s3: don't leak memory or tokens in edge cases for multipart upload

backend/s3/s3.go

@@ -2253,9 +2253,16 @@ func (o *Object) uploadMultipart(ctx context.Context, req *s3.PutObjectInput, si
 		tokens.Get()
 		buf := memPool.Get()
 
+		free := func() {
+			// return the memory and token
+			memPool.Put(buf)
+			tokens.Put()
+		}
+
 		// Fail fast, in case an errgroup managed function returns an error
 		// gCtx is cancelled. There is no point in uploading all the other parts.
 		if gCtx.Err() != nil {
+			free()
 			break
 		}
 
@@ -2264,10 +2271,12 @@ func (o *Object) uploadMultipart(ctx context.Context, req *s3.PutObjectInput, si
 		n, err = readers.ReadFill(in, buf) // this can never return 0, nil
 		if err == io.EOF {
 			if n == 0 && partNum != 1 { // end if no data and if not first chunk
+				free()
 				break
 			}
 			finished = true
 		} else if err != nil {
+			free()
 			return errors.Wrap(err, "multipart upload failed to read source")
 		}
 		buf = buf[:n]
@@ -2276,6 +2285,7 @@ func (o *Object) uploadMultipart(ctx context.Context, req *s3.PutObjectInput, si
 		fs.Debugf(o, "multipart upload starting chunk %d size %v offset %v/%v", partNum, fs.SizeSuffix(n), fs.SizeSuffix(off), fs.SizeSuffix(size))
 		off += int64(n)
 		g.Go(func() (err error) {
+			defer free()
 			partLength := int64(len(buf))
 
 			// create checksum of buffer for integrity checking
@@ -2313,11 +2323,6 @@ func (o *Object) uploadMultipart(ctx context.Context, req *s3.PutObjectInput, si
 
 				return false, nil
 			})
-
-			// return the memory and token
-			memPool.Put(buf)
-			tokens.Put()
-
 			if err != nil {
 				return errors.Wrap(err, "multipart upload failed to upload part")
 			}