Add a test for qrexec-client race condition

Check if the service exit code is correctly retrieved even if the
the service terminates at the exact moment the qrexec-client tries to
send some data.
Try to win the race by initially sending SIGSTOP to the qrexec-client
process, and sending SIGCONT only after preparing both local and remote
data streams. qrexec-client will handle local data stream first, at
which point remote socket is already closed.

Similar issue applies to qrexec-client-vm, but since the implementation
is shared, one test is enough.

QubesOS/qubes-issues#9618

qrexec/tests/socket/daemon.py

@@ -28,6 +28,7 @@
 import time
 import itertools
 import socket
+import signal
 
 import psutil
 import pytest
@@ -664,6 +665,7 @@ def start_client(self, args):
         self.client = subprocess.Popen(
             cmd,
             env=env,
+            stdin=subprocess.PIPE,
             stdout=subprocess.PIPE,
         )
         self.addCleanup(self.stop_client)
@@ -744,6 +746,65 @@ def test_run_vm_command_from_dom0(self):
         self.client.wait()
         self.assertEqual(self.client.returncode, 42)
 
+    def test_run_vm_command_from_dom0_reject_stdin(self):
+        """Test if qrexec-client properly returns remote exit code even if
+            service didn't read all of stdin"""
+        cmd = "user:command"
+        target_domain_name = "target_domain"
+        target_domain_uuid = "d95e1147-2d82-4595-90bb-5a7500cc3196"
+        target_domain = 42
+        target_port = 513
+
+        target_daemon = self.connect_daemon(
+            target_domain, target_domain_name, target_domain_uuid
+        )
+        self.start_client(["-d", target_domain_name, cmd])
+        target_daemon.accept()
+        target_daemon.handshake()
+
+        # negotiate_connection_params
+        self.assertEqual(
+            target_daemon.recv_message(),
+            (
+                qrexec.MSG_EXEC_CMDLINE,
+                struct.pack("<LL", 0, 0) + cmd.encode() + b"\0",
+            ),
+        )
+        target_daemon.send_message(
+            qrexec.MSG_EXEC_CMDLINE,
+            struct.pack("<LL", target_domain, target_port),
+        )
+
+        target = self.connect_target(target_domain, target_port)
+        target.handshake()
+
+        self.client.send_signal(signal.SIGSTOP)
+
+        # select_loop
+        target.send_message(qrexec.MSG_DATA_STDOUT, b"stdout data\n")
+        target.send_message(qrexec.MSG_DATA_STDOUT, b"")
+        target.send_message(qrexec.MSG_DATA_EXIT_CODE, struct.pack("<L", 42))
+        target.close()
+
+        # ...but still send some data
+        self.client.stdin.write(b"A" * 4096)
+        self.client.stdin.write(b"A" * 4096)
+        self.client.stdin.write(b"A" * 4096)
+        self.client.stdin.write(b"A" * 4096)
+        self.client.stdin.write(b"A" * 4096)
+        self.client.stdin.write(b"A" * 4096)
+        self.client.stdin.write(b"A" * 4096)
+        self.client.stdin.write(b"A" * 4096)
+        self.client.stdin.write(b"A" * 4096)
+        self.client.stdin.flush()
+
+        # and only now let the client process both data streams
+        self.client.send_signal(signal.SIGCONT)
+
+        self.assertEqual(self.client.stdout.read(), b"stdout data\n")
+        self.client.wait()
+        self.assertEqual(self.client.returncode, 42)
+
     def test_run_vm_command_from_dom0_with_local_command(self):
         cmd = "user:command"
         local_cmd = "while read x; do echo input: $x; done; exit 44"