trailer: fix leaking strbufs when formatting trailers

When formatting trailer lines we iterate through each of the trailers
and munge their respective token/value pairs according to the trailer
options. When formatting a trailer that has its `item->token` pointer
set we perform the munging in two local buffers. In the case where we
figure out that the value is empty and `trim_empty` is set we just skip
over the trailer item. But the buffers are local to the loop and we
don't release their contents, leading to a memory leak.

Plug this leak by lifting the buffers outside of the loop and releasing
them on function return. This fixes the memory leaks, but also optimizes
the loop as we don't have to reallocate the buffers on every single
iteration.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

t/t7513-interpret-trailers.sh

@@ -5,6 +5,7 @@
 
 test_description='git interpret-trailers'
 
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
 # When we want one trailing space at the end of each line, let's use sed

trailer.c

@@ -1111,16 +1111,18 @@ void format_trailers(const struct process_trailer_options *opts,
 		     struct list_head *trailers,
 		     struct strbuf *out)
 {
+	struct strbuf tok = STRBUF_INIT;
+	struct strbuf val = STRBUF_INIT;
 	size_t origlen = out->len;
 	struct list_head *pos;
 	struct trailer_item *item;
 
 	list_for_each(pos, trailers) {
 		item = list_entry(pos, struct trailer_item, list);
 		if (item->token) {
-			struct strbuf tok = STRBUF_INIT;
-			struct strbuf val = STRBUF_INIT;
+			strbuf_reset(&tok);
 			strbuf_addstr(&tok, item->token);
+			strbuf_reset(&val);
 			strbuf_addstr(&val, item->value);
 
 			/*
@@ -1151,9 +1153,6 @@ void format_trailers(const struct process_trailer_options *opts,
 				if (!opts->separator)
 					strbuf_addch(out, '\n');
 			}
-			strbuf_release(&tok);
-			strbuf_release(&val);
-
 		} else if (!opts->only_trailers) {
 			if (opts->separator && out->len != origlen) {
 				strbuf_addbuf(out, opts->separator);
@@ -1165,6 +1164,9 @@ void format_trailers(const struct process_trailer_options *opts,
 				strbuf_addch(out, '\n');
 		}
 	}
+
+	strbuf_release(&tok);
+	strbuf_release(&val);
 }
 
 void format_trailers_from_commit(const struct process_trailer_options *opts,