+ buf, err := ks.bufferPool.GetContext(ctx)
+ if err != nil {
+ return 0, err
+ }
+ defer ks.bufferPool.Put(buf)
+ streamer := newStreamWriterAt(out, 65536, buf)
+ defer streamer.Close()
+
+ var errToCaller error = os.ErrNotExist
+ for _, mnt := range ks.rendezvous(li.hash, ks.mountsR) {
+ if ctx.Err() != nil {
+ return 0, ctx.Err()
+ }
+ err := mnt.BlockRead(ctx, li.hash, streamer)
+ if err != nil {
+ if streamer.WroteAt() != 0 {
+ // BlockRead encountered an error
+ // after writing some data, so it's
+ // too late to try another
+ // volume. Flush streamer before
+ // calling Wrote() to ensure our
+ // return value accurately reflects
+ // the number of bytes written to
+ // opts.WriteTo.
+ streamer.Close()
+ return streamer.Wrote(), err
+ }
+ if !os.IsNotExist(err) {
+ errToCaller = err
+ }
+ continue
+ }
+ if li.size == 0 {
+ // hashCheckingWriter isn't in use because we
+ // don't know the expected size. All we can do
+ // is check after writing all the data, and
+ // trust the caller is doing a HEAD request so
+ // it's not too late to set an error code in
+ // the response header.
+ err = streamer.Close()
+ if hash := fmt.Sprintf("%x", hashcheck.Sum(nil)); hash != li.hash && err == nil {
+ err = errChecksum
+ }
+ if rw, ok := opts.WriteTo.(http.ResponseWriter); ok {
+ // We didn't set the content-length header
+ // above because we didn't know the block size
+ // until now.
+ rw.Header().Set("Content-Length", fmt.Sprintf("%d", streamer.WroteAt()))
+ }
+ return streamer.WroteAt(), err
+ } else if streamer.WroteAt() != li.size {
+ // If the backend read fewer bytes than
+ // expected but returns no error, we can
+ // classify this as a checksum error (even
+ // though hashCheckWriter doesn't know that
+ // yet, it's just waiting for the next
+ // write). If our caller is serving a GET
+ // request it's too late to do anything about
+ // it anyway, but if it's a HEAD request the
+ // caller can still change the response status
+ // code.
+ return streamer.WroteAt(), errChecksum
+ }
+ // Ensure streamer flushes all buffered data without
+ // errors.
+ err = streamer.Close()
+ return streamer.Wrote(), err
+ }
+ return 0, errToCaller
+}