Extract MySQL protocol utilities for FFTO unit testing (#5499)

New: include/MySQLProtocolUtils.h, lib/MySQLProtocolUtils.cpp

Functions:
- mysql_read_lenenc_int(): reads MySQL length-encoded integers from
  raw buffers (mirrors read_lenenc_int in MySQLFFTO.cpp)
- mysql_build_packet(): constructs MySQL wire-format packets (3-byte
  length + seq_id + payload) for crafting test data

include/MySQLProtocolUtils.h

@@ -0,0 +1,51 @@
+/**
+ * @file MySQLProtocolUtils.h
+ * @brief Pure MySQL protocol utility functions for unit testability.
+ *
+ * Extracted from MySQLFFTO for testing. These are low-level protocol
+ * parsing helpers that operate on raw byte buffers.
+ *
+ * @see FFTO unit testing (GitHub issue #5499)
+ */
+
+#ifndef MYSQL_PROTOCOL_UTILS_H
+#define MYSQL_PROTOCOL_UTILS_H
+
+#include <cstdint>
+#include <cstddef>
+
+/**
+ * @brief Read a MySQL length-encoded integer from a buffer.
+ *
+ * MySQL length-encoded integers use 1-9 bytes:
+ *   - 0x00-0xFA: 1 byte (value itself)
+ *   - 0xFC: 2 bytes follow (uint16)
+ *   - 0xFD: 3 bytes follow (uint24)
+ *   - 0xFE: 8 bytes follow (uint64)
+ *
+ * @param buf  [in/out] Pointer to current position; advanced past the integer.
+ * @param len  [in/out] Remaining buffer length; decremented.
+ * @return Decoded 64-bit integer value (0 on error/truncation).
+ */
+uint64_t mysql_read_lenenc_int(const unsigned char* &buf, size_t &len);
+
+/**
+ * @brief Build a MySQL protocol packet header + payload.
+ *
+ * Constructs a complete MySQL wire-format packet: 3-byte length +
+ * 1-byte sequence number + payload.
+ *
+ * @param payload      Payload data.
+ * @param payload_len  Length of payload.
+ * @param seq_id       Packet sequence number.
+ * @param out_buf      Output buffer (must be at least payload_len + 4).
+ * @return Total packet size (payload_len + 4).
+ */
+size_t mysql_build_packet(
+	const unsigned char *payload,
+	uint32_t payload_len,
+	uint8_t seq_id,
+	unsigned char *out_buf
+);
+
+#endif // MYSQL_PROTOCOL_UTILS_H

lib/Makefile

@@ -109,6 +109,7 @@ _OBJ_CXX := ProxySQL_GloVars.oo network.oo debug.oo configfile.oo Query_Cache.oo
 	TransactionState.oo \
 	HostgroupRouting.oo \
 	PgSQLCommandComplete.oo \
+	MySQLProtocolUtils.oo \
 	proxy_sqlite3_symbols.oo
 
 # TSDB object files

lib/MySQLProtocolUtils.cpp

@@ -0,0 +1,59 @@
+/**
+ * @file MySQLProtocolUtils.cpp
+ * @brief Implementation of MySQL protocol utility functions.
+ *
+ * @see MySQLProtocolUtils.h
+ */
+
+#include "MySQLProtocolUtils.h"
+#include <cstring>
+
+uint64_t mysql_read_lenenc_int(const unsigned char* &buf, size_t &len) {
+	if (len == 0) return 0;
+	uint8_t first_byte = buf[0];
+	buf++; len--;
+	if (first_byte < 0xFB) return first_byte;
+	if (first_byte == 0xFC) {
+		if (len < 2) return 0;
+		uint64_t value = buf[0] | (static_cast<uint64_t>(buf[1]) << 8);
+		buf += 2; len -= 2;
+		return value;
+	}
+	if (first_byte == 0xFD) {
+		if (len < 3) return 0;
+		uint64_t value = buf[0] | (static_cast<uint64_t>(buf[1]) << 8)
+			| (static_cast<uint64_t>(buf[2]) << 16);
+		buf += 3; len -= 3;
+		return value;
+	}
+	if (first_byte == 0xFE) {
+		if (len < 8) return 0;
+		uint64_t value = buf[0] | (static_cast<uint64_t>(buf[1]) << 8)
+			| (static_cast<uint64_t>(buf[2]) << 16)
+			| (static_cast<uint64_t>(buf[3]) << 24)
+			| (static_cast<uint64_t>(buf[4]) << 32)
+			| (static_cast<uint64_t>(buf[5]) << 40)
+			| (static_cast<uint64_t>(buf[6]) << 48)
+			| (static_cast<uint64_t>(buf[7]) << 56);
+		buf += 8; len -= 8;
+		return value;
+	}
+	return 0;
+}
+
+size_t mysql_build_packet(
+	const unsigned char *payload,
+	uint32_t payload_len,
+	uint8_t seq_id,
+	unsigned char *out_buf)
+{
+	// 3-byte length (little-endian) + 1-byte sequence
+	out_buf[0] = payload_len & 0xFF;
+	out_buf[1] = (payload_len >> 8) & 0xFF;
+	out_buf[2] = (payload_len >> 16) & 0xFF;
+	out_buf[3] = seq_id;
+	if (payload && payload_len > 0) {
+		memcpy(out_buf + 4, payload, payload_len);
+	}
+	return payload_len + 4;
+}