Fix issue 1567 (#1570)

### Description

closes #1567

When TLS is enabled, `mc.netConn` is rewritten after the TLS handshak as
detailed here:


https://github.com/go-sql-driver/mysql/blob/d86c4527bae98ccd4e5060f72887520ce30eda5e/packets.go#L355

Therefore, `mc.netConn` should not be accessed within the watcher
goroutine.
Instead, `mc.rawConn` should be initialized prior to invoking
`mc.startWatcher`, and `mc.rawConn` should be used in lieu of
`mc.netConn`.

### Checklist
- [x] Code compiles correctly
- [x] Created tests which fail without the change (if possible)
- [x] All tests passing
- [x] Extended the README / documentation, if necessary
- [x] Added myself / the copyright holder to the AUTHORS file


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Refactor**
	- Improved variable naming for better code readability and maintenance.
	- Enhanced network connection handling logic.
- **New Features**
	- Updated TCP connection handling to better support TCP Keepalives.
- **Tests**
- Added a new test to address and verify the fix for a specific issue
related to TLS, connection pooling, and round trip time estimation.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: shogo82148

connection.go

@@ -153,11 +153,11 @@ func (mc *mysqlConn) cleanup() {
 
 	// Makes cleanup idempotent
 	close(mc.closech)
-	nc := mc.netConn
-	if nc == nil {
+	conn := mc.rawConn
+	if conn == nil {
 		return
 	}
-	if err := nc.Close(); err != nil {
+	if err := conn.Close(); err != nil {
 		mc.log(err)
 	}
 	// This function can be called from multiple goroutines.

connector.go

@@ -102,10 +102,10 @@ func (c *connector) Connect(ctx context.Context) (driver.Conn, error) {
 		nd := net.Dialer{Timeout: mc.cfg.Timeout}
 		mc.netConn, err = nd.DialContext(ctx, mc.cfg.Net, mc.cfg.Addr)
 	}
-
 	if err != nil {
 		return nil, err
 	}
+	mc.rawConn = mc.netConn
 
 	// Enable TCP Keepalives on TCP connections
 	if tc, ok := mc.netConn.(*net.TCPConn); ok {

driver_test.go

@@ -20,6 +20,7 @@ import (
 	"io"
 	"log"
 	"math"
+	mrand "math/rand"
 	"net"
 	"net/url"
 	"os"
@@ -3577,3 +3578,35 @@ func runCallCommand(dbt *DBTest, query, name string) {
 		}
 	}
 }
+
+func TestIssue1567(t *testing.T) {
+	// enable TLS.
+	runTests(t, dsn+"&tls=skip-verify", func(dbt *DBTest) {
+		// disable connection pooling.
+		// data race happens when new connection is created.
+		dbt.db.SetMaxIdleConns(0)
+
+		// estimate round trip time.
+		start := time.Now()
+		if err := dbt.db.PingContext(context.Background()); err != nil {
+			t.Fatal(err)
+		}
+		rtt := time.Since(start)
+		if rtt <= 0 {
+			// In some environments, rtt may become 0, so set it to at least 1ms.
+			rtt = time.Millisecond
+		}
+
+		count := 1000
+		if testing.Short() {
+			count = 10
+		}
+
+		for i := 0; i < count; i++ {
+			timeout := time.Duration(mrand.Int63n(int64(rtt)))
+			ctx, cancel := context.WithTimeout(context.Background(), timeout)
+			dbt.db.PingContext(ctx)
+			cancel()
+		}
+	})
+}

packets.go

@@ -351,7 +351,6 @@ func (mc *mysqlConn) writeHandshakeResponsePacket(authResp []byte, plugin string
 		if err := tlsConn.Handshake(); err != nil {
 			return err
 		}
-		mc.rawConn = mc.netConn
 		mc.netConn = tlsConn
 		mc.buf.nc = tlsConn
 	}