Signal content length to upstream components when present

When the HTTP request contains `Content-Length` header, include it in
the body reader passed down to upstream system components. This is to
enable further optimisations during processing when the total size of a
blob is known beforehand. Examples include piece selection in order to
guarantee that a blob will be fully contained within a piece.

With changes introduced, an upstream component may type-check for
`interface { Size() int64}` and use it to then gain access to the
content length.

Note that based on HTTP standards, `application/octet-stream` content
type is not strictly required to specify `Content-Length` header. Hence
the opportunistic signalling via type-checkers.

api/server/handler.go

@@ -27,6 +27,7 @@ func (m *HttpServer) handlePostBlob(w http.ResponseWriter, r *http.Request) {
 		respondWithJson(w, errResponseNotStreamContentType, http.StatusBadRequest)
 		return
 	}
+	body := r.Body
 	var contentLength uint64
 	if value := r.Header.Get("Content-Length"); value != "" {
 		var err error
@@ -38,9 +39,14 @@ func (m *HttpServer) handlePostBlob(w http.ResponseWriter, r *http.Request) {
 			respondWithJson(w, errResponseContentLengthTooLarge(m.maxBlobLength), http.StatusBadRequest)
 			return
 		}
+		// Wrap body reader to signal content length to upstream components.
+		body = sizerReadCloser{
+			ReadCloser: r.Body,
+			size:       int64(contentLength),
+		}
 	}
-	defer r.Body.Close()
-	desc, err := m.store.Put(r.Context(), r.Body)
+	defer body.Close()
+	desc, err := m.store.Put(r.Context(), body)
 	switch err {
 	case nil:
 	case blob.ErrBlobTooLarge:

api/server/sizer_reader.go

@@ -0,0 +1,14 @@
+package server
+
+import "io"
+
+var _ interface{ Size() int64 } = (*sizerReadCloser)(nil)
+
+type sizerReadCloser struct {
+	io.ReadCloser
+	size int64
+}
+
+func (s sizerReadCloser) Size() int64 {
+	return s.size
+}