clairctl: warn when range requests are not honored

As discussed in #2029 the manifest
generation is efficient only when HTTP "Range: bytes=0-0" header is well
supported by the server. If not, the server might return the full layer,
making the manifest generation very expensive.

We warn the user in case their HTTP server does not support HTTP range
requests.

Signed-off-by: François Rigault <frigo@amadeus.com>

cmd/clairctl/manifest.go

@@ -141,12 +141,22 @@ func Inspect(ctx context.Context, r string) (*claircore.Manifest, error) {
 		if err != nil {
 			return nil, err
 		}
+		// The request is needed to follow any redirection chain that the server sends to a client,
+		// but the actual body is not needed when generating a manifest.
+		// The Range HTTP header allows us to send the request and get a response mostly for free.
 		req.Header.Add("Range", "bytes=0-0")
 		res, err := c.Do(req)
 		if err != nil {
 			return nil, err
 		}
 		res.Body.Close()
+		if res.StatusCode != http.StatusPartialContent {
+			zlog.Warn(ctx).
+				Int("statuscode", res.StatusCode).
+				Int("len", int(res.ContentLength)).
+				Str("url", u.String()).
+				Msg("server might not support requests with Range HTTP header")
+		}
 
 		res.Request.Header.Del("User-Agent")
 		res.Request.Header.Del("Range")