database/sql: allow drivers to only implement Context variants

Drivers shouldn't need to implement both Queryer and QueryerContext,
they should just implement QueryerContext. Same with Execer and
ExecerContext. This CL tests for QueryContext and ExecerContext
first so drivers do not need to implement Queryer and Execer
with an empty definition.

Fixes #21663

Change-Id: Ifbaa71da669f4bc60f8da8c41a04a4afed699a9f
Reviewed-on: https://go-review.googlesource.com/65733
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/database/sql/sql_test.go

@@ -3487,6 +3487,127 @@ func TestNamedValueCheckerSkip(t *testing.T) {
 	}
 }
 
+type ctxOnlyDriver struct {
+	fakeDriver
+}
+
+func (d *ctxOnlyDriver) Open(dsn string) (driver.Conn, error) {
+	conn, err := d.fakeDriver.Open(dsn)
+	if err != nil {
+		return nil, err
+	}
+	return &ctxOnlyConn{fc: conn.(*fakeConn)}, nil
+}
+
+var (
+	_ driver.Conn           = &ctxOnlyConn{}
+	_ driver.QueryerContext = &ctxOnlyConn{}
+	_ driver.ExecerContext  = &ctxOnlyConn{}
+)
+
+type ctxOnlyConn struct {
+	fc *fakeConn
+
+	queryCtxCalled bool
+	execCtxCalled  bool
+}
+
+func (c *ctxOnlyConn) Begin() (driver.Tx, error) {
+	return c.fc.Begin()
+}
+
+func (c *ctxOnlyConn) Close() error {
+	return c.fc.Close()
+}
+
+// Prepare is still part of the Conn interface, so while it isn't used
+// must be defined for compatibility.
+func (c *ctxOnlyConn) Prepare(q string) (driver.Stmt, error) {
+	panic("not used")
+}
+
+func (c *ctxOnlyConn) PrepareContext(ctx context.Context, q string) (driver.Stmt, error) {
+	return c.fc.PrepareContext(ctx, q)
+}
+
+func (c *ctxOnlyConn) QueryContext(ctx context.Context, q string, args []driver.NamedValue) (driver.Rows, error) {
+	c.queryCtxCalled = true
+	return c.fc.QueryContext(ctx, q, args)
+}
+
+func (c *ctxOnlyConn) ExecContext(ctx context.Context, q string, args []driver.NamedValue) (driver.Result, error) {
+	c.execCtxCalled = true
+	return c.fc.ExecContext(ctx, q, args)
+}
+
+// TestQueryExecContextOnly ensures drivers only need to implement QueryContext
+// and ExecContext methods.
+func TestQueryExecContextOnly(t *testing.T) {
+	// Ensure connection does not implment non-context interfaces.
+	var connType driver.Conn = &ctxOnlyConn{}
+	if _, ok := connType.(driver.Execer); ok {
+		t.Fatalf("%T must not implement driver.Execer", connType)
+	}
+	if _, ok := connType.(driver.Queryer); ok {
+		t.Fatalf("%T must not implement driver.Queryer", connType)
+	}
+
+	Register("ContextOnly", &ctxOnlyDriver{})
+	db, err := Open("ContextOnly", "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	conn, err := db.Conn(ctx)
+	if err != nil {
+		t.Fatal("db.Conn", err)
+	}
+	defer conn.Close()
+
+	_, err = conn.ExecContext(ctx, "WIPE")
+	if err != nil {
+		t.Fatal("exec wipe", err)
+	}
+
+	_, err = conn.ExecContext(ctx, "CREATE|keys|v1=string")
+	if err != nil {
+		t.Fatal("exec create", err)
+	}
+	expectedValue := "value1"
+	_, err = conn.ExecContext(ctx, "INSERT|keys|v1=?", expectedValue)
+	if err != nil {
+		t.Fatal("exec insert", err)
+	}
+	rows, err := conn.QueryContext(ctx, "SELECT|keys|v1|")
+	if err != nil {
+		t.Fatal("query select", err)
+	}
+	v1 := ""
+	for rows.Next() {
+		err = rows.Scan(&v1)
+		if err != nil {
+			t.Fatal("rows scan", err)
+		}
+	}
+	rows.Close()
+
+	if v1 != expectedValue {
+		t.Fatalf("expected %q, got %q", expectedValue, v1)
+	}
+
+	coc := conn.dc.ci.(*ctxOnlyConn)
+	if !coc.execCtxCalled {
+		t.Error("ExecContext not called")
+	}
+	if !coc.queryCtxCalled {
+		t.Error("QueryContext not called")
+	}
+}
+
 // badConn implements a bad driver.Conn, for TestBadDriver.
 // The Exec method panics.
 type badConn struct{}