asyncio subprocess spawn does not record the awaiting task

Bug report

Bug description:

The internal subprocess pipe-connect task registers no awaited_by edge

Repro:

import asyncio, sys

async def main():
    spawn = asyncio.create_task(
        asyncio.create_subprocess_exec(sys.executable, '-c', 'pass',
                                       stdin=asyncio.subprocess.PIPE),
        name='spawn')

    await asyncio.sleep(0)
    await asyncio.sleep(0)

    for t in asyncio.all_tasks() - {asyncio.current_task()}:
        asyncio.print_call_graph(t)

    proc = await spawn
    await proc.wait()

asyncio.run(main())

Actual:

* Task(name='spawn', id=0x103e00b90)
  + Call stack:
  |   File '/Users/timofeiivankov/cpython/Lib/asyncio/unix_events.py', line 211, in async _UnixSelectorEventLoop._make_subprocess_transport()
  |   File '/Users/timofeiivankov/cpython/Lib/asyncio/base_events.py', line 1839, in async BaseEventLoop.subprocess_exec()
  |   File '/Users/timofeiivankov/cpython/Lib/asyncio/subprocess.py', line 249, in async create_subprocess_exec()
* Task(name='Task-2', id=0x103b7d0a0)
  + Call stack:
  |   File '/Users/timofeiivankov/cpython/Lib/asyncio/base_events.py', line 1753, in async BaseEventLoop.connect_write_pipe()
  |   File '/Users/timofeiivankov/cpython/Lib/asyncio/base_subprocess.py', line 188, in async BaseSubprocessTransport._connect_pipes()

Expected:

* Task(name='spawn', id=0x105620b90)
  + Call stack:
  |   File '/Users/timofeiivankov/cpython/Lib/asyncio/unix_events.py', line 211, in async _UnixSelectorEventLoop._make_subprocess_transport()
  |   File '/Users/timofeiivankov/cpython/Lib/asyncio/base_events.py', line 1839, in async BaseEventLoop.subprocess_exec()
  |   File '/Users/timofeiivankov/cpython/Lib/asyncio/subprocess.py', line 249, in async create_subprocess_exec()
* Task(name='Task-2', id=0x10539d0a0)
  + Call stack:
  |   File '/Users/timofeiivankov/cpython/Lib/asyncio/base_events.py', line 1753, in async BaseEventLoop.connect_write_pipe()
  |   File '/Users/timofeiivankov/cpython/Lib/asyncio/base_subprocess.py', line 194, in async BaseSubprocessTransport._connect_pipes()
  + Awaited by:
    * Task(name='spawn', id=0x105620b90)
      + Call stack:
      |   File '/Users/timofeiivankov/cpython/Lib/asyncio/unix_events.py', line 211, in async _UnixSelectorEventLoop._make_subprocess_transport()
      |   File '/Users/timofeiivankov/cpython/Lib/asyncio/base_events.py', line 1839, in async BaseEventLoop.subprocess_exec()
      |   File '/Users/timofeiivankov/cpython/Lib/asyncio/subprocess.py', line 249, in async create_subprocess_exec()

Proposed fix: register the edge in BaseSubprocessTransport.__init__ and discard it in doe callback

I have a fix ready for that

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Start in Lib/asyncio/base_subprocess.py at BaseSubprocessTransport.init and inspect the subprocess pipe-connection path shown in the traceback. Run the supplied asyncio reproduction and confirm that the internal pipe-connect task reports an “Awaited by” edge for the spawn task, then verify the edge is discarded by the completion callback.