[PATCH v2 4/5] config: fix a leak in git_config_copy_or_rename_section_in_file

["Rubén Justo" <rjusto@gmail.com>]

A branch can have its configuration spread over several configuration
sections.  This situation was already foreseen in 52d59cc645 (branch:
add a --copy (-c) option to go with --move (-m), 2017-06-18) when
"branch -c" was introduced.

Unfortunately, a leak was also introduced:

   $ git branch foo
   $ cat >> .git/config <<EOF
   [branch "foo"]
   	some-key-a = a value
   [branch "foo"]
   	some-key-b = b value
   [branch "foo"]
   	some-key-c = c value
   EOF
   $ git branch -c foo bar

   Direct leak of 130 byte(s) in 2 object(s) allocated from:
       ... in xrealloc wrapper.c
       ... in strbuf_grow strbuf.c
       ... in strbuf_vaddf strbuf.c
       ... in strbuf_addf strbuf.c
       ... in store_create_section config.c
       ... in git_config_copy_or_rename_section_in_file config.c
       ... in git_config_copy_section_in_file config.c
       ... in git_config_copy_section config.c
       ... in copy_or_rename_branch builtin/branch.c
       ... in cmd_branch builtin/branch.c
       ... in run_builtin git.c

Let's fix it.

Let's also modify the function store_create_section() so that it stops
returning a strbuf, which is an uncommon pattern in our code base, and
instead, starts receiving the strbuf to use from its caller.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
---
 config.c | 29 +++++++++++++++--------------
 1 file changed, 15 insertions(+), 14 deletions(-)

diff --git a/config.c b/config.c
index 39a7d7422c..c5f4b59ef3 100644
--- a/config.c
+++ b/config.c
@@ -3140,37 +3140,36 @@ static int write_error(const char *filename)
 	return 4;
 }
 
-static struct strbuf store_create_section(const char *key,
-					  const struct config_store_data *store)
+static void store_create_section(const char *key,
+				const struct config_store_data *store,
+				struct strbuf *sb)
 {
 	const char *dot;
 	size_t i;
-	struct strbuf sb = STRBUF_INIT;
 
 	dot = memchr(key, '.', store->baselen);
 	if (dot) {
-		strbuf_addf(&sb, "[%.*s \"", (int)(dot - key), key);
+		strbuf_addf(sb, "[%.*s \"", (int)(dot - key), key);
 		for (i = dot - key + 1; i < store->baselen; i++) {
 			if (key[i] == '"' || key[i] == '\\')
-				strbuf_addch(&sb, '\\');
-			strbuf_addch(&sb, key[i]);
+				strbuf_addch(sb, '\\');
+			strbuf_addch(sb, key[i]);
 		}
-		strbuf_addstr(&sb, "\"]\n");
+		strbuf_addstr(sb, "\"]\n");
 	} else {
-		strbuf_addch(&sb, '[');
-		strbuf_add(&sb, key, store->baselen);
-		strbuf_addstr(&sb, "]\n");
+		strbuf_addch(sb, '[');
+		strbuf_add(sb, key, store->baselen);
+		strbuf_addstr(sb, "]\n");
 	}
-
-	return sb;
 }
 
 static ssize_t write_section(int fd, const char *key,
 			     const struct config_store_data *store)
 {
-	struct strbuf sb = store_create_section(key, store);
+	struct strbuf sb = STRBUF_INIT;
 	ssize_t ret;
 
+	store_create_section(key, store, &sb);
 	ret = write_in_full(fd, sb.buf, sb.len);
 	strbuf_release(&sb);
 
@@ -3833,7 +3832,9 @@ static int git_config_copy_or_rename_section_in_file(const char *config_filename
 						output[0] = '\t';
 					}
 				} else {
-					copystr = store_create_section(new_name, &store);
+					strbuf_reset(&copystr);
+					store_create_section(new_name, &store,
+							     &copystr);
 				}
 			}
 			remove = 0;
-- 
2.40.1