datastore/query.go
func (t *Iterator) Cursor() (Cursor, error) {
	if t.err != nil && t.err != Done {
		return Cursor{}, t.err
	}
	// If we are at either end of the current batch of results,
	// return the compiled cursor at that end.
	skipped := t.res.GetSkippedResults()
	if t.i == 0 && skipped == 0 {
		if t.prevCC == nil {
			// A nil pointer (of type *pb.CompiledCursor) means no constraint:
			// passing it as the end cursor of a new query means unlimited results
			// (glossing over the integer limit parameter for now).
			// A non-nil pointer to an empty pb.CompiledCursor means the start:
			// passing it as the end cursor of a new query means 0 results.
			// If prevCC was nil, then the original query had no start cursor, but
			// Iterator.Cursor should return "the start" instead of unlimited.
			return Cursor{&zeroCC}, nil
		}
		return Cursor{t.prevCC}, nil
	}
	if t.i == len(t.res.Result) {
		return Cursor{t.res.CompiledCursor}, nil
	}
	// Otherwise, re-run the query offset to this iterator's position, starting from
	// the most recent compiled cursor. This is done on a best-effort basis, as it
	// is racy; if a concurrent process has added or removed entities, then the
	// cursor returned may be inconsistent.
	q := t.q.clone()
	q.start = t.prevCC
	q.offset = skipped + int32(t.i)
	q.limit = 0
	q.keysOnly = len(q.projection) == 0
	t1 := q.Run(t.c)
	_, _, err := t1.next()
	if err != Done {
		if err == nil {
			err = fmt.Errorf("datastore: internal error: zero-limit query did not have zero results")
		}
		return Cursor{}, err
	}
	return Cursor{t1.res.CompiledCursor}, nil
}
func (t *Iterator) Cursor() (Cursor, error) {
	if t.err != nil && t.err != Done {
		return Cursor{}, t.err
	}
	// If we are at either end of the current batch of results,
	// return the compiled cursor at that end.
	skipped := t.res.GetSkippedResults()
	if t.i == 0 && skipped == 0 {
		if t.prevCC == nil {
			// A nil pointer (of type *pb.CompiledCursor) means no constraint:
			// passing it as the end cursor of a new query means unlimited results
			// (glossing over the integer limit parameter for now).
			// A non-nil pointer to an empty pb.CompiledCursor means the start:
			// passing it as the end cursor of a new query means 0 results.
			// If prevCC was nil, then the original query had no start cursor, but
			// Iterator.Cursor should return "the start" instead of unlimited.
			return Cursor{&zeroCC}, nil
		}
		return Cursor{t.prevCC}, nil
	}
	if t.i == len(t.res.Result) {
		return Cursor{t.res.CompiledCursor}, nil
	}
	// Otherwise, re-run the query offset to this iterator's position, starting from
	// the most recent compiled cursor. This is done on a best-effort basis, as it
	// is racy; if a concurrent process has added or removed entities, then the
	// cursor returned may be inconsistent.
	q := t.q.clone()
	q.start = t.prevCC
	q.offset = skipped + int32(t.i)
	q.limit = 0
	q.keysOnly = len(q.projection) == 0
	t1 := q.Run(t.c)
	_, _, err := t1.next()
	if err != Done {
		if err == nil {
			err = fmt.Errorf("datastore: internal error: zero-limit query did not have zero results")
		}
		return Cursor{}, err
	}
	return Cursor{t1.res.CompiledCursor}, nil
}
// Cursor returns a cursor for the iterator's current location.