database/sql: ensure a Stmt from a Conn executes on the same driver.Conn

Ensure a Stmt prepared on a Conn executes on the same driver.Conn.
This also removes another instance of duplicated prepare logic
as a side effect.

Fixes #20647

Change-Id: Ia00a19e4dd15e19e4d754105babdff5dc127728f
Reviewed-on: https://go-review.googlesource.com/45391Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/database/sql/sql_test.go

@@ -877,7 +877,7 @@ func TestStatementClose(t *testing.T) {
 		msg  string
 	}{
 		{&Stmt{stickyErr: want}, "stickyErr not propagated"},
-		{&Stmt{tx: &Tx{}, txds: &driverStmt{Locker: &sync.Mutex{}, si: stubDriverStmt{want}}}, "driverStmt.Close() error not propagated"},
+		{&Stmt{cg: &Tx{}, cgds: &driverStmt{Locker: &sync.Mutex{}, si: stubDriverStmt{want}}}, "driverStmt.Close() error not propagated"},
 	}
 	for _, test := range tests {
 		if err := test.stmt.Close(); err != want {
@@ -3231,6 +3231,42 @@ func TestIssue18719(t *testing.T) {
 	cancel()
 }
 
+func TestIssue20647(t *testing.T) {
+	db := newTestDB(t, "people")
+	defer closeDB(t, db)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	conn, err := db.Conn(ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer conn.Close()
+
+	stmt, err := conn.PrepareContext(ctx, "SELECT|people|name|")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer stmt.Close()
+
+	rows1, err := stmt.QueryContext(ctx)
+	if err != nil {
+		t.Fatal("rows1", err)
+	}
+	defer rows1.Close()
+
+	rows2, err := stmt.QueryContext(ctx)
+	if err != nil {
+		t.Fatal("rows2", err)
+	}
+	defer rows2.Close()
+
+	if rows1.dc != rows2.dc {
+		t.Fatal("stmt prepared on Conn does not use same connection")
+	}
+}
+
 func TestConcurrency(t *testing.T) {
 	list := []struct {
 		name string