diff --git a/trio/_subprocess/unix_pipes.py b/trio/_subprocess/unix_pipes.py
index 7d7c94891e..a7ab987132 100644
--- a/trio/_subprocess/unix_pipes.py
+++ b/trio/_subprocess/unix_pipes.py
@@ -46,28 +46,63 @@ def __del__(self):
 class PipeSendStream(_PipeMixin, SendStream):
     """Represents a send stream over an os.pipe object."""
 
-    async def send_all(self, data: bytes):
-        # we have to do this no matter what
-        await _core.checkpoint()
+    async def send_some(self, data: bytes, next_offset: int) -> int:
+        """Write data from ``data`` beginning at offset ``next_offset``
+        to the pipe until all data is written or an exception occurs.
+        This will block until some data can be written, but will return
+        a short write rather than blocking again after a previous
+        successful write.
+
+        Returns:
+          If some data is written, returns the index of the first byte in
+          ``data`` that was not written, or ``len(data)`` if all bytes
+          were written.
+
+        Raises:
+          If no data is written, raises the exception that caused the first
+          write to fail: :exc:`Cancelled`, :exc:`BrokenResourceError`, or
+          :exc:`OSError`.
+
+        """
+
         if self._closed:
+            await _core.checkpoint()
             raise _core.ClosedResourceError("this pipe is already closed")
 
         if not data:
-            return
+            await _core.checkpoint()
+            return 0
+
+        await _core.checkpoint_if_cancelled()
 
-        length = len(data)
         # adapted from the SocketStream code
         with memoryview(data) as view:
-            total_sent = 0
-            while total_sent < length:
-                with view[total_sent:] as remaining:
+            # First write: block and raise exceptions
+            with view[next_offset:] as remaining:
+                try:
+                    next_offset += os.write(self._pipe, remaining)
+                except BrokenPipeError as e:
+                    await _core.cancel_shielded_checkpoint()
+                    raise BrokenResourceError from e
+                except BlockingIOError:
+                    await self.wait_send_all_might_not_block()
+                else:
+                    await _core.cancel_shielded_checkpoint()
+
+            # Later writes: return a short write instead
+            while next_offset < len(data):
+                with view[next_offset:] as remaining:
                     try:
-                        total_sent += os.write(self._pipe, remaining)
-                    except BrokenPipeError as e:
-                        await _core.checkpoint()
-                        raise BrokenResourceError from e
-                    except BlockingIOError:
-                        await self.wait_send_all_might_not_block()
+                        next_offset += os.write(self._pipe, remaining)
+                    except OSError:  # includes BlockingIOError
+                        break
+
+        return next_offset
+
+    async def send_all(self, data: bytes) -> None:
+        next_offset = await self.send_some(data, 0)
+        while next_offset < len(data):
+            next_offset = await self.send_some(data, next_offset)
 
     async def wait_send_all_might_not_block(self) -> None:
         if self._closed:
diff --git a/trio/tests/subprocess/test_unix_pipes.py b/trio/tests/subprocess/test_unix_pipes.py
index b8cc2d496a..3186f9aed6 100644
--- a/trio/tests/subprocess/test_unix_pipes.py
+++ b/trio/tests/subprocess/test_unix_pipes.py
@@ -1,11 +1,12 @@
 import errno
 import select
+import random
 
 import os
 import pytest
 
 from trio._core.tests.tutil import gc_collect_harder
-from ... import _core
+from ... import _core, move_on_after, sleep, BrokenResourceError
 from ...testing import (wait_all_tasks_blocked, check_one_way_stream)
 
 posix = os.name == "posix"
@@ -146,3 +147,43 @@ async def make_clogged_pipe():
 
 async def test_pipe_fully():
     await check_one_way_stream(make_pipe, make_clogged_pipe)
+
+
+async def test_pipe_send_some(autojump_clock):
+    write, read = await make_pipe()
+    data = bytearray(random.randint(0, 255) for _ in range(2**18))
+    next_send_offset = 0
+    received = bytearray()
+
+    async def sender():
+        nonlocal next_send_offset
+        with move_on_after(2.0):
+            while next_send_offset < len(data):  # pragma: no branch
+                next_send_offset = await write.send_some(
+                    data, next_send_offset
+                )
+        await write.aclose()
+
+    async def reader():
+        nonlocal received
+        await wait_all_tasks_blocked()
+        while True:
+            await sleep(0.1)
+            chunk = await read.receive_some(4096)
+            if chunk == b"":
+                break
+            received.extend(chunk)
+
+    async with _core.open_nursery() as n:
+        n.start_soon(sender)
+        n.start_soon(reader)
+
+    assert received == data[:next_send_offset]
+
+    await read.aclose()
+
+    write, read = await make_pipe()
+    await read.aclose()
+    with pytest.raises(BrokenResourceError):
+        await write.send_some(data, next_send_offset)
+    await write.aclose()