cvsclient: Handle error in fetch_rlog when path does not exist

When attempting to fetch the rlog for a path that does not exist in
the repository, the CVS server will respond with the following lines:

E cvs rlog: could not read RCS file for <path>
ok

That error case was not handled in fetch_rlog so ensure it returns None
when encountering it.

The issue was spotted when the loader attempts to fetch more rlog data from
Attic directories. The paths of these Attic directories are computed from
those of the files in the repositories but it exist cases where those
directories do not exist.

swh/loader/cvs/cvsclient.py

@@ -339,9 +339,12 @@ class CVSClient:
             if response is None:
                 raise CVSProtocolError("No response from CVS server")
             if response[0:2] == b"E ":
-                if len(path) > 0 and response[-11:] == b" - ignored\n":
+                if len(path) > 0 and (
+                    response.endswith(b" - ignored\n")
+                    or b"could not read RCS file" in response
+                ):
                     response = self.conn_read_line()
-                    if response != b"error  \n":
+                    if response not in (b"error  \n", b"ok\n"):
                         raise CVSProtocolError(
                             "Invalid response from CVS server: %s" % response
                         )

swh/loader/cvs/tests/test_cvsclient.py

+# Copyright (C) 2022  The Software Heritage developers
+# See the AUTHORS file at the top-level directory of this distribution
+# License: GNU Affero General Public License version 3, or any later version
+# See top-level LICENSE file for more information
+
+from urllib.parse import urlparse
+
+from swh.loader.cvs.cvsclient import CVSClient
+
+
+def test_cvs_client_rlog_could_not_read_rcs_file(mocker):
+
+    url = "ssh://anoncvs@anoncvs.example.org/cvsroot/src"
+    file = "src/README.txt"
+
+    mocker.patch("swh.loader.cvs.cvsclient.socket")
+    conn_read_line = mocker.patch.object(CVSClient, "conn_read_line")
+    conn_read_line.side_effect = [
+        # server response lines when client is initialized
+        b"Valid-requests ",
+        b"ok\n",
+        # server response lines when attempting to fetch rlog of a file
+        # but RCS file is missing
+        f"E cvs rlog: could not read RCS file for {file}\n".encode(),
+        b"ok\n",
+    ]
+
+    client = CVSClient(urlparse(url))
+
+    assert client.fetch_rlog(file.encode()) is None