Merge branch 'abderrahim/etag' into 'master'

_downloadablefilesource.py: don't download the file if etag matches

See merge request BuildStream/buildstream!1833

src/buildstream/plugins/sources/_downloadablefilesource.py

@@ -178,7 +178,11 @@ def _ensure_mirror(self):
                 with contextlib.closing(opener.open(request)) as response:
                     info = response.info()
 
-                    etag = info["ETag"] if "ETag" in info else None
+                    # some servers don't honor the 'If-None-Match' header
+                    if self.ref and etag and info["ETag"] == etag:
+                        return self.ref
+
+                    etag = info["ETag"]
 
                     filename = info.get_filename(default_name)
                     filename = os.path.basename(filename)