cvsclient: handle files which lack a trailing newline

CVS uses \n as a protocol message separator, which forces us
to read protocol message line-by-line. File content sent by
the server has a length known and is transmitted in bytes.
The server appends a final "ok\n" message (or perhaps an error
message) when it is done sending file contents.

Properly handle the case where this final message gets buffered
along with file contents and is not delimited from file contents
by \n because the file lacks a trailing newline. Previously, the
final protocol message ended up being written out to file contents
in this case.

Found while testing ingestion of the GNU dino CVS repository from
cvs.savannah.gnu.org/sources/dino.

swh/loader/cvs/cvsclient.py

@@ -335,23 +335,33 @@ class CVSClient:
             % (self.cvs_module_name, self.cvs_module_name, rev, path)
         )
         while True:
-            if have_bytecount and bytecount > 0:
+            if have_bytecount:
+                if bytecount < 0:
+                    raise CVSProtocolError("server sent too much file content data")
                 response = self.conn_read_line(require_newline=False)
                 if response is None:
                     raise CVSProtocolError("Incomplete response from CVS server")
-                co_output.write(response)
-                bytecount -= len(response)
-                if bytecount < 0:
-                    raise CVSProtocolError(
-                        "Overlong response from " "CVS server: %s" % response
-                    )
-                continue
+                if len(response) > bytecount:
+                    # When a file lacks a final newline we receive a line which
+                    # contains file content as well as CVS protocol response data.
+                    # Split last line of file content from CVS protocol data...
+                    co_output.write(response[:bytecount])
+                    response = response[bytecount:]
+                    bytecount = 0
+                    # ...and process the CVS protocol response below.
+                else:
+                    co_output.write(response)
+                    bytecount -= len(response)
+                    continue
             else:
                 response = self.conn_read_line()
             if response[0:2] == b"E ":
                 raise CVSProtocolError("Error from CVS server: %s" % response)
-            if have_bytecount and bytecount == 0 and response == b"ok\n":
-                break
+            if response == b"ok\n":
+                if have_bytecount:
+                    break
+                else:
+                    raise CVSProtocolError("server sent 'ok' but no file contents")
             if skip_line:
                 skip_line = False
                 continue