Fix typo in docs.

filter/dscache/doc.go

 // Copyright 2015 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 // Package dscache provides a transparent cache for RawDatastore which is
 // backed by Memcache.
 //
 // Inspiration
 //
 // Although this is not a port of any particular implementation, it takes
@@ skipping 142 matching lines @@
 //   - push a new version of the application disabling the cache filter
 //     by setting InstanceEnabledStatic to false in an init() function.
 //
 // On every dscache.FilterRDS invocation, it takes the opportunity to fetch this
 // datastore value, if it hasn't been fetched in the last
 // GlobalEnabledCheckInterval time (5 minutes). This equates to essentially once
 // per http request, per 5 minutes, per instance.
 //
 // AppEngine's memcache reserves the right to evict keys at any moment. This is
 // especially true for shared memcache, which is subject to pressures outside of
-// your application. When when eviction happens due to memory pressure,
+// your application. When eviction happens due to memory pressure,
 // least-recently-used values are evicted first.
 //
 // https://cloud.google.com/appengine/docs/go/memcache/#Go_How_cached_data_expires
 //
 // Eviction presents a potential race window, as lock items that were put in
 // memcache may be evicted prior to the locked operations completing (or
 // failing), causing concurrent Get operations to cache bad data indefinitely.
 //
 // In practice, a dedicated memcache will be safe. LRU-based eviction means that
 // that locks recently added will almost certainly not be evicted before their
 // operations are complete, and a dedicated memcache lowers eviction pressure to
 // a single application's operation. Production systems that have data integrity
 // requirements are encouraged to use a dedicated memcache.
 //
 // Note that flusing memcache of a running application may also induce this
 // race. Flushes should be performed with this concern in mind.
 //
 // TODO: A potential mitigation to lock eviction poisoning is to use memcache
 // Statistics to identify the oldest memcache item and use that age to bound
 // the lifetime of cached datastore entries. This would cause dscache items
 // created around the time of a flush to expire quickly (instead of never),
 // bounding the period of time when poisoned data may reside in the cache.
 package dscache