From 9599348f9b668feafd0a05db012e8806cda8389f Mon Sep 17 00:00:00 2001
From: "Nathaniel J. Smith" <njs@pobox.com>
Date: Tue, 12 May 2020 12:48:23 -0700
Subject: [PATCH] make trio.Process.returncode automatically update itself as
 needed

---
 newsfragments/1315.feature.rst |  4 +++
 trio/_subprocess.py            | 47 +++++++++++++++++++---------------
 trio/tests/test_subprocess.py  | 11 ++++++++
 3 files changed, 42 insertions(+), 20 deletions(-)
 create mode 100644 newsfragments/1315.feature.rst

diff --git a/newsfragments/1315.feature.rst b/newsfragments/1315.feature.rst
new file mode 100644
index 0000000000..824a793e74
--- /dev/null
+++ b/newsfragments/1315.feature.rst
@@ -0,0 +1,4 @@
+The `trio.Process.returncode` attribute is now automatically updated
+as needed, instead of only when you call `~trio.Process.poll` or
+`~trio.Process.wait`. Also, ``repr(process_object)`` now always
+contains up-to-date information about the process status.
diff --git a/trio/_subprocess.py b/trio/_subprocess.py
index 6983c839c7..ab6ab76a7e 100644
--- a/trio/_subprocess.py
+++ b/trio/_subprocess.py
@@ -133,19 +133,20 @@ def __init__(self, popen, stdin, stdout, stderr):
         self.pid = self._proc.pid
 
     def __repr__(self):
-        if self.returncode is None:
+        returncode = self.returncode
+        if returncode is None:
             status = "running with PID {}".format(self.pid)
         else:
-            if self.returncode < 0:
-                status = "exited with signal {}".format(-self.returncode)
+            if returncode < 0:
+                status = "exited with signal {}".format(-returncode)
             else:
-                status = "exited with status {}".format(self.returncode)
+                status = "exited with status {}".format(returncode)
         return "<trio.Process {!r}: {}>".format(self.args, status)
 
     @property
     def returncode(self):
-        """The exit status of the process (an integer), or ``None`` if it is
-        not yet known to have exited.
+        """The exit status of the process (an integer), or ``None`` if it's
+        still running.
 
         By convention, a return code of zero indicates success.  On
         UNIX, negative values indicate termination due to a signal,
@@ -153,10 +154,16 @@ def returncode(self):
         Windows, a process that exits due to a call to
         :meth:`Process.terminate` will have an exit status of 1.
 
-        Accessing this attribute does not check for termination;
-        use :meth:`poll` or :meth:`wait` for that.
+        Unlike the standard library `subprocess.Popen.returncode`, you don't
+        have to call `poll` or `wait` to update this attribute; it's
+        automatically updated as needed, and will always give you the latest
+        information.
+
         """
-        return self._proc.returncode
+        result = self._proc.poll()
+        if result is not None:
+            self._close_pidfd()
+        return result
 
     async def aclose(self):
         """Close any pipes we have to the process (both input and output)
@@ -175,7 +182,7 @@ async def aclose(self):
         try:
             await self.wait()
         finally:
-            if self.returncode is None:
+            if self._proc.returncode is None:
                 self.kill()
                 with trio.CancelScope(shield=True):
                     await self.wait()
@@ -205,20 +212,20 @@ async def wait(self):
                 # actually block for a tiny fraction of a second.
                 self._proc.wait()
                 self._close_pidfd()
-        assert self.returncode is not None
-        return self.returncode
+        assert self._proc.returncode is not None
+        return self._proc.returncode
 
     def poll(self):
-        """Check if the process has exited yet.
+        """Returns the exit status of the process (an integer), or ``None`` if
+        it's still running.
+
+        Note that on Trio (unlike the standard library `subprocess.Popen`),
+        ``process.poll()`` and ``process.returncode`` always give the same
+        result. See `returncode` for more details. This method is only
+        included to make it easier to port code from `subprocess`.
 
-        Returns:
-          The exit status of the process, or ``None`` if it is still
-          running; see :attr:`returncode`.
         """
-        result = self._proc.poll()
-        if result is not None:
-            self._close_pidfd()
-        return result
+        return self.returncode
 
     def send_signal(self, sig):
         """Send signal ``sig`` to the process.
diff --git a/trio/tests/test_subprocess.py b/trio/tests/test_subprocess.py
index b9290f0401..ebc838b0ed 100644
--- a/trio/tests/test_subprocess.py
+++ b/trio/tests/test_subprocess.py
@@ -47,6 +47,7 @@ async def test_basic():
         assert repr(proc) == repr_template.format(
             "running with PID {}".format(proc.pid)
         )
+    assert proc._pidfd is None
     assert proc.returncode == 0
     assert repr(proc) == repr_template.format("exited with status 0")
 
@@ -58,6 +59,16 @@ async def test_basic():
     )
 
 
+async def test_auto_update_returncode():
+    p = await open_process(SLEEP(9999))
+    assert p.returncode is None
+    p.kill()
+    p._proc.wait()
+    assert p.returncode is not None
+    assert p._pidfd is None
+    assert p.returncode is not None
+
+
 async def test_multi_wait():
     async with await open_process(SLEEP(10)) as proc:
         # Check that wait (including multi-wait) tolerates being cancelled