From 4407451942c5d09e0862058ee3349bd2fd536cf8 Mon Sep 17 00:00:00 2001
From: Zac Hatfield-Dodds <zac@anthropic.com>
Date: Tue, 16 Sep 2025 21:30:52 -0700
Subject: [PATCH] fix broken-channel bug

---
 newsfragments/3331.bugfix.rst   |  3 +++
 src/trio/_channel.py            | 11 ++++++--
 src/trio/_tests/test_channel.py | 46 ++++++++++++++++++++++++++++++++-
 3 files changed, 57 insertions(+), 3 deletions(-)
 create mode 100644 newsfragments/3331.bugfix.rst

diff --git a/newsfragments/3331.bugfix.rst b/newsfragments/3331.bugfix.rst
new file mode 100644
index 000000000..da30bc9d5
--- /dev/null
+++ b/newsfragments/3331.bugfix.rst
@@ -0,0 +1,3 @@
+Fixed a bug where iterating over an ``@as_safe_channel``-derived ``ReceiveChannel``
+would raise `~trio.BrokenResourceError` if the channel was closed by another task.
+It now shuts down cleanly.
diff --git a/src/trio/_channel.py b/src/trio/_channel.py
index 2afca9d7c..1398e766e 100644
--- a/src/trio/_channel.py
+++ b/src/trio/_channel.py
@@ -17,7 +17,7 @@
 import trio
 
 from ._abc import ReceiveChannel, ReceiveType, SendChannel, SendType, T
-from ._core import Abort, RaiseCancelT, Task, enable_ki_protection
+from ._core import Abort, BrokenResourceError, RaiseCancelT, Task, enable_ki_protection
 from ._util import (
     MultipleExceptionError,
     NoPublicConstructor,
@@ -577,12 +577,19 @@ async def _move_elems_to_channel(
                 while True:
                     # wait for receiver to call next on the aiter
                     await send_semaphore.acquire()
+                    if not send_chan._state.open_receive_channels:
+                        # skip the possibly-expensive computation in the generator,
+                        # if we know it will be impossible to send the result.
+                        break
                     try:
                         value = await agen.__anext__()
                     except StopAsyncIteration:
                         return
                     # Send the value to the channel
-                    await send_chan.send(value)
+                    try:
+                        await send_chan.send(value)
+                    except BrokenResourceError:
+                        break  # closed since we checked above
             finally:
                 # work around `.aclose()` not suppressing GeneratorExit in an
                 # ExceptionGroup:
diff --git a/src/trio/_tests/test_channel.py b/src/trio/_tests/test_channel.py
index 85cd982a4..cfdf904fd 100644
--- a/src/trio/_tests/test_channel.py
+++ b/src/trio/_tests/test_channel.py
@@ -434,7 +434,7 @@ async def test_as_safe_channel_broken_resource() -> None:
     @as_safe_channel
     async def agen() -> AsyncGenerator[int]:
         yield 1
-        yield 2
+        yield 2  # pragma: no cover
 
     async with agen() as recv_chan:
         assert await recv_chan.__anext__() == 1
@@ -695,3 +695,47 @@ async def agen(ex: type[BaseException]) -> AsyncGenerator[None]:
         async with agen(ValueError) as g:
             async for _ in g:
                 break
+
+
+async def test_as_safe_channel_close_between_iteration() -> None:
+    @as_safe_channel
+    async def agen() -> AsyncGenerator[None]:
+        while True:
+            yield
+
+    async with agen() as chan, trio.open_nursery() as nursery:
+
+        async def close_channel() -> None:
+            await trio.lowlevel.checkpoint()
+            await chan.aclose()
+
+        nursery.start_soon(close_channel)
+        with pytest.raises(trio.ClosedResourceError):
+            async for _ in chan:
+                pass
+
+
+async def test_as_safe_channel_close_before_iteration() -> None:
+    @as_safe_channel
+    async def agen() -> AsyncGenerator[None]:
+        raise AssertionError("should be unreachable")  # pragma: no cover
+        yield  # pragma: no cover
+
+    async with agen() as chan:
+        await chan.aclose()
+        with pytest.raises(trio.ClosedResourceError):
+            await chan.receive()
+
+
+async def test_as_safe_channel_close_during_iteration() -> None:
+    @as_safe_channel
+    async def agen() -> AsyncGenerator[None]:
+        await chan.aclose()
+        while True:
+            yield
+
+    for _ in range(10):  # 20% missed-alarm rate, so run ten times
+        async with agen() as chan:
+            with pytest.raises(trio.ClosedResourceError):
+                async for _ in chan:
+                    pass