[PATCH 1/3] graph: fix case that hit assert()

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

From: Derrick Stolee <dstolee@microsoft.com>

A failure was reported in "git log --graph --all" with the new
graph-rendering logic. Create a test case that matches the
topology of that example and uses an explicit ref ordering instead
of the "--all" option. The test would fail with the following error:

	graph.c:1228: graph_output_collapsing_line: Assertion
		      `graph->mapping[i - 3] == target' failed.

The situation is a little complicated, so let's break it down.

The assert was introduced by eaf158f8 ("graph API: Use horizontal
lines for more compact graphs", 2009-04-21), which is quite old.
This assert is trying to say that when we complete a horizontal
line with a single slash, it is because we have reached our target.

This assertion is hit when we have two collapsing lines from the
same merge commit, as follows:

    | | | | *
    | |_|_|/|
    |/| | |/
    | | |/|
    | |/| |
    | * | |
    * | | |

It is actually the _second_ collapsing line that hits this assert.
The reason we are in this code path is because we are collapsing
the first line, and it in that case we are hitting our target now
that the horizontal line is complete. However, the second line
cannot be a horizontal line, so it will collapse without horizontal
lines. In this case, it is inappropriate to assert that we have
reached our target, as we need to continue for another column
before reaching the target. Dropping the assert is safe here.

Second, the horizontal lines in that first line drop their coloring.
This is due to a use of graph_line_addch() instead of
graph_line_write_column(). Using a ternary operator to pick the
character is nice for compact code, but we actually need a column
to provide the color.

Helped-by: Jeff King <peff@peff.net>
Reported-by: Bradley Smith <brad@brad-smith.co.uk>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 graph.c                      | 17 ++++++++-------
 t/t4215-log-skewed-merges.sh | 42 ++++++++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+), 8 deletions(-)

diff --git a/graph.c b/graph.c
index 66ae18add8..aaf97069bd 100644
--- a/graph.c
+++ b/graph.c
@@ -1063,7 +1063,7 @@ static void graph_output_post_merge_line(struct git_graph *graph, struct graph_l
 	int i, j;
 
 	struct commit_list *first_parent = first_interesting_parent(graph);
-	int seen_parent = 0;
+	struct column *parent_col = NULL;
 
 	/*
 	 * Output the post-merge row
@@ -1117,12 +1117,17 @@ static void graph_output_post_merge_line(struct git_graph *graph, struct graph_l
 			graph_line_addch(line, ' ');
 		} else {
 			graph_line_write_column(line, col, '|');
-			if (graph->merge_layout != 0 || i != graph->commit_index - 1)
-				graph_line_addch(line, seen_parent ? '_' : ' ');
+			if (graph->merge_layout != 0 || i != graph->commit_index - 1) {
+				if (parent_col)
+					graph_line_write_column(
+						line, parent_col, '_');
+				else
+					graph_line_addch(line, ' ');
+			}
 		}
 
 		if (col_commit == first_parent->item)
-			seen_parent = 1;
+			parent_col = col;
 	}
 
 	/*
@@ -1219,13 +1224,9 @@ static void graph_output_collapsing_line(struct git_graph *graph, struct graph_l
 			 *
 			 * The space just to the left of this
 			 * branch should always be empty.
-			 *
-			 * The branch to the left of that space
-			 * should be our eventual target.
 			 */
 			assert(graph->mapping[i - 1] > target);
 			assert(graph->mapping[i - 2] < 0);
-			assert(graph->mapping[i - 3] == target);
 			graph->mapping[i - 2] = target;
 			/*
 			 * Mark this branch as the horizontal edge to
diff --git a/t/t4215-log-skewed-merges.sh b/t/t4215-log-skewed-merges.sh
index 18709a723e..ddf6f6f5d3 100755
--- a/t/t4215-log-skewed-merges.sh
+++ b/t/t4215-log-skewed-merges.sh
@@ -240,4 +240,46 @@ test_expect_success 'log --graph with octopus merge with column joining its penu
 	EOF
 '
 
+test_expect_success 'log --graph with multiple tips' '
+	git checkout --orphan 6_1 &&
+	test_commit 6_A &&
+	git branch 6_2 &&
+	git branch 6_4 &&
+	test_commit 6_B &&
+	git branch 6_3 &&
+	test_commit 6_C &&
+	git checkout 6_2 && test_commit 6_D &&
+	git checkout 6_3 && test_commit 6_E &&
+	git checkout -b 6_5 6_1 &&
+	git merge --no-ff 6_2 -m 6_F &&
+	git checkout 6_4 && test_commit 6_G &&
+	git checkout 6_3 &&
+	git merge --no-ff 6_4 -m 6_H &&
+	git checkout 6_1 &&
+	git merge --no-ff 6_2 -m 6_I &&
+
+	check_graph 6_1 6_3 6_5 <<-\EOF
+	*   6_I
+	|\
+	| | *   6_H
+	| | |\
+	| | | * 6_G
+	| | * | 6_E
+	| | | | * 6_F
+	| |_|_|/|
+	|/| | |/
+	| | |/|
+	| |/| |
+	| * | | 6_D
+	| | |/
+	| |/|
+	* | | 6_C
+	| |/
+	|/|
+	* | 6_B
+	|/
+	* 6_A
+	EOF
+'
+
 test_done
-- 
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