Allow bmemcached to be optional for memcache_pool

This allows oslo_cache.memcache_pool to be used without the
python-binary-memcached package being installed, as it is
only required if sasl_enabled is set to True.

Closes-Bug: #1991250
Change-Id: I7e6cc83864be68e946d86b1f4b44847b95ea8b05

oslo_cache/backends/memcache_pool.py

@@ -19,7 +19,13 @@ import functools
 
 from dogpile.cache.backends import memcached as memcached_backend
 
-from oslo_cache import _bmemcache_pool
+try:
+    from oslo_cache import _bmemcache_pool
+except ImportError as e:
+    if str(e) == "No module named 'bmemcached'":
+        _bmemcache_pool = None
+    else:
+        raise
 from oslo_cache import _memcache_pool
 
 
@@ -57,6 +63,8 @@ class PooledMemcachedBackend(memcached_backend.MemcachedBackend):
     def __init__(self, arguments):
         super(PooledMemcachedBackend, self).__init__(arguments)
         if arguments.get('sasl_enabled', False):
+            if not _bmemcache_pool:
+                raise ImportError("python-binary-memcached package is missing")
             self.client_pool = _bmemcache_pool.BMemcacheClientPool(
                 self.url,
                 arguments,

releasenotes/notes/bug-1991250-23bc3463273e5a91.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    [`bug 1991250 <https://bugs.launchpad.net/oslo.cache/+bug/1991250>`_]
+    The python-binary-memcached package is only required if sasl_enabled=True.
+    When sasl_enabled=False (default), the memcache_pool backend can be used
+    without the python-binary-memcached package installed.