From 75725a8148d647b5524d1381758e6c23f586a99e Mon Sep 17 00:00:00 2001
From: Joshua Oreman <oremanj@gmail.com>
Date: Tue, 5 Feb 2019 13:18:58 -0800
Subject: [PATCH 1/2] Cancel a cancel scope upon entry if its deadline is in
 the past

Fixes #320. Now that #901 is implemented, the concerns discussed in #835 don't apply.
---
 newsfragments/320.misc.rst   |  4 ++++
 trio/_core/_run.py           |  2 ++
 trio/_core/tests/test_run.py | 15 ++++++++-------
 trio/_core/tests/tutil.py    |  2 +-
 4 files changed, 15 insertions(+), 8 deletions(-)
 create mode 100644 newsfragments/320.misc.rst

diff --git a/newsfragments/320.misc.rst b/newsfragments/320.misc.rst
new file mode 100644
index 0000000000..3f799b9eb0
--- /dev/null
+++ b/newsfragments/320.misc.rst
@@ -0,0 +1,4 @@
+Entering a cancel scope whose deadline is in the past now immediately
+cancels it, so :exc:`~trio.Cancelled` will be raised by the first
+checkpoint in the cancel scope rather than the second one.
+This also affects constructs like ``with trio.move_on_after(0):``.
diff --git a/trio/_core/_run.py b/trio/_core/_run.py
index e00295a1ca..6faeced12f 100644
--- a/trio/_core/_run.py
+++ b/trio/_core/_run.py
@@ -201,6 +201,8 @@ def __enter__(self):
             )
         self._scope_task = task
         self.cancelled_caught = False
+        if current_time() >= self._deadline:
+            self.cancel_called = True
         with self._might_change_effective_deadline():
             self._add_task(task)
         return self
diff --git a/trio/_core/tests/test_run.py b/trio/_core/tests/test_run.py
index e03f6da03a..a6ed154ab0 100644
--- a/trio/_core/tests/test_run.py
+++ b/trio/_core/tests/test_run.py
@@ -407,12 +407,13 @@ async def main():
 
     _core.run(main, instruments=[r1, r2])
 
-    # It sleeps 5 times, so it runs 6 times
+    # It sleeps 5 times, so it runs 6 times.  Note that checkpoint()
+    # reschedules the task immediately upon yielding, before the
+    # after_task_step event fires.
     expected = (
-        [("before_run",)] +
-        6 * [("schedule", task),
-             ("before", task),
-             ("after", task)] + [("after_run",)]
+        [("before_run",), ("schedule", task)] +
+        [("before", task), ("schedule", task), ("after", task)] * 5 +
+        [("before", task), ("after", task), ("after_run",)]
     )
     assert len(r1.record) > len(r2.record) > len(r3.record)
     assert r1.record == r2.record + r3.record
@@ -444,15 +445,15 @@ async def main():
         ("schedule", tasks["t2"]),
         {
             ("before", tasks["t1"]),
+            ("schedule", tasks["t1"]),
             ("after", tasks["t1"]),
             ("before", tasks["t2"]),
+            ("schedule", tasks["t2"]),
             ("after", tasks["t2"])
         },
         {
-            ("schedule", tasks["t1"]),
             ("before", tasks["t1"]),
             ("after", tasks["t1"]),
-            ("schedule", tasks["t2"]),
             ("before", tasks["t2"]),
             ("after", tasks["t2"])
         },
diff --git a/trio/_core/tests/tutil.py b/trio/_core/tests/tutil.py
index dc93794bef..d5d22d525e 100644
--- a/trio/_core/tests/tutil.py
+++ b/trio/_core/tests/tutil.py
@@ -52,7 +52,7 @@ def gc_collect_harder():
 
 
 # template is like:
-#   [1, {2.1, 2.2}, 3] -> matches [1, 2.1, 3] or [1, 2.2, 3]
+#   [1, {2.1, 2.2}, 3] -> matches [1, 2.1, 2.2, 3] or [1, 2.2, 2.1, 3]
 def check_sequence_matches(seq, template):
     i = 0
     for pattern in template:

From 0f90296fe4617e8d4565553cec47f3a630bc9670 Mon Sep 17 00:00:00 2001
From: Joshua Oreman <oremanj@gmail.com>
Date: Tue, 5 Feb 2019 13:42:43 -0800
Subject: [PATCH 2/2] yapf

---
 trio/_core/tests/test_run.py | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/trio/_core/tests/test_run.py b/trio/_core/tests/test_run.py
index a6ed154ab0..c6d3298bcd 100644
--- a/trio/_core/tests/test_run.py
+++ b/trio/_core/tests/test_run.py
@@ -411,9 +411,14 @@ async def main():
     # reschedules the task immediately upon yielding, before the
     # after_task_step event fires.
     expected = (
-        [("before_run",), ("schedule", task)] +
-        [("before", task), ("schedule", task), ("after", task)] * 5 +
-        [("before", task), ("after", task), ("after_run",)]
+        [("before_run",),
+         ("schedule", task)] +
+        [("before", task),
+         ("schedule", task),
+         ("after", task)] * 5 + [
+             ("before", task),
+             ("after", task), ("after_run",)
+         ]
     )
     assert len(r1.record) > len(r2.record) > len(r3.record)
     assert r1.record == r2.record + r3.record