Multipart download enhancements (#4021)

* fix(api): set blob response Content-Type from OCI descriptor

Blob HEAD responses had no Content-Type and GET responses echoed the
request's Accept header verbatim, which produced missing or malformed
media types and left multipart/byteranges parts without a per-part
Content-Type. This breaks OCI distribution-spec conformance and
consumers like stargz-snapshotter that need a well-formed layer media
type.

Add a blobResponseMediaType helper that resolves the descriptor's
MediaType via GetBlobDescriptorFromRepo and falls back to
application/octet-stream. Use it in CheckBlob (HEAD), GetBlob full
(200), GetBlob single-range (206), and per-part in
writeMultipartRanges (206 multipart). Lookup is deferred until after
the blob is known to exist.

Cover the new behaviour with mock-based unit tests in routes_test.go
and end-to-end assertions in TestPullRange.

Signed-off-by: Andrei Aaron <andreifdaaron@gmail.com>

* perf(api): stream multipart blob ranges lazily with precomputed Content-Length

writeMultipartRanges previously opened every range reader up front
and emitted no Content-Length, so an N-range request held N
concurrent storage readers (and their fds / read buffers) per
response window and forced chunked encoding on HTTP/1.1 — neither
friendly to proxies nor to fan-out scenarios like stargz lazy pulls.

Signed-off-by: Andrei Aaron <andreifdaaron@gmail.com>

---------

Signed-off-by: Andrei Aaron <andreifdaaron@gmail.com>

Author: andaaron

pkg/api/controller_test.go

@@ -11247,6 +11247,10 @@ func TestPullRange(t *testing.T) {
 			So(err, ShouldBeNil)
 			So(resp.Header().Get("Accept-Ranges"), ShouldEqual, "bytes")
 			So(resp.StatusCode(), ShouldEqual, http.StatusOK)
+			// HEAD on an unreferenced blob (no manifest/index) must still
+			// advertise a valid Content-Type. The descriptor lookup fails
+			// here, so we expect the application/octet-stream fallback.
+			So(resp.Header().Get("Content-Type"), ShouldEqual, "application/octet-stream")
 		})
 
 		Convey("Get a range of bytes", func() {
@@ -11255,6 +11259,11 @@ func TestPullRange(t *testing.T) {
 			So(resp.StatusCode(), ShouldEqual, http.StatusPartialContent)
 			So(resp.Header().Get("Content-Length"), ShouldEqual, strconv.Itoa(len(content)))
 			So(resp.Body(), ShouldResemble, content)
+			// Single-range 206: descriptor lookup fails (blob is not
+			// referenced by any manifest), so Content-Type falls back to
+			// application/octet-stream rather than echoing the request
+			// Accept header.
+			So(resp.Header().Get("Content-Type"), ShouldEqual, "application/octet-stream")
 
 			resp, err = resty.R().SetHeader("Range", "bytes=0-100").Get(blobLoc)
 			So(err, ShouldBeNil)
@@ -11318,6 +11327,11 @@ func TestPullRange(t *testing.T) {
 			part, err := multipartReader.NextPart()
 			So(err, ShouldBeNil)
 			So(part.Header.Get("Content-Range"), ShouldEqual, "bytes 0-1/10")
+			// Per-part Content-Type reflects the descriptor-aware
+			// resolution. For an unreferenced blob this is the
+			// application/octet-stream fallback; for a real OCI layer it
+			// would carry the manifest's layer media type.
+			So(part.Header.Get("Content-Type"), ShouldEqual, "application/octet-stream")
 
 			partBody, err := io.ReadAll(part)
 			So(err, ShouldBeNil)
@@ -11326,6 +11340,7 @@ func TestPullRange(t *testing.T) {
 			part, err = multipartReader.NextPart()
 			So(err, ShouldBeNil)
 			So(part.Header.Get("Content-Range"), ShouldEqual, "bytes 4-6/10")
+			So(part.Header.Get("Content-Type"), ShouldEqual, "application/octet-stream")
 
 			partBody, err = io.ReadAll(part)
 			So(err, ShouldBeNil)