Test SSL_get0_alpn_selected on both client and server.

On the server, we echo the selected ALPN back out of the same getter as
the client.

Change-Id: I11978b9cd0a3e7c611f162e9cf5054e52005a195
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/43045
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

ssl/test/bssl_shim.cc

@@ -524,16 +524,18 @@ static bool CheckHandshakeProperties(SSL *ssl, bool is_resume,
     }
   }
 
-  if (!config->is_server) {
-    const uint8_t *alpn_proto;
-    unsigned alpn_proto_len;
-    SSL_get0_alpn_selected(ssl, &alpn_proto, &alpn_proto_len);
-    if (alpn_proto_len != config->expect_alpn.size() ||
-        OPENSSL_memcmp(alpn_proto, config->expect_alpn.data(),
-                       alpn_proto_len) != 0) {
-      fprintf(stderr, "negotiated alpn proto mismatch\n");
-      return false;
-    }
+  // On the server, the protocol selected in the ALPN callback must be echoed
+  // out of |SSL_get0_alpn_selected|. On the client, it should report what the
+  // test expected.
+  const std::string &expect_alpn =
+      config->is_server ? config->select_alpn : config->expect_alpn;
+  const uint8_t *alpn_proto;
+  unsigned alpn_proto_len;
+  SSL_get0_alpn_selected(ssl, &alpn_proto, &alpn_proto_len);
+  if (alpn_proto_len != expect_alpn.size() ||
+      OPENSSL_memcmp(alpn_proto, expect_alpn.data(), alpn_proto_len) != 0) {
+    fprintf(stderr, "negotiated alpn proto mismatch\n");
+    return false;
   }
 
   if (!config->expect_quic_transport_params.empty() && expect_handshake_done) {