sock: tcp: fix sock_tcp_read() example

[benpicco]

Contribution description

sock_tcp_read() will return 0 if the connection was closed, we need to check for this.

Testing procedure

Issues/PRs references

#16494 (comment)

[maribu]

Note that the example is currently conaistent with the API doc. According to the current API doc, there is no way to detect when a TCP connection is closed. That obviously is a design flaw in need to be fixed. I trust that the implementation already has been fixed, but fixing the was forgotten. Care to add a commit to fiz this? Also, the @retval Doxygen command would make sense for documenting the individual magic return constants.

[benpicco]

TBH I'm a bit surprised it does not return -ECONNABORTED when the remote side closes the connection.

[maribu]

Maybe it is worth to discuss the correct behaviour first.

But note: Both POSIX read() and recv() return 0 at end of file/stream. For that reason, they had to introduce non-error error-codes for nonblocking operation to indicate that no data is available yet. This kind of proves that not using an error code for EOF / closed connection was a bad idea. But still, since the SOCK API was modeled after BSD sockets, I think there will be some confusion with different behaviour.

[brummer-simon]

TBH I'm a bit surprised it does not return -ECONNABORTED when the remote side closes the connection.

-ECONNABORTED is used as an error condition. Closing a connection normally is not an error. TCP assumes that a connection can be closed from any participating peer at any time regardless of what the peer is currently doing.

Think of recv for example:
-ECONNABORTED is returned if there was no message received from the peer within the last to minutes assuming the connection broke or the peer crashed.

[fjmolinas]

LGTM and @brummer-simon is fine with the change. ACK.