test/host: add check for call with fds

Create a new test that checks method calls that pass file
descriptors in both directions.

test/mod/test_host.py

@@ -4,7 +4,9 @@
 # Runtime Tests for Host Services
 #
 
+import os
 import sys
+import tempfile
 from typing import Any
 
 import pytest
@@ -16,12 +18,20 @@ from osbuild.util.jsoncomm import FdSet
 class ServiceTest(host.Service):
 
     def dispatch(self, method: str, args: Any, fds: FdSet):
-        ret, fds = None, None
+        ret = None
 
         if method == "exception":
             raise ValueError("Remote Exception")
         if method == "echo":
             ret = args
+        elif method == "echo-fd":
+            ret = args
+            with tempfile.TemporaryFile("w+") as f:
+                with os.fdopen(fds.steal(0)) as d:
+                    f.write(d.read())
+                f.seek(0)
+                fds = [os.dup(f.fileno())]
+
         elif method == "identify":
             ret = self.id
         else:
@@ -51,6 +61,35 @@ def test_basic():
             client.stop()
 
 
+def test_pass_fd():
+    with host.ServiceManager() as mgr:
+        for i in range(3):
+            client = mgr.start(str(i), __file__)
+
+            args = ["an", "argument"]
+            data = "osbuild\n"
+
+            with tempfile.TemporaryFile("w+") as f:
+                f.write(data)
+                f.seek(0)
+
+                res, fds = client.call_with_fds("echo-fd", args, fds=[f.fileno()])
+
+            assert args == res
+            with os.fdopen(fds.steal(0)) as d:
+                assert data == d.read()
+
+            remote_id = client.call("identify")
+            assert remote_id == str(i)
+
+            with pytest.raises(ValueError, match=f"{str(i)}"):
+                _ = mgr.start(str(i), __file__)
+
+        for i in range(3):
+            client = mgr.services[str(i)]
+            client.stop()
+
+
 def test_exception():
     with host.ServiceManager() as mgr:
         client = mgr.start("exception", __file__)