From a28f85f655c5882ad15a5281d8e69bb8a939bf5a Mon Sep 17 00:00:00 2001
From: Eryx-Z <Eryx-Z@users.noreply.github.com>
Date: Sat, 17 Oct 2026 08:09:07 +0800
Subject: [PATCH 1/2] Index send_all memoryviews by byte count, not element
 count

SocketStream and FdStream treated send()/os.write() return values as
element offsets into the memoryview. For contiguous views with
itemsize > 1, a partial write skipped data and returned success early.

Cast nonempty contiguous views to a 1-D bytes view before slicing.

Fixes #3543.
---
 src/trio/_highlevel_socket.py      |  5 +++++
 src/trio/_tests/test_unix_pipes.py | 26 ++++++++++++++++++++++++++
 src/trio/_unix_pipes.py            |  7 ++++++-
 3 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/src/trio/_highlevel_socket.py b/src/trio/_highlevel_socket.py
index 142ab11e0..8bcf12b86 100644
--- a/src/trio/_highlevel_socket.py
+++ b/src/trio/_highlevel_socket.py
@@ -117,6 +117,11 @@ async def send_all(self, data: bytes | bytearray | memoryview) -> None:
                             raise trio.ClosedResourceError("socket was already closed")
                         await trio.lowlevel.checkpoint()
                         return
+                    # send() returns a byte count; index memoryviews in bytes too.
+                    # Contiguous multi-byte / multi-dim views must be cast first
+                    # (#3543). Leave empty views alone (cast can fail on strided).
+                    if data.contiguous:
+                        data = data.cast("B")
                     total_sent = 0
                     while total_sent < len(data):
                         with data[total_sent:] as remaining:
diff --git a/src/trio/_tests/test_unix_pipes.py b/src/trio/_tests/test_unix_pipes.py
index f81004049..3499120bc 100644
--- a/src/trio/_tests/test_unix_pipes.py
+++ b/src/trio/_tests/test_unix_pipes.py
@@ -286,3 +286,29 @@ async def test_bizarro_OSError_from_receive() -> None:
 @skip_if_fbsd_pipes_broken
 async def test_pipe_fully() -> None:
     await check_one_way_stream(make_pipe, make_clogged_pipe)
+
+
+async def test_send_all_memoryview_itemsize_gt_1() -> None:
+    """Partial writes must advance by byte count, not element count (#3543)."""
+    import os
+
+    import trio
+
+    payload = bytes(range(256)) * 64  # 16 KiB
+    read_fd, write_fd = os.pipe()
+    received = bytearray()
+    async with trio.lowlevel.FdStream(write_fd) as sender, trio.lowlevel.FdStream(
+        read_fd,
+    ) as receiver:
+
+        async def send() -> None:
+            with memoryview(payload).cast("H") as data:
+                await sender.send_all(data)
+            await sender.aclose()
+
+        async with trio.open_nursery() as nursery:
+            nursery.start_soon(send)
+            async for chunk in receiver:
+                received.extend(chunk)
+
+    assert received == payload
diff --git a/src/trio/_unix_pipes.py b/src/trio/_unix_pipes.py
index 598feb61e..6a253a41a 100644
--- a/src/trio/_unix_pipes.py
+++ b/src/trio/_unix_pipes.py
@@ -134,9 +134,14 @@ async def send_all(self, data: bytes | bytearray | memoryview) -> None:
             if self._fd_holder.closed:
                 raise trio.ClosedResourceError("file was already closed")
             await trio.lowlevel.checkpoint()
-            length = len(data)
             # adapted from the SocketStream code
             with memoryview(data) as view:
+                # os.write returns a byte count; index memoryviews in bytes too.
+                # Contiguous multi-byte / multi-dim views must be cast first
+                # (#3543). Leave empty views alone (cast can fail on strided).
+                if view.contiguous and view:
+                    view = view.cast("B")
+                length = len(view)
                 sent = 0
                 while sent < length:
                     with view[sent:] as remaining:

From 0e9a80c1fdb9edfeb789d1024fa492660660f60a Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Sat, 17 Oct 2026 00:10:01 +0000
Subject: [PATCH 2/2] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 src/trio/_tests/test_unix_pipes.py | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/src/trio/_tests/test_unix_pipes.py b/src/trio/_tests/test_unix_pipes.py
index 3499120bc..ded203d73 100644
--- a/src/trio/_tests/test_unix_pipes.py
+++ b/src/trio/_tests/test_unix_pipes.py
@@ -297,9 +297,12 @@ async def test_send_all_memoryview_itemsize_gt_1() -> None:
     payload = bytes(range(256)) * 64  # 16 KiB
     read_fd, write_fd = os.pipe()
     received = bytearray()
-    async with trio.lowlevel.FdStream(write_fd) as sender, trio.lowlevel.FdStream(
-        read_fd,
-    ) as receiver:
+    async with (
+        trio.lowlevel.FdStream(write_fd) as sender,
+        trio.lowlevel.FdStream(
+            read_fd,
+        ) as receiver,
+    ):
 
         async def send() -> None:
             with memoryview(payload).cast("H") as data: