fix(console): a serial console worker holds only its own terminal

pty.fork returns an inheritable master descriptor, so each new console
worker, and the ssh it may start, held the terminals of every other open
session until it ended. A worker now closes every descriptor above its own
terminal before it starts.

console/socketiod

@@ -52,6 +52,9 @@ def connect(sid, environ, auth):
     pid, fd = pty.fork()
     if pid == 0:
         try:
+            # pty.fork's master is inheritable: without this each worker would
+            # hold the other sessions' consoles open
+            os.closerange(3, os.sysconf("SC_OPEN_MAX"))
             os.execv(sys.executable, [sys.executable, CONSOLECALLBACK, str(instance.pk)])
         finally:
             os._exit(1)

console/tests/test_socketiod.py

@@ -1,6 +1,7 @@
 """socketiod: who may connect, and each session's console kept to itself."""
 
 import os
+import tempfile
 import time
 from importlib.machinery import SourceFileLoader
 from importlib.util import module_from_spec, spec_from_loader
@@ -129,6 +130,30 @@ class SocketiodTestCase(Base):
         self.d.os.kill.assert_called_once_with(pid, self.d.signal.SIGKILL)
 
 
+class WorkerIsolationTestCase(Base):
+    """A real fork: a session's worker holds no other session's console."""
+
+    @override_settings(SERIAL_CONSOLE_ENABLED=True)
+    def test_a_worker_has_only_its_own_terminal(self):
+        d = load_socketiod()
+        other = Instance.objects.create(compute=self.compute, name="vm-b", uuid="11111111-0000-0000-0000-000000000002")
+        UserInstance.objects.create(user=self.user, instance=other)
+        with tempfile.NamedTemporaryFile("w", suffix=".py", delete=False) as script:
+            # stands in for consolecallback: prints the descriptors it holds
+            script.write("import os, time\nprint(sorted(os.listdir('/proc/self/fd')))\ntime.sleep(5)\n")
+        self.addCleanup(os.unlink, script.name)
+        with patch.object(d, "CONSOLECALLBACK", script.name), patch.object(d.sio, "start_background_task"), \
+                patch.object(d.db, "close_old_connections"):
+            d.connect("first", {}, {"token": self.token()})
+            d.connect("second", {}, {"token": self.token(vm=other)})
+        self.addCleanup(lambda: [d.stop(sid) for sid in list(d.sessions)])
+        output = b""
+        while b"]" not in output:
+            output += os.read(d.sessions["second"]["fd"], 1024)
+        # 0-2: its terminal, 3: the listing itself
+        self.assertEqual(output.decode().strip(), "['0', '1', '2', '3']")
+
+
 def _open(fd):
     try:
         os.fstat(fd)