Guard error log underflow in parser

src/RequestParser.cpp

@@ -444,10 +444,12 @@ RequestParser::Status RequestParser::parse(Buffer* buf, int& pos, bool split)
         SString<64> bufHex;
         bufHex.printHex(buf->data() + start, buf->length() - start);
         SString<16> errHex;
-        errHex.printHex(cursor - 1, end - cursor + 1);
+        // Clamp errStart to the buffer head to avoid underflow on first-byte errors.
+        const char* errStart = cursor > buf->data() ? cursor - 1 : cursor;
+        errHex.printHex(errStart, end - errStart);
         logDebug("request parse error %d state %d buf:%s errpos %d err:%s",
                 error, mState, bufHex.data(),
-                pos - 1 - start, errHex.data());
+                (errStart - buf->data()) - start, errHex.data());
         return ParseError;
     }
     return Normal;

test/request_parser_error_log.py

@@ -0,0 +1,32 @@
+#!/usr/bin/env python3
+#
+# Trigger parser errors at the first byte to verify safe error logging.
+#
+
+import socket
+from test_util import parse_args, make_client, exit_with_result
+
+
+def run_test(host, port):
+    try:
+        sock = socket.create_connection((host, port), timeout=1.0)
+        sock.sendall(b"!")
+        sock.close()
+    except Exception as exc:
+        print("WARN: invalid byte send failed:", exc)
+
+    try:
+        c = make_client(host, port)
+        if c.ping() is not True:
+            print("FAIL: ping after invalid byte")
+            return False
+    except Exception as exc:
+        print("FAIL: ping after invalid byte:", exc)
+        return False
+    return True
+
+
+if __name__ == "__main__":
+    args = parse_args("Request parser error logging test")
+    success = run_test(args.host, args.port)
+    exit_with_result(success, "request parser error logging", "request parser error logging")

test/run.sh

@@ -141,6 +141,7 @@ TESTS=(
     "test/pubsub_parser_reset.py"
     "test/null_response_handling.py"
     "test/request_parser_boundary.py"
+    "test/request_parser_error_log.py"
     "test/pubsub_long_name.py"
     "test/pubsub_large_message.py"
     "test/transaction_forbid.py"