Python's fcntl module already provides some support for making support for file locking optional. For example, constants like F_SETFL are only defined if present. Unfortunately, the accompanying functions 'flock()' and 'lockf()' are present unconditionally.

CloudABI, a sandboxed runtime environment for UNIX (https://mail.python.org/pipermail/python-dev/2016-July/145708.html), doesn't implement these ways of doing file locking, for the reason that these operations do not fit nicely within its process model. These locks are per-process; not per-descriptor. This means that processes cannot safely be composed or decomposed.

Attached is a relatively small patch to make flock() and lockf() optional, only compiling them when the necessary APIs are present. The LOCK_* constants are now also defined optionally.