From 91f6899d7599bf3de2495f8ebcf6823752a9d33b Mon Sep 17 00:00:00 2001
From: "Masih H. Derkani" <m@derkani.org>
Date: Tue, 8 Aug 2023 13:14:17 +0100
Subject: [PATCH] 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      | 10 ++++++++--
 api/server/sizer_reader.go | 14 ++++++++++++++
 2 files changed, 22 insertions(+), 2 deletions(-)
 create mode 100644 api/server/sizer_reader.go

diff --git a/api/server/handler.go b/api/server/handler.go
index 4b3cf28..7bf82bc 100644
--- a/api/server/handler.go
+++ b/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:
diff --git a/api/server/sizer_reader.go b/api/server/sizer_reader.go
new file mode 100644
index 0000000..d6d27b0
--- /dev/null
+++ b/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
+}