Fix infinite loop if Reader returns io.ErrUnexpectedEOF

Apparently some HTTP transactions can return io.ErrUnexpectedEOF, eg
https://groups.google.com/forum/#!topic/golang-nuts/2wDPB42-rZ0

Before this change, this would result in S3Manager sending a lot of 0
sized parts, eventually causing the error "TotalPartsExceeded:
exceeded total allowed configured MaxUploadParts".

service/s3/s3manager/upload.go

@@ -508,6 +508,7 @@ func (u *multiuploader) upload(firstBuf io.ReadSeeker) (*UploadOutput, error) {
 
 	// Read and queue the rest of the parts
 	for u.geterr() == nil {
+		num++
 		// This upload exceeded maximum number of supported parts, error now.
 		if num > int64(u.ctx.MaxUploadParts) || num > int64(MaxUploadParts) {
 			var msg string
@@ -521,7 +522,6 @@ func (u *multiuploader) upload(firstBuf io.ReadSeeker) (*UploadOutput, error) {
 			u.seterr(awserr.New("TotalPartsExceeded", msg, nil))
 			break
 		}
-		num++
 
 		buf, err := u.nextReader()
 		if err == io.EOF {
@@ -530,7 +530,9 @@ func (u *multiuploader) upload(firstBuf io.ReadSeeker) (*UploadOutput, error) {
 
 		ch <- chunk{buf: buf, num: num}
 
-		if err != nil && err != io.ErrUnexpectedEOF {
+		if err == io.ErrUnexpectedEOF {
+			break
+		} else if err != nil {
 			u.seterr(awserr.New(
 				"ReadRequestBody",
 				"read multipart upload data failed",

service/s3/s3manager/upload_test.go

@@ -391,11 +391,15 @@ func TestUploadOrderReadFail2(t *testing.T) {
 type sizedReader struct {
 	size int
 	cur  int
+	err  error
 }
 
 func (s *sizedReader) Read(p []byte) (n int, err error) {
 	if s.cur >= s.size {
-		return 0, io.EOF
+		if s.err == nil {
+			s.err = io.EOF
+		}
+		return 0, s.err
 	}
 
 	n = len(p)
@@ -429,6 +433,52 @@ func TestUploadOrderMultiBufferedReader(t *testing.T) {
 	assert.Equal(t, []int{1024 * 1024 * 2, 1024 * 1024 * 5, 1024 * 1024 * 5}, parts)
 }
 
+func TestUploadOrderMultiBufferedReaderUnexpectedEOF(t *testing.T) {
+	s, ops, args := loggingSvc(emptyList)
+	mgr := s3manager.NewUploaderWithClient(s)
+	_, err := mgr.Upload(&s3manager.UploadInput{
+		Bucket: aws.String("Bucket"),
+		Key:    aws.String("Key"),
+		Body:   &sizedReader{size: 1024 * 1024 * 12, err: io.ErrUnexpectedEOF},
+	})
+
+	assert.NoError(t, err)
+	assert.Equal(t, []string{"CreateMultipartUpload", "UploadPart", "UploadPart", "UploadPart", "CompleteMultipartUpload"}, *ops)
+
+	// Part lengths
+	parts := []int{
+		buflen(val((*args)[1], "Body")),
+		buflen(val((*args)[2], "Body")),
+		buflen(val((*args)[3], "Body")),
+	}
+	sort.Ints(parts)
+	assert.Equal(t, []int{1024 * 1024 * 2, 1024 * 1024 * 5, 1024 * 1024 * 5}, parts)
+}
+
+// TestUploadOrderMultiBufferedReaderEOF tests the edge case where the
+// file size is the same as part size, which means nextReader will
+// return io.EOF rather than io.ErrUnexpectedEOF
+func TestUploadOrderMultiBufferedReaderEOF(t *testing.T) {
+	s, ops, args := loggingSvc(emptyList)
+	mgr := s3manager.NewUploaderWithClient(s)
+	_, err := mgr.Upload(&s3manager.UploadInput{
+		Bucket: aws.String("Bucket"),
+		Key:    aws.String("Key"),
+		Body:   &sizedReader{size: 1024 * 1024 * 10, err: io.EOF},
+	})
+
+	assert.NoError(t, err)
+	assert.Equal(t, []string{"CreateMultipartUpload", "UploadPart", "UploadPart", "CompleteMultipartUpload"}, *ops)
+
+	// Part lengths
+	parts := []int{
+		buflen(val((*args)[1], "Body")),
+		buflen(val((*args)[2], "Body")),
+	}
+	sort.Ints(parts)
+	assert.Equal(t, []int{1024 * 1024 * 5, 1024 * 1024 * 5}, parts)
+}
+
 func TestUploadOrderMultiBufferedReaderExceedTotalParts(t *testing.T) {
 	s, ops, _ := loggingSvc([]string{"UploadPart"})
 	mgr := s3manager.NewUploaderWithClient(s, func(u *s3manager.Uploader) {