Upstream: flush low-level buffers on write retry. If the data to write is bigger than what the socket can send, and the reminder is smaller than NGX_SSL_BUFSIZE, then SSL_write() fails with SSL_ERROR_WANT_WRITE. The reminder of payload however is successfully copied to the low-level buffer and all the output chain buffers are flushed. This means that retry logic doesn't work because ngx_http_upstream_process_non_buffered_request() checks only if there's anything in the output chain buffers and ignores the fact that something may be buffered in low-level parts of the stack. Signed-off-by: Patryk Lesiewicz <patryk@google.com>