diff: store graph prefix buf in git_graph struct

The diffopt output_prefix interface makes it the callback's job to
handle ownership of the memory it returns, keeping it valid while
callers are using it and then eventually freeing it when we are done
diffing.

In diff_output_prefix_callback() we handle this with a static strbuf,
effectively "leaking" it when the diff is done (but not triggering any
leak detectors because it's technically still reachable). This has not
been a big problem in practice, but it is a problem for libification:
two diffs running in the same process could stomp on each other's prefix
buffers.

Since we only need the strbuf when we are formatting graph padding, we
can give ownership of the strbuf to the git_graph struct, letting us
free it when that struct is no longer in use.

Signed-off-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: peff

graph.c

@@ -307,24 +307,28 @@ struct git_graph {
 	 * stored as an index into the array column_colors.
 	 */
 	unsigned short default_column_color;
+
+	/*
+	 * Scratch buffer for generating prefixes to be used with
+	 * diff_output_prefix_callback().
+	 */
+	struct strbuf prefix_buf;
 };
 
 static const char *diff_output_prefix_callback(struct diff_options *opt, void *data)
 {
 	struct git_graph *graph = data;
-	static struct strbuf msgbuf = STRBUF_INIT;
 
 	assert(opt);
 
 	if (!graph)
 		return opt->line_prefix;
 
-	strbuf_reset(&msgbuf);
+	strbuf_reset(&graph->prefix_buf);
 	if (opt->line_prefix)
-		strbuf_addstr(&msgbuf, opt->line_prefix);
-	if (graph)
-		graph_padding_line(graph, &msgbuf);
-	return msgbuf.buf;
+		strbuf_addstr(&graph->prefix_buf, opt->line_prefix);
+	graph_padding_line(graph, &graph->prefix_buf);
+	return graph->prefix_buf.buf;
 }
 
 static const struct diff_options *default_diffopt;
@@ -394,6 +398,7 @@ struct git_graph *graph_init(struct rev_info *opt)
 	 * The diff output prefix callback, with this we can make
 	 * all the diff output to align with the graph lines.
 	 */
+	strbuf_init(&graph->prefix_buf, 0);
 	opt->diffopt.output_prefix = diff_output_prefix_callback;
 	opt->diffopt.output_prefix_data = graph;
 
@@ -409,6 +414,7 @@ void graph_clear(struct git_graph *graph)
 	free(graph->new_columns);
 	free(graph->mapping);
 	free(graph->old_mapping);
+	strbuf_release(&graph->prefix_buf);
 	free(graph);
 }