Let Manifest copy constructor take const& other

The lock is mutable so we can still lock the other without
passing other as a non-const reference

Change-Id: I8c47676f23b399a2c158414866ce6818509ea9c2
Reviewed-on: https://review.couchbase.org/c/kv_engine/+/227320
Reviewed-by: Vesko Karaganev <vesko.karaganev@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>

Author: trondn

engines/ep/src/collections/vbucket_manifest.cc

@@ -89,7 +89,7 @@ Manifest::~Manifest() {
     }
 }
 
-Manifest::Manifest(Manifest& other) : manager(other.manager) {
+Manifest::Manifest(const Manifest& other) : manager(other.manager) {
     // Prevent other from being modified while we copy.
     std::unique_lock<Manifest::mutex_type> wlock(other.rwlock);
 

engines/ep/src/collections/vbucket_manifest.h

@@ -144,14 +144,8 @@ class Manifest {
      * Copy ctor - more of a logical copy as we can't copy the lock guarding
      * the manifest. Used by NexusKVStore to generate a manifest for the
      * secondary KVStore to update stats against when flushing.
-     *
-     * Interestingly this isn't const because we want to take the write lock
-     * (wlock()) to prevent updates to the manifest. It's probably fine to
-     * take a read lock instead but because the read handles can update various
-     * stats which will be copied it's better to err on the side of caution and
-     * take the write lock.
      */
-    Manifest(Manifest& other);
+    Manifest(const Manifest& other);
 
     /**
      * @return ReadHandle, no iterator is held on the collection container