union: fix deadlock when one part of a multi-upload fails

Before this fix, rclone would deadlock when uploading two files at
once, if one errored. This caused the other file to block in the multi
reader and never complete.

This fix drains the input buffer on error which allows the other
upload to complete.

See: https://forum.rclone.org/t/union-with-create-policy-all-copy-stuck-when-first-union-fails/29601

backend/union/entry.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"sync"
 	"time"
 
@@ -84,6 +85,10 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 			err := o.Update(ctx, readers[i], src, options...)
 			if err != nil {
 				errs[i] = fmt.Errorf("%s: %w", o.UpstreamFs().Name(), err)
+				if len(entries) > 1 {
+					// Drain the input buffer to allow other uploads to continue
+					_, _ = io.Copy(ioutil.Discard, readers[i])
+				}
 			}
 		} else {
 			errs[i] = fs.ErrorNotAFile

backend/union/union.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"path"
 	"path/filepath"
 	"strings"
@@ -486,6 +487,10 @@ func (f *Fs) put(ctx context.Context, in io.Reader, src fs.ObjectInfo, stream bo
 		}
 		if err != nil {
 			errs[i] = fmt.Errorf("%s: %w", u.Name(), err)
+			if len(upstreams) > 1 {
+				// Drain the input buffer to allow other uploads to continue
+				_, _ = io.Copy(ioutil.Discard, readers[i])
+			}
 			return
 		}
 		objs[i] = u.WrapObject(o)