Fix TUS offset queries in production environments

Previously, `mod_wsgi` would convert `HEAD` requests into `GET`, which
would be rejected, so clients were unable to resume an upload that failed
midway through.

To make use of this, update the SDK code to enable upload resumption.

CHANGELOG.md

@@ -61,6 +61,8 @@ non-ascii paths while adding files from "Connected file share" (issue #4428)
   (<https://github.com/opencv/cvat/issues/5048>)
 - Changing an object causes current z layer to be set to the maximum (<https://github.com/opencv/cvat/pull/5145>)
 - Create manifest with cvat/server docker container command (<https://github.com/opencv/cvat/pull/5172>)
+- Fixed upload resumption in production environments
+  (<https://github.com/opencv/cvat/issues/4839>)
 
 ### Security
 - TDB

cvat-sdk/cvat_sdk/core/uploading.py

@@ -13,6 +13,7 @@
 import urllib3
 
 from cvat_sdk.api_client.api_client import ApiClient, Endpoint
+from cvat_sdk.api_client.exceptions import ApiException
 from cvat_sdk.api_client.rest import RESTClientObject
 from cvat_sdk.core.helpers import StreamWithProgress, expect_status
 from cvat_sdk.core.progress import ProgressReporter
@@ -211,18 +212,31 @@ def get_offset(self):
                 This is different from the instance attribute 'offset' because this makes an
                 http request to the tus server to retrieve the offset.
                 """
-                # FIXME: traefik changes HEAD to GET for some reason, and it breaks the protocol
-
-                # Assume we are starting from scratch. This effectively disallows us to resume
-                # old file uploading
-                return 0
-
-                # resp = self._api_client.rest_client.HEAD(self.url, headers=self.headers)
-                # offset = resp.headers.get("upload-offset")
-                # if offset is None:
-                #     msg = "Attempt to retrieve offset fails with status {}".format(resp.status_code)
-                #     raise tus_uploader.TusCommunicationError(msg, resp.status_code, resp.content)
-                # return int(offset)
+                try:
+                    resp = self._api_client.rest_client.HEAD(self.url, headers=self.headers)
+                except ApiException as ex:
+                    if ex.status == 405:  # Method Not Allowed
+                        # In CVAT up to version 2.2.0, HEAD requests were internally
+                        # converted to GET by mod_wsgi, and subsequently rejected by the server.
+                        # For compatibility with old servers, we'll handle such rejections by
+                        # restarting the upload from the beginning.
+                        return 0
+
+                    raise tus_uploader.TusCommunicationError(
+                        f"Attempt to retrieve offset failed with status {ex.status}",
+                        ex.status,
+                        ex.body,
+                    ) from ex
+
+                offset = resp.headers.get("upload-offset")
+                if offset is None:
+                    raise tus_uploader.TusCommunicationError(
+                        f"Attempt to retrieve offset failed with status {resp.status}",
+                        resp.status,
+                        resp.data,
+                    )
+
+                return int(offset)
 
         # Add headers required by CVAT server
         headers = {}

mod_wsgi.conf

@@ -2,3 +2,15 @@ LoadModule xsendfile_module /usr/lib/apache2/modules/mod_xsendfile.so
 XSendFile On
 XSendFilePath ${HOME}/data/
 XSendFilePath ${HOME}/static/
+
+# The presence of an Apache output filter (mod_xsendfile) causes mod_wsgi
+# to internally convert HEAD requests to GET before passing them to the
+# application, for reasons explained here:
+# <http://blog.dscpl.com.au/2009/10/wsgi-issues-with-http-head-requests.html>.
+# However, we need HEAD requests passed through as-is, because the TUS
+# protocol requires them. It should be safe to disable this functionality in
+# our case, because mod_xsendfile does not examine the response body (it
+# either passes it through or discards it entirely based on the headers),
+# so it shouldn't matter whether the application omits the body in response
+# to a HEAD request.
+WSGIMapHEADToGET Off