union: fix writing with the all policy - fixes #4534

Before this change writing with the all policy deadlocked while
uploading.

This change fixes the problem by fixing the multi reader, closing the
pipes at the correct time with the correct error. This is factored
into a new function as it was used twice.

This patch also adds a new test which tests the all policies.

backend/union/entry.go

@@ -1,7 +1,6 @@
 package union
 
 import (
-	"bufio"
 	"context"
 	"io"
 	"sync"
@@ -67,31 +66,9 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 		obj := entries[0].(*upstream.Object)
 		return obj.Update(ctx, in, src, options...)
 	}
-	// Get multiple reader
-	readers := make([]io.Reader, len(entries))
-	writers := make([]io.Writer, len(entries))
-	errs := Errors(make([]error, len(entries)+1))
-	for i := range entries {
-		r, w := io.Pipe()
-		bw := bufio.NewWriter(w)
-		readers[i], writers[i] = r, bw
-		defer func() {
-			err := w.Close()
-			if err != nil {
-				panic(err)
-			}
-		}()
-	}
-	go func() {
-		mw := io.MultiWriter(writers...)
-		es := make([]error, len(writers)+1)
-		_, es[len(es)-1] = io.Copy(mw, in)
-		for i, bw := range writers {
-			es[i] = bw.(*bufio.Writer).Flush()
-		}
-		errs[len(entries)] = Errors(es).Err()
-	}()
 	// Multi-threading
+	readers, errChan := multiReader(len(entries), in)
+	errs := Errors(make([]error, len(entries)+1))
 	multithread(len(entries), func(i int) {
 		if o, ok := entries[i].(*upstream.Object); ok {
 			err := o.Update(ctx, readers[i], src, options...)
@@ -100,6 +77,7 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 			errs[i] = fs.ErrorNotAFile
 		}
 	})
+	errs[len(entries)] = <-errChan
 	return errs.Err()
 }