mailinfo: also free strbuf lists when clearing mailinfo

mailinfo.p_hdr_info/s_hdr_info are null-terminated lists of strbuf's,
with entries pointing either to NULL or an allocated strbuf. Therefore
we need to free those strbuf's (and not just the data they contain)
whenever we're done with a given entry. (See handle_header() where those
new strbufs are malloc'd.)

Once we no longer need the list (and not just its entries) we can switch
over to strbuf_list_free() instead of manually iterating over the list,
which takes care of those additional details for us. We can only do this
in clear_mailinfo() - in handle_commit_message() we are only clearing the
array contents but want to reuse the array itself, hence we can't use
strbuf_list_free() there.

However, strbuf_list_free() cannot handle a NULL input, and the lists we
are freeing might be NULL. Therefore we add a NULL check in
strbuf_list_free() to make it safe to use with a NULL input (which is a
pattern used by some of the other *_free() functions around git).

Leak output from t0023:

Direct leak of 72 byte(s) in 3 object(s) allocated from:
    #0 0x49a85d in malloc ../projects/compiler-rt/lib/asan/asan_malloc_linux.cpp:145:3
    #1 0x9ac9f4 in do_xmalloc wrapper.c:41:8
    #2 0x9ac9ca in xmalloc wrapper.c:62:9
    #3 0x7f6cf7 in handle_header mailinfo.c:205:10
    #4 0x7f5abf in check_header mailinfo.c:583:4
    #5 0x7f5524 in mailinfo mailinfo.c:1197:3
    #6 0x4dcc95 in parse_mail builtin/am.c:1167:6
    #7 0x4d9070 in am_run builtin/am.c:1732:12
    #8 0x4d5b7a in cmd_am builtin/am.c:2398:3
    #9 0x4cd91d in run_builtin git.c:467:11
    #10 0x4cb5f3 in handle_builtin git.c:719:3
    #11 0x4ccf47 in run_argv git.c:808:4
    #12 0x4caf49 in cmd_main git.c:939:19
    #13 0x69e43e in main common-main.c:52:11
    #14 0x7fc1fadfa349 in __libc_start_main (/lib64/libc.so.6+0x24349)

SUMMARY: AddressSanitizer: 72 byte(s) leaked in 3 allocation(s).

Signed-off-by: Andrzej Hunt <ajrhunt@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mailinfo.c

@@ -821,7 +821,7 @@ static int handle_commit_msg(struct mailinfo *mi, struct strbuf *line)
 		for (i = 0; header[i]; i++) {
 			if (mi->s_hdr_data[i])
 				strbuf_release(mi->s_hdr_data[i]);
-			mi->s_hdr_data[i] = NULL;
+			FREE_AND_NULL(mi->s_hdr_data[i]);
 		}
 		return 0;
 	}
@@ -1236,22 +1236,14 @@ void setup_mailinfo(struct mailinfo *mi)
 
 void clear_mailinfo(struct mailinfo *mi)
 {
-	int i;
-
 	strbuf_release(&mi->name);
 	strbuf_release(&mi->email);
 	strbuf_release(&mi->charset);
 	strbuf_release(&mi->inbody_header_accum);
 	free(mi->message_id);
 
-	if (mi->p_hdr_data)
-		for (i = 0; mi->p_hdr_data[i]; i++)
-			strbuf_release(mi->p_hdr_data[i]);
-	free(mi->p_hdr_data);
-	if (mi->s_hdr_data)
-		for (i = 0; mi->s_hdr_data[i]; i++)
-			strbuf_release(mi->s_hdr_data[i]);
-	free(mi->s_hdr_data);
+	strbuf_list_free(mi->p_hdr_data);
+	strbuf_list_free(mi->s_hdr_data);
 
 	while (mi->content < mi->content_top) {
 		free(*(mi->content_top));

strbuf.c

@@ -209,6 +209,8 @@ void strbuf_list_free(struct strbuf **sbs)
 {
 	struct strbuf **s = sbs;
 
+	if (!s)
+		return;
 	while (*s) {
 		strbuf_release(*s);
 		free(*s++);