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

CHANGELOG.rst

@@ -25,6 +25,10 @@ Deprecations:
 Changes:
 ^^^^^^^^
 
+- Fixed the inability of ``OpenSSL.SSL.Connection.sendall()`` to
+  keep with sending data over the wire after ``SSL_ERROR_WANT_READ``
+  or ``SSL_ERROR_WANT_WRITE`` is returned by ``SSL_write()``.
+  `#176 <https://github.com/pyca/pyopenssl/pull/176>`_
 - Added a new optional ``chain`` parameter to ``OpenSSL.crypto.X509StoreContext()``
   where additional untrusted certificates can be specified to help chain building.
   `#948 <https://github.com/pyca/pyopenssl/pull/948>`_

src/OpenSSL/SSL.py

@@ -1670,7 +1670,14 @@ def sendall(self, buf, flags=0):
                 result = _lib.SSL_write(
                     self._ssl, data + total_sent, min(left_to_send, 2147483647)
                 )
-                self._raise_ssl_error(self._ssl, result)
+                try:
+                    self._raise_ssl_error(self._ssl, result)
+                except (WantReadError, WantWriteError):
+                    # NOTE: The use of SSL_MODE_ENABLE_PARTIAL_WRITE
+                    # NOTE: above guarrantees that in case of failure
+                    # NOTE: no bytes have been written so we don't need
+                    # NOTE: to update the counters, just need to retry.
+                    continue
                 total_sent += result
                 left_to_send -= result