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Retry on `WANT_WRITE` & `WANT_READ` in `sendall()` #954

Draft

webknjaz wants to merge 1 commit into pyca:main from webknjaz:bugfixes/176-sendall-want-write-read-processing

Commits on Jan 23, 2024

Retry on WANT_WRITE & WANT_READ in sendall()

This change introduces retries in `OpenSSL.SSL.Connection.sendall()`
when `WANT_WRITE_ERROR` or `WANT_READ_ERROR` happen.

It relies on `SSL_MODE_ENABLE_PARTIAL_WRITE` being set on the context,
that changes the mode of `SSL_write()` to return errors only if zero
bytes has been sent making it safe to retry in these cases.

Ideally, the calling code is supposed to `poll()`/`select()` the
socket to know when it's okay to attempt the next retry (hence it is
readable or writable) but it's not available in the `sendall()` method
and just retrying the operation is good enough.

Fixes pyca#176

Refs:
* http://openssl.6102.n7.nabble.com/SSL-MODE-ACCEPT-MOVING-WRITE-BUFFER-td6421.html
* https://stackoverflow.com/a/28992313/595220
* https://www.openssl.org/docs/manmaster/man3/SSL_write.html
* https://stackoverflow.com/a/20817394/595220

webknjaz committed Jan 23, 2024