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.

Comece pela documentação de selectors e pelo comportamento de BufferedIOBase referenciado na issue; em seguida, compare exemplos que usam sys.stdin e socket.makefile(). Determine se o escopo é um aviso na documentação ou suporte da implementação para dados armazenados em buffer. Considera-se concluído quando o comportamento escolhido estiver documentado claramente, incluindo a recomendação os.read(fileobj.fileno(), size) caso a alteração permaneça somente na documentação.