database/sql: add support for decimal interface

Add support for scanning decimal types into values. If the dest
supports the decimal composer interface and the src supports
the decimal decomposer, set the value of the decimal when Scanning.

Add support for sending decimal decomposer interface values
as parameters.

For #30870

Change-Id: Ic5dbf9069df8d56405852b17542a9188d55c2947
Reviewed-on: https://go-review.googlesource.com/c/go/+/174181
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 104 insertions, 0 deletions

diff --git a/src/database/sql/convert_test.go b/src/database/sql/convert_test.go
index 412f0b1823..8a82891c25 100644
--- a/src/database/sql/convert_test.go
+++ b/src/database/sql/convert_test.go
@@ -494,3 +494,107 @@ func TestDriverArgs(t *testing.T) {
 		}
 	}
 }
+
+type dec struct {
+	form        byte
+	neg         bool
+	coefficient [16]byte
+	exponent    int32
+}
+
+func (d dec) Decompose(buf []byte) (form byte, negative bool, coefficient []byte, exponent int32) {
+	coef := make([]byte, 16)
+	copy(coef, d.coefficient[:])
+	return d.form, d.neg, coef, d.exponent
+}
+
+func (d *dec) Compose(form byte, negative bool, coefficient []byte, exponent int32) error {
+	switch form {
+	default:
+		return fmt.Errorf("unknown form %d", form)
+	case 1, 2:
+		d.form = form
+		d.neg = negative
+		return nil
+	case 0:
+	}
+	d.form = form
+	d.neg = negative
+	d.exponent = exponent
+
+	// This isn't strictly correct, as the extra bytes could be all zero,
+	// ignore this for this test.
+	if len(coefficient) > 16 {
+		return fmt.Errorf("coefficent too large")
+	}
+	copy(d.coefficient[:], coefficient)
+
+	return nil
+}
+
+type decFinite struct {
+	neg         bool
+	coefficient [16]byte
+	exponent    int32
+}
+
+func (d decFinite) Decompose(buf []byte) (form byte, negative bool, coefficient []byte, exponent int32) {
+	coef := make([]byte, 16)
+	copy(coef, d.coefficient[:])
+	return 0, d.neg, coef, d.exponent
+}
+
+func (d *decFinite) Compose(form byte, negative bool, coefficient []byte, exponent int32) error {
+	switch form {
+	default:
+		return fmt.Errorf("unknown form %d", form)
+	case 1, 2:
+		return fmt.Errorf("unsupported form %d", form)
+	case 0:
+	}
+	d.neg = negative
+	d.exponent = exponent
+
+	// This isn't strictly correct, as the extra bytes could be all zero,
+	// ignore this for this test.
+	if len(coefficient) > 16 {
+		return fmt.Errorf("coefficent too large")
+	}
+	copy(d.coefficient[:], coefficient)
+
+	return nil
+}
+
+func TestDecimal(t *testing.T) {
+	list := []struct {
+		name string
+		in   decimalDecompose
+		out  dec
+		err  bool
+	}{
+		{name: "same", in: dec{exponent: -6}, out: dec{exponent: -6}},
+
+		// Ensure reflection is not used to assign the value by using different types.
+		{name: "diff", in: decFinite{exponent: -6}, out: dec{exponent: -6}},
+
+		{name: "bad-form", in: dec{form: 200}, err: true},
+	}
+	for _, item := range list {
+		t.Run(item.name, func(t *testing.T) {
+			out := dec{}
+			err := convertAssign(&out, item.in)
+			if item.err {
+				if err == nil {
+					t.Fatalf("unexpected nil error")
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			if !reflect.DeepEqual(out, item.out) {
+				t.Fatalf("got %#v want %#v", out, item.out)
+			}
+		})
+	}
+}