database/sql: support returning query database types

Creates a ColumnType structure that can be extended in to future.
Allow drivers to implement what makes sense for the database.

Fixes #16652

Change-Id: Ieb1fd64eac1460107b1d3474eba5201fa300a4ec
Reviewed-on: https://go-review.googlesource.com/29961
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql.go

@@ -18,6 +18,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"reflect"
 	"runtime"
 	"sort"
 	"sync"
@@ -996,8 +997,8 @@ const maxBadConnRetries = 2
 // The caller must call the statement's Close method
 // when the statement is no longer needed.
 //
-// The provided context is for the preparation of the statment, not for the execution of
-// the statement.
+// The provided context is used for the preparation of the statement, not for the
+// execution of the statement.
 func (db *DB) PrepareContext(ctx context.Context, query string) (*Stmt, error) {
 	var stmt *Stmt
 	var err error
@@ -2033,6 +2034,107 @@ func (rs *Rows) Columns() ([]string, error) {
 	return rs.rowsi.Columns(), nil
 }
 
+// ColumnTypes returns column information such as column type, length,
+// and nullable. Some information may not be available from some drivers.
+func (rs *Rows) ColumnTypes() ([]*ColumnType, error) {
+	if rs.isClosed() {
+		return nil, errors.New("sql: Rows are closed")
+	}
+	if rs.rowsi == nil {
+		return nil, errors.New("sql: no Rows available")
+	}
+	return rowsColumnInfoSetup(rs.rowsi), nil
+}
+
+// ColumnType contains the name and type of a column.
+type ColumnType struct {
+	name string
+
+	hasNullable       bool
+	hasLength         bool
+	hasPrecisionScale bool
+
+	nullable     bool
+	length       int64
+	databaseType string
+	precision    int64
+	scale        int64
+	scanType     reflect.Type
+}
+
+// Name returns the name or alias of the column.
+func (ci *ColumnType) Name() string {
+	return ci.name
+}
+
+// Length returns the column type length for variable length column types such
+// as text and binary field types. If the type length is unbounded the value will
+// be math.MaxInt64 (any database limits will still apply).
+// If the column type is not variable length, such as an int, or if not supported
+// by the driver ok is false.
+func (ci *ColumnType) Length() (length int64, ok bool) {
+	return ci.length, ci.hasLength
+}
+
+// DecimalSize returns the scale and precision of a decimal type.
+// If not applicable or if not supported ok is false.
+func (ci *ColumnType) DecimalSize() (precision, scale int64, ok bool) {
+	return ci.precision, ci.scale, ci.hasPrecisionScale
+}
+
+// ScanType returns a Go type suitable for scanning into using Rows.Scan.
+// If a driver does not support this property ScanType will return
+// the type of an empty interface.
+func (ci *ColumnType) ScanType() reflect.Type {
+	return ci.scanType
+}
+
+// Nullable returns whether the column may be null.
+// If a driver does not support this property ok will be false.
+func (ci *ColumnType) Nullable() (nullable, ok bool) {
+	return ci.nullable, ci.hasNullable
+}
+
+// DatabaseTypeName returns the database system name of the column type. If an empty
+// string is returned the driver type name is not supported.
+// Consult your driver documentation for a list of driver data types. Length specifiers
+// are not included.
+// Common type include "VARCHAR", "TEXT", "NVARCHAR", "DECIMAL", "BOOL", "INT", "BIGINT".
+func (ci *ColumnType) DatabaseTypeName() string {
+	return ci.databaseType
+}
+
+func rowsColumnInfoSetup(rowsi driver.Rows) []*ColumnType {
+	names := rowsi.Columns()
+
+	list := make([]*ColumnType, len(names))
+	for i := range list {
+		ci := &ColumnType{
+			name: names[i],
+		}
+		list[i] = ci
+
+		if prop, ok := rowsi.(driver.RowsColumnTypeScanType); ok {
+			ci.scanType = prop.ColumnTypeScanType(i)
+		} else {
+			ci.scanType = reflect.TypeOf(new(interface{})).Elem()
+		}
+		if prop, ok := rowsi.(driver.RowsColumnTypeDatabaseTypeName); ok {
+			ci.databaseType = prop.ColumnTypeDatabaseTypeName(i)
+		}
+		if prop, ok := rowsi.(driver.RowsColumnTypeLength); ok {
+			ci.length, ci.hasLength = prop.ColumnTypeLength(i)
+		}
+		if prop, ok := rowsi.(driver.RowsColumnTypeNullable); ok {
+			ci.nullable, ci.hasNullable = prop.ColumnTypeNullable(i)
+		}
+		if prop, ok := rowsi.(driver.RowsColumnTypePrecisionScale); ok {
+			ci.precision, ci.scale, ci.hasPrecisionScale = prop.ColumnTypePrecisionScale(i)
+		}
+	}
+	return list
+}
+
 // Scan copies the columns in the current row into the values pointed
 // at by dest. The number of values in dest must be the same as the
 // number of columns in Rows.