jsoncom: gracefully report EMSGSIZE errors

When `jsoncomm` fails because the message is too big it currently
does not indicate just how big the message was. This commit adds
this information so that it's easier for us to determine what to
do about it.

We could also include a pointer to `/proc/sys/net/core/wmem_defaults`
but it seems we want to not require fiddling with that so let's
not do it for now.

See also osbuild#1838

osbuild/util/jsoncomm.py

@@ -404,7 +404,13 @@ def send(self, payload: object, *, fds: Optional[list] = None):
         if fds:
             cmsg.append((socket.SOL_SOCKET, socket.SCM_RIGHTS, array.array("i", fds)))
 
-        n = self._socket.sendmsg([serialized], cmsg, 0)
+        try:
+            n = self._socket.sendmsg([serialized], cmsg, 0)
+        except OSError as exc:
+            if exc.errno == errno.EMSGSIZE:
+                raise BufferError(
+                    f"jsoncomm message size {len(serialized)} is too big") from exc
+            raise exc
         assert n == len(serialized)
 
     def send_and_recv(self, payload: object, *, fds: Optional[list] = None):

test/mod/test_util_jsoncomm.py

@@ -3,12 +3,15 @@
 #
 
 import asyncio
+import errno
 import os
 import pathlib
 import tempfile
 import unittest
 from concurrent import futures
 
+import pytest
+
 from osbuild.util import jsoncomm
 
 
@@ -216,3 +219,12 @@ def test_send_and_recv(self):
         self.assertEqual(ping, pong)
         pong, _, _ = a.recv()
         self.assertEqual(ping, pong)
+
+    def test_send_and_recv_tons_of_data_still_errors(self):
+        a, _ = jsoncomm.Socket.new_pair()
+
+        ping = {"data": "1" * 1_000_000}
+        with pytest.raises(BufferError) as exc:
+            a.send(ping)
+        assert str(exc.value) == "jsoncomm message size 1000012 is too big"
+        assert exc.value.__cause__.errno == errno.EMSGSIZE