MB-41916: Make engineFileDescriptors atomic

This should fix a data race when we create a new bucket and set the
field.

Change-Id: I3b0d6bff7f86876e052aff1c6288c2160f6ffc60
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/137779
Tested-by: Build Bot <[email protected]>
Reviewed-by: Dave Rigby <[email protected]>

Author: BenHuddleston

engines/ep/src/environment.h

@@ -34,7 +34,7 @@ struct Environment {
      * The maximum number of file descriptors that the engines can
      * (collectively) use
      */
-    size_t engineFileDescriptors = 0;
+    std::atomic<size_t> engineFileDescriptors = 0;
 
     /**
      * We want to reserve /some/ files for each engine for tasks such as

engines/ep/src/kvstore_config.cc

@@ -94,7 +94,7 @@ KVStoreConfig::KVStoreConfig(uint16_t _maxVBuckets,
       backend(std::move(_backend)),
       shardId(_shardId),
       logger(globalBucketLogger.get()) {
-    auto env = Environment::get();
+    auto& env = Environment::get();
     maxFileDescriptors = env.getMaxBackendFileDescriptors();
 }