Add SSLKEYLOGFILE support

src/h2load.cc

@@ -75,6 +75,15 @@ bool recorded(const std::chrono::steady_clock::time_point &t) {
 }
 } // namespace
 
+namespace {
+std::ofstream keylog_file;
+void keylog_callback(const SSL *ssl, const char *line) {
+  keylog_file.write(line, strlen(line));
+  keylog_file.put('\n');
+  keylog_file.flush();
+}
+} // namespace
+
 Config::Config()
     : ciphers(tls::DEFAULT_CIPHER_LIST),
       tls13_ciphers("TLS_AES_128_GCM_SHA256:TLS_AES_256_GCM_SHA384:TLS_"
@@ -2723,6 +2732,14 @@ int main(int argc, char **argv) {
   SSL_CTX_set_alpn_protos(ssl_ctx, proto_list.data(), proto_list.size());
 #endif // OPENSSL_VERSION_NUMBER >= 0x10002000L
 
+  auto keylog_filename = getenv("SSLKEYLOGFILE");
+  if (keylog_filename) {
+    keylog_file.open(keylog_filename, std::ios_base::app);
+    if (keylog_file) {
+      SSL_CTX_set_keylog_callback(ssl_ctx, keylog_callback);
+    }
+  }
+
   std::string user_agent = "h2load nghttp2/" NGHTTP2_VERSION;
   Headers shared_nva;
   shared_nva.emplace_back(":scheme", config.scheme);