vendor: update buildkit to v0.8

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

vendor/github.com/containerd/containerd/content/helpers.go

@@ -229,9 +229,47 @@ func seekReader(r io.Reader, offset, size int64) (io.Reader, error) {
 	return r, nil
 }
 
+// copyWithBuffer is very similar to  io.CopyBuffer https://golang.org/pkg/io/#CopyBuffer
+// but instead of using Read to read from the src, we use ReadAtLeast to make sure we have
+// a full buffer before we do a write operation to dst to reduce overheads associated
+// with the write operations of small buffers.
 func copyWithBuffer(dst io.Writer, src io.Reader) (written int64, err error) {
-	buf := bufPool.Get().(*[]byte)
-	written, err = io.CopyBuffer(dst, src, *buf)
-	bufPool.Put(buf)
+	// If the reader has a WriteTo method, use it to do the copy.
+	// Avoids an allocation and a copy.
+	if wt, ok := src.(io.WriterTo); ok {
+		return wt.WriteTo(dst)
+	}
+	// Similarly, if the writer has a ReadFrom method, use it to do the copy.
+	if rt, ok := dst.(io.ReaderFrom); ok {
+		return rt.ReadFrom(src)
+	}
+	bufRef := bufPool.Get().(*[]byte)
+	defer bufPool.Put(bufRef)
+	buf := *bufRef
+	for {
+		nr, er := io.ReadAtLeast(src, buf, len(buf))
+		if nr > 0 {
+			nw, ew := dst.Write(buf[0:nr])
+			if nw > 0 {
+				written += int64(nw)
+			}
+			if ew != nil {
+				err = ew
+				break
+			}
+			if nr != nw {
+				err = io.ErrShortWrite
+				break
+			}
+		}
+		if er != nil {
+			// If an EOF happens after reading fewer than the requested bytes,
+			// ReadAtLeast returns ErrUnexpectedEOF.
+			if er != io.EOF && er != io.ErrUnexpectedEOF {
+				err = er
+			}
+			break
+		}
+	}
 	return
 }

vendor/github.com/containerd/containerd/content/local/store_unix.go

@@ -1,4 +1,4 @@
-// +build linux solaris darwin freebsd
+// +build linux solaris darwin freebsd netbsd
 
 /*
    Copyright The containerd Authors.

vendor/github.com/containerd/containerd/content/proxy/content_reader.go

@@ -40,7 +40,13 @@ func (ra *remoteReaderAt) ReadAt(p []byte, off int64) (n int, err error) {
 		Offset: off,
 		Size_:  int64(len(p)),
 	}
-	rc, err := ra.client.Read(ra.ctx, rr)
+	// we need a child context with cancel, or the eventually called
+	// grpc.NewStream will leak the goroutine until the whole thing is cleared.
+	// See comment at https://godoc.org/google.golang.org/grpc#ClientConn.NewStream
+	childCtx, cancel := context.WithCancel(ra.ctx)
+	// we MUST cancel the child context; see comment above
+	defer cancel()
+	rc, err := ra.client.Read(childCtx, rr)
 	if err != nil {
 		return 0, err
 	}