Add CBB_zero to set a CBB to the zero state.

One tedious thing about using CBB is that you can't safely CBB_cleanup
until CBB_init is successful, which breaks the general 'goto err' style
of cleanup. This makes it possible:

  CBB_zero ~ EVP_MD_CTX_init
  CBB_init ~ EVP_DigestInit
  CBB_cleanup ~ EVP_MD_CTX_cleanup

Change-Id: I085ecc4405715368886dc4de02285a47e7fc4c52
Reviewed-on: https://boringssl-review.googlesource.com/5267
Reviewed-by: Adam Langley <agl@google.com>

2 files changed, 14 insertions, 1 deletions

diff --git a/crypto/bytestring/bytestring_test.cc b/crypto/bytestring/bytestring_test.cc
index 870d7c6a..e987e1b9 100644
--- a/crypto/bytestring/bytestring_test.cc
+++ b/crypto/bytestring/bytestring_test.cc
@@ -649,6 +649,14 @@ static bool TestASN1Uint64() {
   return true;
 }
 
+static int TestZero() {
+  CBB cbb;
+  CBB_zero(&cbb);
+  // Calling |CBB_cleanup| on a zero-state |CBB| must not crash.
+  CBB_cleanup(&cbb);
+  return 1;
+}
+
 int main(void) {
   CRYPTO_library_init();
 
@@ -665,7 +673,8 @@ int main(void) {
       !TestCBBASN1() ||
       !TestBerConvert() ||
       !TestASN1Uint64() ||
-      !TestGetOptionalASN1Bool()) {
+      !TestGetOptionalASN1Bool() ||
+      !TestZero()) {
     return 1;
   }
 
diff --git a/crypto/bytestring/cbb.c b/crypto/bytestring/cbb.c
index 290d16fa..b9291ce9 100644
--- a/crypto/bytestring/cbb.c
+++ b/crypto/bytestring/cbb.c
@@ -20,6 +20,10 @@
 #include <openssl/mem.h>
 
 
+void CBB_zero(CBB *cbb) {
+  memset(cbb, 0, sizeof(CBB));
+}
+
 static int cbb_init(CBB *cbb, uint8_t *buf, size_t cap) {
   struct cbb_buffer_st *base;