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>

src/database/sql/sql_test.go

@@ -3606,7 +3606,7 @@ type nvcConn struct {
 	skipNamedValueCheck bool
 }
 
-type decimal struct {
+type decimalInt struct {
 	value int
 }
 
@@ -3630,7 +3630,7 @@ func (c *nvcConn) CheckNamedValue(nv *driver.NamedValue) error {
 			nv.Value = "OUT:*string"
 		}
 		return nil
-	case decimal, []int64:
+	case decimalInt, []int64:
 		return nil
 	case doNotInclude:
 		return driver.ErrRemoveArgument
@@ -3659,13 +3659,13 @@ func TestNamedValueChecker(t *testing.T) {
 	}
 
 	o1 := ""
-	_, err = db.ExecContext(ctx, "INSERT|keys|dec1=?A,str1=?,out1=?O1,array1=?", Named("A", decimal{123}), "hello", Named("O1", Out{Dest: &o1}), []int64{42, 128, 707}, doNotInclude{})
+	_, err = db.ExecContext(ctx, "INSERT|keys|dec1=?A,str1=?,out1=?O1,array1=?", Named("A", decimalInt{123}), "hello", Named("O1", Out{Dest: &o1}), []int64{42, 128, 707}, doNotInclude{})
 	if err != nil {
 		t.Fatal("exec insert", err)
 	}
 	var (
 		str1 string
-		dec1 decimal
+		dec1 decimalInt
 		arr1 []int64
 	)
 	err = db.QueryRowContext(ctx, "SELECT|keys|dec1,str1,array1|").Scan(&dec1, &str1, &arr1)
@@ -3675,7 +3675,7 @@ func TestNamedValueChecker(t *testing.T) {
 
 	list := []struct{ got, want interface{} }{
 		{o1, "from-server"},
-		{dec1, decimal{123}},
+		{dec1, decimalInt{123}},
 		{str1, "hello"},
 		{arr1, []int64{42, 128, 707}},
 	}
@@ -3708,7 +3708,7 @@ func TestNamedValueCheckerSkip(t *testing.T) {
 		t.Fatal("exec create", err)
 	}
 
-	_, err = db.ExecContext(ctx, "INSERT|keys|dec1=?A", Named("A", decimal{123}))
+	_, err = db.ExecContext(ctx, "INSERT|keys|dec1=?A", Named("A", decimalInt{123}))
 	if err == nil {
 		t.Fatalf("expected error with bad argument, got %v", err)
 	}