add iter scanner interface and method

Add Scanner method to Iter which returns a type which allows scanning
result rows in a similar method to that provided by database/sql in the
standard library. This makes it easier to handle and differentiate
Scanning marshalling errors from other errors.

session.go

@@ -1027,6 +1027,128 @@ func (iter *Iter) Columns() []ColumnInfo {
 	return iter.meta.columns
 }
 
+type Scanner interface {
+	Next() bool
+	Scan(...interface{}) error
+	Err() error
+}
+
+type iterScanner struct {
+	iter *Iter
+	cols [][]byte
+}
+
+// Next advances the row pointer to point at the next row, the row is valid until
+// the next call of Next. It returns true if there is a row which is available to be
+// scanned into with Scan.
+// Next must be called before every call to Scan.
+func (is *iterScanner) Next() bool {
+	iter := is.iter
+	if iter.err != nil {
+		return false
+	}
+
+	if iter.pos >= iter.numRows {
+		if iter.next != nil {
+			is.iter = iter.next.fetch()
+			return is.Next()
+		}
+		return false
+	}
+
+	cols := make([][]byte, len(iter.meta.columns))
+	for i := 0; i < len(cols); i++ {
+		col, err := iter.readColumn()
+		if err != nil {
+			iter.err = err
+			return false
+		}
+		cols[i] = col
+	}
+	is.cols = cols
+	iter.pos++
+
+	return true
+}
+
+func scanColumn(p []byte, col ColumnInfo, dest []interface{}) (int, error) {
+	if dest[0] == nil {
+		return 1, nil
+	}
+
+	if col.TypeInfo.Type() == TypeTuple {
+		// this will panic, actually a bug, please report
+		tuple := col.TypeInfo.(TupleTypeInfo)
+
+		count := len(tuple.Elems)
+		// here we pass in a slice of the struct which has the number number of
+		// values as elements in the tuple
+		if err := Unmarshal(col.TypeInfo, p, dest[:count]); err != nil {
+			return 0, err
+		}
+		return count, nil
+	} else {
+		if err := Unmarshal(col.TypeInfo, p, dest[0]); err != nil {
+			return 0, err
+		}
+		return 1, nil
+	}
+}
+
+// Scan copies the current row's columns into dest. If the length of dest does not equal
+// the number of columns returned in the row an error is returned. If an error is encountered
+// when unmarshalling a column into the value in dest an error is returned and the row is invalidated
+// until the next call to Next.
+// Next must be called before calling Scan, if it is not an error is returned.
+func (is *iterScanner) Scan(dest ...interface{}) error {
+	if is.cols == nil {
+		return errors.New("gocql: Scan called without calling Next")
+	}
+
+	iter := is.iter
+	// currently only support scanning into an expand tuple, such that its the same
+	// as scanning in more values from a single column
+	if len(dest) != iter.meta.actualColCount {
+		return fmt.Errorf("gocql: not enough columns to scan into: have %d want %d", len(dest), iter.meta.actualColCount)
+	}
+
+	// i is the current position in dest, could posible replace it and just use
+	// slices of dest
+	i := 0
+	var err error
+	for _, col := range iter.meta.columns {
+		var n int
+		n, err = scanColumn(is.cols[i], col, dest[i:])
+		if err != nil {
+			break
+		}
+		i += n
+	}
+
+	is.cols = nil
+
+	return err
+}
+
+// Err returns the if there was one during iteration that resulted in iteration being unable to complete.
+// Err will also release resources held by the iterator and should not used after being called.
+func (is *iterScanner) Err() error {
+	iter := is.iter
+	is.iter = nil
+	is.cols = nil
+	return iter.Close()
+}
+
+// Scanner returns a row Scanner which provides an interface to scan rows in a manner which is
+// similar to database/sql. The iter should NOT be used again after calling this method.
+func (iter *Iter) Scanner() Scanner {
+	if iter == nil {
+		return nil
+	}
+
+	return &iterScanner{iter: iter}
+}
+
 func (iter *Iter) readColumn() ([]byte, error) {
 	return iter.framer.readBytesInternal()
 }
@@ -1066,37 +1188,19 @@ func (iter *Iter) Scan(dest ...interface{}) bool {
 	// i is the current position in dest, could posible replace it and just use
 	// slices of dest
 	i := 0
-	for c := range iter.meta.columns {
-		col := &iter.meta.columns[c]
+	for _, col := range iter.meta.columns {
 		colBytes, err := iter.readColumn()
 		if err != nil {
 			iter.err = err
 			return false
 		}
 
-		if dest[i] == nil {
-			i++
-			continue
-		}
-
-		switch col.TypeInfo.Type() {
-		case TypeTuple:
-			// this will panic, actually a bug, please report
-			tuple := col.TypeInfo.(TupleTypeInfo)
-
-			count := len(tuple.Elems)
-			// here we pass in a slice of the struct which has the number number of
-			// values as elements in the tuple
-			iter.err = Unmarshal(col.TypeInfo, colBytes, dest[i:i+count])
-			i += count
-		default:
-			iter.err = Unmarshal(col.TypeInfo, colBytes, dest[i])
-			i++
-		}
-
-		if iter.err != nil {
+		n, err := scanColumn(colBytes, col, dest[i:])
+		if err != nil {
+			iter.err = err
 			return false
 		}
+		i += n
 	}
 
 	iter.pos++