The method should be low-level coroutine, implemented on loops with native sendfile support only (UnixEventLoop).

The signature should be close to socket.sendfile() method

Next step is implementing loop.sendfile(transport, ...) with fallback to sending by chunks if sendfile is not supported.

New changeset 6b5a27975a415108a5eac12ee302bf2b3233f4d4 by Andrew Svetlov in branch 'master':
bpo-32410: Implement loop.sock_sendfile() (#4976)
https://github.com/python/cpython/commit/6b5a27975a415108a5eac12ee302bf2b3233f4d4

New changeset 7464e87a6511d3626b04c9833a262a77b1f21e23 by Andrew Svetlov in branch 'master':
bpo-32410: Make SendfileNotAvailableError exception public (#5243)
https://github.com/python/cpython/commit/7464e87a6511d3626b04c9833a262a77b1f21e23

Andrew, test_cancel2 test fails 1 out of 10 times on my machine (log below).  Could you please take a look?

Also, can you rename sendfile tests to 'test_sock_sendfile_*'?


{pydev} ~/d/p/cpython (master %) » ./python.exe -m test test_asyncio -v -m test_cancel2
== CPython 3.7.0a4+ (heads/master:9d411c119f, Jan 23 2018, 15:12:10) [Clang 9.0.0 (clang-900.0.39.2)]
== Darwin-17.3.0-x86_64-i386-64bit little-endian
== cwd: /Users/yury/dev/pydev/cpython/build/test_python_65406
== CPU count: 8
== encodings: locale=UTF-8, FS=utf-8
Run tests sequentially
0:00:00 load avg: 1.80 [1/1] test_asyncio
test_cancel2 (test.test_asyncio.test_unix_events.SelectorEventLoopUnixSockSendfileTests) ... ERROR

======================================================================
ERROR: test_cancel2 (test.test_asyncio.test_unix_events.SelectorEventLoopUnixSockSendfileTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/yury/dev/pydev/cpython/Lib/test/test_asyncio/test_unix_events.py", line 487, in cleanup
    proto.transport.close()
AttributeError: 'NoneType' object has no attribute 'close'

----------------------------------------------------------------------
Ran 1 test in 0.010s

FAILED (errors=1)
Task was destroyed but it is pending!
task: <Task pending coro=<BaseSelectorEventLoop._accept_connection2() done, defined at /Users/yury/dev/pydev/cpython/Lib/asyncio/selector_events.py:200> wait_for=<Future pending cb=[<TaskWakeupMethWrapper object at 0x10731c2e8>()]>>
test test_asyncio failed
test_asyncio failed

New changeset 0a5e71b4c70aab87125a54d7a59765e18d7583a4 by Andrew Svetlov in branch 'master':
bpo-32410: Improve sock_sendfile tests (#5294)
https://github.com/python/cpython/commit/0a5e71b4c70aab87125a54d7a59765e18d7583a4

New changeset 7165754b6b5f3b7c07050d921fa1c58bba5f0ff1 by Yury Selivanov in branch 'master':
bpo-32410: Avoid blocking on file IO in sendfile fallback code (GH-7172)
https://github.com/python/cpython/commit/7165754b6b5f3b7c07050d921fa1c58bba5f0ff1

New changeset 420092e255389008b399efa61db300fe44356ee8 by Ned Deily (Miss Islington (bot)) in branch '3.7':
bpo-32410: Avoid blocking on file IO in sendfile fallback code (GH-7172) (#7182)
https://github.com/python/cpython/commit/420092e255389008b399efa61db300fe44356ee8