pipes from subprocess_exec do not have extra info 'pipe' (incompatible with vanilla asyncio)

• uvloop version:
  0.19.0

• Python version:
  3.10.12

• Platform:
  linux

• Can you reproduce the bug with PYTHONASYNCIODEBUG in env?:
  yes

• Does uvloop behave differently from vanilla asyncio? How?:
  Yes, this is the point of this issue report. So after you get your process transport from subprocess_exec(...) you then can use get_pipe_transport() to access the transport associated with stdin. With vanilla asyncio you can then call get_extra_info('pipe') on that transport to access the pipe directly. With uvloop get_extra_info('pipe') on the same transport returns None.

I can see in the source that when a pipe transport is created via loop.create_write_pipe(...) we have:

        transp = WriteUnixTransport.new(self, proto, None, waiter)
        transp._add_extra_info('pipe', pipe)

However no transp._add_extra_info(...) is done on the WriteUnixTransport that is created inside the UVProcessTransport code. There are ways to work around this (don't depend on getting access to that pipe) however this works fine in the asyncio loop implementation. I imagine it can be done without too much pain by wrapping the raw file descriptor created in the process transport code however I'm not super fluent in cython so I didn't attempt a patch.

从 UVProcessTransport 代码中创建子进程管道的 WriteUnixTransport 的位置开始，并将其与会添加管道额外信息的 loop.create_write_pipe() 进行比较。验证 get_pipe_transport() 返回的子进程 stdin transport 是否通过 get_extra_info('pipe') 暴露底层管道，使其行为与 vanilla asyncio 一致。