test: add shared fake_process fixture for complete Popen mocking

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/conftest.py

@@ -138,3 +138,28 @@ def test_db(tmp_path):
     conn.execute("PRAGMA journal_mode = WAL")
     yield conn
     conn.close()
+
+
+@pytest.fixture
+def fake_process():
+    """Factory for complete subprocess.Popen replacements.
+
+    Hand-rolled Popen mocks keep missing two things: __enter__ (subprocess.run
+    wraps Popen in a context manager) and a communicate() tuple. Use this
+    factory instead of building MagicMock processes inline.
+    """
+
+    def _make(returncode=0, stdout="", stderr="", running=True, pid=12345):
+        proc = MagicMock()
+        proc.poll.return_value = None if running else returncode
+        proc.returncode = returncode
+        proc.pid = pid
+        proc.wait.return_value = returncode
+        proc.communicate.return_value = (stdout, stderr)
+        proc.stdout.read.return_value = stdout
+        proc.stderr.read.return_value = stderr
+        proc.stdin = MagicMock()
+        proc.__enter__.return_value = proc
+        return proc
+
+    return _make

tests/test_conftest_fixtures.py

@@ -0,0 +1,24 @@
+"""Tests for shared conftest fixtures."""
+
+import subprocess
+from unittest.mock import patch
+
+
+class TestFakeProcess:
+    def test_works_with_subprocess_run(self, fake_process):
+        """subprocess.run() must unpack communicate() and enter the context manager."""
+        proc = fake_process(stdout="hello", stderr="", returncode=0)
+        with patch("subprocess.Popen", return_value=proc):
+            result = subprocess.run(["anything"], capture_output=True, text=True, timeout=5)
+        assert result.stdout == "hello"
+
+    def test_running_process_defaults(self, fake_process):
+        proc = fake_process()
+        assert proc.poll() is None  # still running
+        assert proc.pid == 12345
+        assert proc.communicate() == ("", "")
+
+    def test_exited_process(self, fake_process):
+        proc = fake_process(running=False, returncode=1, stderr=b"device busy")
+        assert proc.poll() == 1
+        assert proc.stderr.read() == b"device busy"