fix(console): a serial console worker's failure reaches socketiod's log

- The worker told why a console did not open or closed only in the user's
  terminal (as users see errors: a host that cannot be reached is not
  named). It now also logs the full reason to socketiod's own stderr, the
  journal or docker logs, which socketiod hands it as a descriptor.
- A refused token says the link has expired or the user may no longer
  open the console: a valid link for a user who lost access was told only
  that it had expired.
- A send that takes nothing (0 bytes) waits like a busy stream instead of
  spinning.

console/tests/test_socketiod.py

@@ -242,8 +242,8 @@ class WorkerIsolationTestCase(Base):
         while b"]" not in output:
             select.select([fd], [], [], 5)
             output += os.read(fd, 1024)
-        # 0-2: its terminal, 3: the listing itself
-        self.assertEqual(output.decode().strip(), "['0', '1', '2', '3']")
+        # 0-2: its terminal, 3: socketiod's log, 4: the listing itself
+        self.assertEqual(output.decode().strip(), "['0', '1', '2', '3', '4']")
 
 
 def _open(fd):

console/tests/test_socketiod_child.py

@@ -70,8 +70,9 @@ def start_socketiod(test):
     port = free_port()
     env = dict(os.environ, DJANGO_SETTINGS_MODULE="socketiod_test_settings",
                PYTHONPATH=os.pathsep.join([tmp.name, str(ROOT)]))
+    test.log = Path(tmp.name) / "socketiod.log"  # its stderr, as the journal would have it
     server = subprocess.Popen([sys.executable, str(SOCKETIOD), "-H", "127.0.0.1", "-p", str(port)],
-                              cwd=ROOT, env=env, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
+                              cwd=ROOT, env=env, stdout=subprocess.DEVNULL, stderr=test.log.open("w"))
     test.addCleanup(lambda: (server.kill(), server.wait()))
     end = time.monotonic() + 20
     while time.monotonic() < end:
@@ -125,6 +126,10 @@ class SocketiodProcessTestCase(TransactionTestCase):
             time.sleep(0.05)
         self.assertEqual(consoles(self.server.pid), [])
         self.assertIsNone(self.server.poll())
+        # the operator finds why in socketiod's log, with the details the user is not shown
+        log = self.log.read_text()
+        for name in self.tokens:
+            self.assertIn("serial console of VM %s did not open" % name, log)
 
     def test_a_client_without_a_valid_token_is_refused(self):
         for token in ("", "22-no-such-vm"):