Fix git log --graph -u hangs (#5193)

This fixes https://github.com/git-for-windows/git/issues/5185 by
backporting https://github.com/gitgitgadget/git/pull/1806 (which, sadly,
seems not to have made it into Git v2.47.0).

diff-lib.c

@@ -701,7 +701,7 @@ int index_differs_from(struct repository *r,
 	return (has_changes != 0);
 }
 
-static struct strbuf *idiff_prefix_cb(struct diff_options *opt UNUSED, void *data)
+static const char *idiff_prefix_cb(struct diff_options *opt UNUSED, void *data)
 {
 	return data;
 }
@@ -716,7 +716,7 @@ void show_interdiff(const struct object_id *oid1, const struct object_id *oid2,
 	opts.output_format = DIFF_FORMAT_PATCH;
 	opts.output_prefix = idiff_prefix_cb;
 	strbuf_addchars(&prefix, ' ', indent);
-	opts.output_prefix_data = &prefix;
+	opts.output_prefix_data = prefix.buf;
 	diff_setup_done(&opts);
 
 	diff_tree_oid(oid1, oid2, "", &opts);

diff.c

@@ -2317,12 +2317,10 @@ const char *diff_get_color(int diff_use_color, enum color_diff ix)
 
 const char *diff_line_prefix(struct diff_options *opt)
 {
-	struct strbuf *msgbuf;
-	if (!opt->output_prefix)
-		return "";
+	if (opt->output_prefix)
+		return opt->output_prefix(opt, opt->output_prefix_data);
 
-	msgbuf = opt->output_prefix(opt, opt->output_prefix_data);
-	return msgbuf->buf;
+	return "";
 }
 
 static unsigned long sane_truncate_line(char *line, unsigned long len)

diff.h

@@ -94,7 +94,7 @@ typedef void (*add_remove_fn_t)(struct diff_options *options,
 typedef void (*diff_format_fn_t)(struct diff_queue_struct *q,
 		struct diff_options *options, void *data);
 
-typedef struct strbuf *(*diff_prefix_fn_t)(struct diff_options *opt, void *data);
+typedef const char *(*diff_prefix_fn_t)(struct diff_options *opt, void *data);
 
 #define DIFF_FORMAT_RAW		0x0001
 #define DIFF_FORMAT_DIFFSTAT	0x0002

graph.c

@@ -314,7 +314,7 @@ struct git_graph {
 	unsigned short default_column_color;
 };
 
-static struct strbuf *diff_output_prefix_callback(struct diff_options *opt, void *data)
+static const char *diff_output_prefix_callback(struct diff_options *opt, void *data)
 {
 	struct git_graph *graph = data;
 	static struct strbuf msgbuf = STRBUF_INIT;
@@ -327,7 +327,7 @@ static struct strbuf *diff_output_prefix_callback(struct diff_options *opt, void
 			   opt->line_prefix_length);
 	if (graph)
 		graph_padding_line(graph, &msgbuf);
-	return &msgbuf;
+	return msgbuf.buf;
 }
 
 static const struct diff_options *default_diffopt;

line-log.c

@@ -897,16 +897,6 @@ static void print_line(const char *prefix, char first,
 		fputs("\\ No newline at end of file\n", file);
 }
 
-static char *output_prefix(struct diff_options *opt)
-{
-	if (opt->output_prefix) {
-		struct strbuf *sb = opt->output_prefix(opt, opt->output_prefix_data);
-		return sb->buf;
-	} else {
-		return xstrdup("");
-	}
-}
-
 static void dump_diff_hacky_one(struct rev_info *rev, struct line_log_data *range)
 {
 	unsigned int i, j = 0;
@@ -916,7 +906,7 @@ static void dump_diff_hacky_one(struct rev_info *rev, struct line_log_data *rang
 	struct diff_ranges *diff = &range->diff;
 
 	struct diff_options *opt = &rev->diffopt;
-	char *prefix = output_prefix(opt);
+	const char *prefix = diff_line_prefix(opt);
 	const char *c_reset = diff_get_color(opt->use_color, DIFF_RESET);
 	const char *c_frag = diff_get_color(opt->use_color, DIFF_FRAGINFO);
 	const char *c_meta = diff_get_color(opt->use_color, DIFF_METAINFO);
@@ -1003,7 +993,6 @@ static void dump_diff_hacky_one(struct rev_info *rev, struct line_log_data *rang
 out:
 	free(p_ends);
 	free(t_ends);
-	free(prefix);
 }
 
 /*
@@ -1012,10 +1001,9 @@ out:
  */
 static void dump_diff_hacky(struct rev_info *rev, struct line_log_data *range)
 {
-	char *prefix = output_prefix(&rev->diffopt);
+	const char *prefix = diff_line_prefix(&rev->diffopt);
 
 	fprintf(rev->diffopt.file, "%s\n", prefix);
-	free(prefix);
 
 	while (range) {
 		dump_diff_hacky_one(rev, range);

log-tree.c

@@ -923,10 +923,10 @@ int log_tree_diff_flush(struct rev_info *opt)
 			 */
 			int pch = DIFF_FORMAT_DIFFSTAT | DIFF_FORMAT_PATCH;
 			if (opt->diffopt.output_prefix) {
-				struct strbuf *msg = NULL;
+				const char *msg;
 				msg = opt->diffopt.output_prefix(&opt->diffopt,
 					opt->diffopt.output_prefix_data);
-				fwrite(msg->buf, msg->len, 1, opt->diffopt.file);
+				fwrite(msg, strlen(msg), 1, opt->diffopt.file);
 			}
 
 			/*

range-diff.c

@@ -480,7 +480,7 @@ static void patch_diff(const char *a, const char *b,
 	diff_flush(diffopt);
 }
 
-static struct strbuf *output_prefix_cb(struct diff_options *opt UNUSED, void *data)
+static const char *output_prefix_cb(struct diff_options *opt UNUSED, void *data)
 {
 	return data;
 }
@@ -508,7 +508,7 @@ static void output(struct string_list *a, struct string_list *b,
 	opts.flags.suppress_hunk_header_line_count = 1;
 	opts.output_prefix = output_prefix_cb;
 	strbuf_addstr(&indent, "    ");
-	opts.output_prefix_data = &indent;
+	opts.output_prefix_data = indent.buf;
 	diff_setup_done(&opts);
 
 	/*

t/t4211-line-log.sh

@@ -337,4 +337,32 @@ test_expect_success 'zero-width regex .* matches any function name' '
 	test_cmp expect actual
 '
 
+test_expect_success 'show line-log with graph' '
+	qz_to_tab_space >expect <<-EOF &&
+	* $head_oid Modify func2() in file.c
+	|Z
+	| diff --git a/file.c b/file.c
+	| --- a/file.c
+	| +++ b/file.c
+	| @@ -6,4 +6,4 @@
+	|  int func2()
+	|  {
+	| -    return F2;
+	| +    return F2 + 2;
+	|  }
+	* $root_oid Add func1() and func2() in file.c
+	ZZ
+	  diff --git a/file.c b/file.c
+	  --- /dev/null
+	  +++ b/file.c
+	  @@ -0,0 +6,4 @@
+	  +int func2()
+	  +{
+	  +    return F2;
+	  +}
+	EOF
+	git log --graph --oneline -L:func2:file.c >actual &&
+	test_cmp expect actual
+'
+
 test_done