tests: Add first_stage_test with closed STDIN

Without the next commit, this hangs forever in the select call.

Signed-off-by: Marc Hartmayer <[email protected]>

Author: mhartmay

tests/first_stage_test.py

@@ -1,4 +1,5 @@
 import subprocess
+import sys
 
 import mitogen.core
 import mitogen.parent
@@ -82,3 +83,34 @@ def test_eof_too_early(self):
             b("Error -3 while decompressing data"),  # Unknown compression method
             stderr,
         )
+
+    def test_timeout_error(self):
+        options = mitogen.parent.Options(max_message_size=123)
+        conn = mitogen.parent.Connection(options, self.router)
+        conn.context = mitogen.core.Context(None, 123)
+        args = conn.get_boot_command()
+
+        # The boot command should write an ECO marker to stdout, read the
+        # preamble from stdin, then execute it.
+
+        # This test attaches closes stdin to create a specific failure
+        # 1. Fork child tries to read from STDIN, but fails as it is closed
+        # 2. Fork child raises TimeoutError
+        # 3. Fork child's file descriptors (write pipes) are closed by the OS
+        # 4. Fork parent does `dup(<read pipe>, <stdin>)` and `exec(<python>)`
+        # 5. Python reads `b''` (i.e. EOF) from stdin (a closed pipe)
+        # 6. Python runs `''` (a valid script) and exits with success
+
+        proc = mitogen.parent.popen(args=args,
+                                    stdout=subprocess.PIPE,
+                                    stderr=subprocess.PIPE,
+                                    preexec_fn=sys.stdin.close
+                                    )
+        stdout, stderr = proc.communicate()
+        self.assertEqual(0, proc.returncode)
+        self.assertEqual(stdout,
+                         mitogen.parent.BootstrapProtocol.EC0_MARKER+b('\n'))
+        self.assertIn(
+            b("TimeoutError"),
+            stderr,
+        )