@@ -2897,6 +2907,27 @@ enum commit_action simplify_commit(struct rev_info *revs, struct commit *commit)
 	return action;
 }
 
+static void track_linear(struct rev_info *revs, struct commit *commit)
+{
+	if (revs->track_first_time) {
+		revs->linear = 1;
+		revs->track_first_time = 0;
+	} else {
+		struct commit_list *p;
+		for (p = revs->previous_parents; p; p = p->next)
+			if (p->item == NULL || /* first commit */
+			    !hashcmp(p->item->object.sha1, commit->object.sha1))
+				break;
+		revs->linear = p != NULL;
+	}
+	if (revs->reverse) {
+		if (revs->linear)
+			commit->object.flags |= TRACK_LINEAR;
+	}
+	free_commit_list(revs->previous_parents);
+	revs->previous_parents = copy_commit_list(commit->parents);
+}
+
 static struct commit *get_revision_1(struct rev_info *revs)
 {
 	if (!revs->commits)
@@ -2936,6 +2967,8 @@ static struct commit *get_revision_1(struct rev_info *revs)
 			die("Failed to simplify parents of commit %s",
 			    sha1_to_hex(commit->object.sha1));
 		default:
+			if (revs->track_linear)
+				track_linear(revs, commit);
 			return commit;
 		}
 	} while (revs->commits);
@@ -3102,14 +3135,23 @@ struct commit *get_revision(struct rev_info *revs)
 		revs->reverse_output_stage = 1;
 	}
 
-	if (revs->reverse_output_stage)
-		return pop_commit(&revs->commits);
+	if (revs->reverse_output_stage) {
+		c = pop_commit(&revs->commits);
+		if (revs->track_linear)
+			revs->linear = !!(c && c->object.flags & TRACK_LINEAR);
+		return c;
+	}
 
 	c = get_revision_internal(revs);
 	if (c && revs->graph)
 		graph_update(revs->graph, c);
-	if (!c)
+	if (!c) {
 		free_saved_parents(revs);
+		if (revs->previous_parents) {
+			free_commit_list(revs->previous_parents);
+			revs->previous_parents = NULL;
+		}
+	}
 	return c;
 }
 

revision.h

@@ -19,7 +19,8 @@
 #define SYMMETRIC_LEFT	(1u<<8)
 #define PATCHSAME	(1u<<9)
 #define BOTTOM		(1u<<10)
-#define ALL_REV_FLAGS	((1u<<11)-1)
+#define TRACK_LINEAR	(1u<<26)
+#define ALL_REV_FLAGS	(((1u<<11)-1) | TRACK_LINEAR)
 
 #define DECORATE_SHORT_REFS	1
 #define DECORATE_FULL_REFS	2
@@ -138,6 +139,10 @@ struct rev_info {
 			preserve_subject:1;
 	unsigned int	disable_stdin:1;
 	unsigned int	leak_pending:1;
+	/* --show-linear-break */
+	unsigned int	track_linear:1,
+			track_first_time:1,
+			linear:1;
 
 	enum date_mode date_mode;
 
@@ -196,6 +201,9 @@ struct rev_info {
 
 	/* copies of the parent lists, for --full-diff display */
 	struct saved_parents *saved_parents_slab;
+
+	struct commit_list *previous_parents;
+	const char *break_bar;
 };
 
 extern int ref_excluded(struct string_list *, const char *path);