Confusing behavior of selectors with buffered file objects

Using selectors with buffered file objects is confusing because the selector will only report whether there is something to read in the underlying file descriptor and ignore the internal buffer. This is especially relevant because buffered file objects are used in many places in the standard library, e.g. sys.stdin or socket.makefile().

I propose to add a corresponding warning to the docs, e.g. like this:

Selectors only look at the underlying file descriptors, so they do not know about data that is buffered in python code. Buffered file objects are used in many places in the standard library, e.g. sys.stdin or socket.makefile(). In those cases, it is recommended to bypass the buffering by using os.read(fileobj.fileno(), size) instead of fileobj.read(size).

Even better would be add code to handle buffered file objects. Unfortunately, I think that BufferedIOBase does not expose whether it has data in its buffer.

先从 selectors 文档和 issue 中引用的 BufferedIOBase 行为入手，然后比较使用 sys.stdin 和 socket.makefile() 的示例。确定范围是文档警告，还是对缓冲数据的实现支持。如果更改仍仅限于文档，则还应包括 os.read(fileobj.fileno(), size) 的建议；当所选行为已得到清晰记录时，即视为完成。