Using HTTP2 compliant cipher suites by default.

- Added a way to override the cipher suites with an environment
  variable so that we can still do interop testing with java7.
- Takes care of #681.

src/core/security/security_context.c

@@ -52,20 +52,33 @@
 
 /* -- Constants. -- */
 
-/* Defines the cipher suites that we accept. All these cipher suites are
-   compliant with TLS 1.2 and use an RSA public key. We prefer GCM over CBC
-   and ECDHE-RSA over just RSA. */
-#define GRPC_SSL_CIPHER_SUITES                                                 \
-  "ECDHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:AES128-GCM-SHA256:" \
-  "AES256-GCM-SHA384:ECDHE-RSA-AES128-SHA256:ECDHE-RSA-AES256-SHA384:AES128-"  \
-  "SHA256:AES256-SHA256"
-
 #ifndef INSTALL_PREFIX
 static const char *installed_roots_path = "/usr/share/grpc/roots.pem";
 #else
 static const char *installed_roots_path = INSTALL_PREFIX "/share/grpc/roots.pem";
 #endif
 
+/* -- Cipher suites. -- */
+
+/* Defines the cipher suites that we accept by default. All these cipher suites
+   are compliant with HTTP2. */
+#define GRPC_SSL_CIPHER_SUITES                                                 \
+  "ECDHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:AES128-GCM-SHA256:" \
+  "AES256-GCM-SHA384:ECDHE-RSA-AES128-SHA256:ECDHE-RSA-AES256-SHA384"
+
+static gpr_once cipher_suites_once = GPR_ONCE_INIT;
+static const char *cipher_suites = NULL;
+
+static void init_cipher_suites(void) {
+  char *overridden = gpr_getenv("GRPC_SSL_CIPHER_SUITES");
+  cipher_suites = overridden != NULL ? overridden : GRPC_SSL_CIPHER_SUITES;
+}
+
+static const char *ssl_cipher_suites(void) {
+  gpr_once_init(&cipher_suites_once, init_cipher_suites);
+  return cipher_suites;
+}
+
 /* -- Common methods. -- */
 
 grpc_security_status grpc_security_context_create_handshaker(
@@ -487,7 +500,7 @@ grpc_security_status grpc_ssl_channel_security_context_create(
   result = tsi_create_ssl_client_handshaker_factory(
       config->pem_private_key, config->pem_private_key_size,
       config->pem_cert_chain, config->pem_cert_chain_size, pem_root_certs,
-      pem_root_certs_size, GRPC_SSL_CIPHER_SUITES, alpn_protocol_strings,
+      pem_root_certs_size, ssl_cipher_suites(), alpn_protocol_strings,
       alpn_protocol_string_lengths, num_alpn_protocols, &c->handshaker_factory);
   if (result != TSI_OK) {
     gpr_log(GPR_ERROR, "Handshaker factory creation failed with %s.",
@@ -541,7 +554,7 @@ grpc_security_status grpc_ssl_server_security_context_create(
       (const unsigned char **)config->pem_cert_chains,
       config->pem_cert_chains_sizes, config->num_key_cert_pairs,
       config->pem_root_certs, config->pem_root_certs_size,
-      GRPC_SSL_CIPHER_SUITES, alpn_protocol_strings,
+      ssl_cipher_suites(), alpn_protocol_strings,
       alpn_protocol_string_lengths, num_alpn_protocols, &c->handshaker_factory);
   if (result != TSI_OK) {
     gpr_log(GPR_ERROR, "Handshaker factory creation failed with %s.",