fix: prevent connection pollution on context cancel in BeginTx

main.go

@@ -1,7 +1,88 @@
 package main
 
-import "fmt"
+import (
+	"context"
+	"database/sql/driver"
+	"errors"
+	"fmt"
+)
+
+// conn represents a physical MySQL connection.
+type conn struct {
+	id     int
+	closed bool
+}
+
+// BeginTx implements driver.ConnBeginTx.
+//
+// When the context is canceled during transaction initiation, the connection's
+// transaction state is indeterminate — START TRANSACTION may have already been
+// sent to the server. Returning only the context error causes database/sql to
+// treat the connection as healthy and return it to the pool, where subsequent
+// queries inherit the open transaction.
+//
+// By returning driver.ErrBadConn, we signal database/sql to discard this
+// connection and open a fresh one, preventing connection pollution.
+func (c *conn) BeginTx(ctx context.Context, opts driver.TxOptions) (driver.Tx, error) {
+	// Simulate network roundtrip for START TRANSACTION
+	done := make(chan struct{})
+	var tx driver.Tx
+
+	go func() {
+		tx = &transaction{conn: c}
+		close(done)
+	}()
+
+	select {
+	case <-ctx.Done():
+		// Context canceled — transaction state is indeterminate.
+		// Attempt ROLLBACK to clean up server-side state. If ROLLBACK
+		// fails because the context is expired, discard the connection.
+		if err := c.rollback(ctx); err != nil {
+			return nil, driver.ErrBadConn
+		}
+		return nil, ctx.Err()
+	case <-done:
+		if err := ctx.Err(); err != nil {
+			c.rollback(context.Background())
+			return nil, driver.ErrBadConn
+		}
+		return tx, nil
+	}
+}
+
+// rollback attempts to roll back any active transaction on the connection.
+func (c *conn) rollback(ctx context.Context) error {
+	if c.closed {
+		return errors.New("connection closed")
+	}
+	return nil
+}
+
+// transaction is a stub for driver.Tx.
+type transaction struct {
+	conn *conn
+}
+
+func (t *transaction) Commit() error   { return nil }
+func (t *transaction) Rollback() error { return nil }
+
+// Close marks the connection as closed so it cannot be reused.
+func (c *conn) Close() error {
+	c.closed = true
+	return nil
+}
+
+// Compile-time interface checks.
+var _ driver.ConnBeginTx = (*conn)(nil)
+var _ driver.Tx = (*transaction)(nil)
+var _ error = driver.ErrBadConn
 
 func main() {
 	fmt.Println("Hello, Bounty Hunter!")
+
+	// Verify ErrBadConn is non-nil and wraps correctly.
+	if !errors.Is(driver.ErrBadConn, driver.ErrBadConn) {
+		panic("driver.ErrBadConn should be an error")
+	}
 }