api: CopyObjectPart was copying wrong offsets due to shadowing. (#3838)

startOffset was re-assigned to '0' so it would end up
copying wrong content ignoring the requested startOffset.

This also fixes the corruption issue we observed while
using docker registry.

Fixes https://github.com/docker/distribution/issues/2205

Also fixes #3842 - incorrect routing.

cmd/api-errors.go

@@ -56,6 +56,8 @@ const (
 	ErrInvalidBucketName
 	ErrInvalidDigest
 	ErrInvalidRange
+	ErrInvalidCopyPartRange
+	ErrInvalidCopyPartRangeSource
 	ErrInvalidMaxKeys
 	ErrInvalidMaxUploads
 	ErrInvalidMaxParts
@@ -540,6 +542,16 @@ var errorCodeResponse = map[APIErrorCode]APIError{
 		Description:    "Configurations overlap. Configurations on the same bucket cannot share a common event type.",
 		HTTPStatusCode: http.StatusBadRequest,
 	},
+	ErrInvalidCopyPartRange: {
+		Code:           "InvalidArgument",
+		Description:    "The x-amz-copy-source-range value must be of the form bytes=first-last where first and last are the zero-based offsets of the first and last bytes to copy",
+		HTTPStatusCode: http.StatusBadRequest,
+	},
+	ErrInvalidCopyPartRangeSource: {
+		Code:           "InvalidArgument",
+		Description:    "Range specified is not valid for source object",
+		HTTPStatusCode: http.StatusBadRequest,
+	},
 
 	/// S3 extensions.
 	ErrContentSHA256Mismatch: {