CDRIVER-770: Return how much we did write since its OK to return short writes

When we can't finish writing our entire buffered iovec, we must still
return how much we have successfully written so far, in case any of
the previous buffers finished.
Then we can regroup and recalculate from where we left off and retry in
case of an [WSA]EWOULDBLOCK or friends.

Author: bjori

src/mongoc/mongoc-stream-tls.c

@@ -579,6 +579,7 @@ _mongoc_stream_tls_writev (mongoc_stream_t *stream,
    BSON_ASSERT (tls);
    BSON_ASSERT (iov);
    BSON_ASSERT (iovcnt);
+   ENTRY;
 
    tls->timeout_msec = timeout_msec;
 
@@ -624,18 +625,22 @@ _mongoc_stream_tls_writev (mongoc_stream_t *stream,
              * if we didn't buffer and have to send out of the iovec */
 
             child_ret = _mongoc_stream_tls_write (tls, to_write, to_write_len);
+            if (child_ret != to_write_len) {
+               MONGOC_DEBUG("Got child_ret: %ld while to_write_len is: %ld",
+                     child_ret, to_write_len);
+            }
 
             if (child_ret < 0) {
-               /* Buffer write failed, just return the error */
-               return child_ret;
+               TRACE("Returning what I had (%ld) as apposed to the error (%ld, errno:%d)", ret, child_ret, errno);
+               RETURN (ret);
             }
 
             ret += child_ret;
 
             if (child_ret < to_write_len) {
                /* we timed out, so send back what we could send */
 
-               return ret;
+               RETURN (ret);
             }
 
             to_write = NULL;
@@ -646,10 +651,12 @@ _mongoc_stream_tls_writev (mongoc_stream_t *stream,
    if (buf_head != buf_tail) {
       /* If we have any bytes buffered, send */
 
+      TRACE("buffered writing %ld", buf_tail - buf_head);
       child_ret = _mongoc_stream_tls_write (tls, buf_head, buf_tail - buf_head);
+      TRACE("Got %ld written", child_ret);
 
       if (child_ret < 0) {
-         return child_ret;
+         RETURN (child_ret);
       }
 
       ret += child_ret;
@@ -659,7 +666,7 @@ _mongoc_stream_tls_writev (mongoc_stream_t *stream,
       mongoc_counter_streams_egress_add (ret);
    }
 
-   return ret;
+   RETURN (ret);
 }