move gotd fork into repo. (#111)

- update to latest telegram layer
- remove some references to fields in tg.Entities that don't exist in
the schema
- originally added here:
https://github.com/beeper/td/commit/820929062a2ba0104397bc01235ab58a9cff780e
  - referenced here
-
https://github.com/mautrix/telegramgo/commit/124f0967ed195b5a380c9bd02e170ada9710dde3
-
https://github.com/mautrix/telegramgo/commit/4205047aab2e0639217148b5d125bfaab668bd8e

pkg/gotd/syncio/writer_at.go

@@ -0,0 +1,97 @@
+package syncio
+
+import (
+	"io"
+	"sync"
+
+	"github.com/go-faster/errors"
+)
+
+// WriterAt is synchronized io.WriterAt.
+type WriterAt struct {
+	w   io.WriterAt
+	mux sync.Mutex
+}
+
+// NewWriterAt creates new WriterAt.
+func NewWriterAt(w io.WriterAt) *WriterAt {
+	return &WriterAt{w: w}
+}
+
+// WriteAt implements io.WriterAt.
+func (s *WriterAt) WriteAt(p []byte, off int64) (n int, err error) {
+	s.mux.Lock()
+	n, err = s.w.WriteAt(p, off)
+	s.mux.Unlock()
+
+	return
+}
+
+// BufWriterAt is synchronized buffer which implements io.WriterAt.
+type BufWriterAt struct {
+	buf []byte
+	mux sync.RWMutex
+}
+
+// Bytes returns copy of data from buffer.
+func (b *BufWriterAt) Bytes() (r []byte) {
+	b.mux.RLock()
+	defer b.mux.RUnlock()
+
+	return append(make([]byte, 0, len(b.buf)), b.buf...)
+}
+
+// Len returns buffer available data size.
+func (b *BufWriterAt) Len() int {
+	b.mux.RLock()
+	defer b.mux.RUnlock()
+
+	return len(b.buf)
+}
+
+// ReadAt implements io.ReaderAt.
+func (b *BufWriterAt) ReadAt(p []byte, off int64) (n int, err error) {
+	if off < 0 {
+		return 0, errors.Errorf("invalid offset %d", off)
+	}
+
+	b.mux.RLock()
+	defer b.mux.RUnlock()
+
+	l := int64(len(b.buf))
+	switch {
+	case off >= l:
+		return 0, nil
+	case off+int64(len(p)) >= l:
+		r := b.buf[off:]
+		copy(p, r)
+		return len(r), nil
+	}
+
+	from := off
+	to := off + int64(len(p))
+	copy(p, b.buf[from:to])
+	return len(p), nil
+}
+
+// WriteAt implements io.WriterAt.
+func (b *BufWriterAt) WriteAt(p []byte, off int64) (n int, err error) {
+	if off < 0 {
+		return 0, errors.Errorf("invalid offset %d", off)
+	}
+
+	b.mux.Lock()
+	defer b.mux.Unlock()
+
+	ends := len(p) + int(off)
+	if len(b.buf) < ends {
+		newBuf := make([]byte, ends)
+		copy(newBuf, b.buf)
+		b.buf = newBuf
+	}
+
+	from := off
+	to := off + int64(len(p))
+	copy(b.buf[from:to], p)
+	return len(p), nil
+}