SERVER-22114 Avoid doing unnecessary repeated calls to DBConfig::_load

(cherry picked from commit 0f85256eefa78b1b74b9e6ae0d1c646853f6f22f)

Conflicts:
	src/mongo/s/config.cpp
	src/mongo/s/config.h

2 files changed, 31 insertions, 13 deletions

diff --git a/src/mongo/s/config.cpp b/src/mongo/s/config.cpp
index f6d0e01c151..3f7586e70bb 100644
--- a/src/mongo/s/config.cpp
+++ b/src/mongo/s/config.cpp
@@ -367,13 +367,15 @@ ChunkManagerPtr DBConfig::getChunkManager(const string& ns, bool shouldReload, b
     ChunkVersion oldVersion;
     ChunkManagerPtr oldManager;
 
+    const auto currentReloadIteration = _reloadCount.load();
+
     {
         scoped_lock lk(_lock);
 
         bool earlyReload = !_collections[ns].isSharded() && (shouldReload || forceReload);
         if (earlyReload) {
-            // this is to catch cases where there this is a new sharded collection
-            _reload();
+            // This is to catch cases where there this is a new sharded collection
+            _loadIfNeeded(currentReloadIteration);
         }
 
         CollectionInfo& ci = _collections[ns];
@@ -530,11 +532,16 @@ void DBConfig::unserialize(const BSONObj& from) {
 }
 
 bool DBConfig::load() {
+    const auto currentReloadIteration = _reloadCount.load();
     scoped_lock lk(_lock);
-    return _load();
+    return _loadIfNeeded(currentReloadIteration);
 }
 
-bool DBConfig::_load() {
+bool DBConfig::_loadIfNeeded(Counter reloadIteration) {
+    if (reloadIteration != _reloadCount.load()) {
+        return true;
+    }
+
     ScopedDbConnection conn(configServer.modelServer(), 30.0);
 
     BSONObj dbObj = conn->findOne(DatabaseType::ConfigNS, BSON(DatabaseType::name(_name)));
@@ -580,6 +587,8 @@ bool DBConfig::_load() {
 
     conn.done();
 
+    _reloadCount.fetchAndAdd(1);
+
     return true;
 }
 
@@ -617,10 +626,11 @@ void DBConfig::_save(bool db, bool coll) {
 
 bool DBConfig::reload() {
     bool successful = false;
+    const auto currentReloadIteration = _reloadCount.load();
 
     {
         scoped_lock lk(_lock);
-        successful = _reload();
+        successful = _loadIfNeeded(currentReloadIteration);
     }
 
     //
@@ -634,11 +644,6 @@ bool DBConfig::reload() {
     return successful;
 }
 
-bool DBConfig::_reload() {
-    // TODO: i don't think is 100% correct
-    return _load();
-}
-
 bool DBConfig::dropDatabase(string& errmsg) {
     /**
      * 1) make sure everything is up
diff --git a/src/mongo/s/config.h b/src/mongo/s/config.h
index d638a3a483b..a187320f55d 100644
--- a/src/mongo/s/config.h
+++ b/src/mongo/s/config.h
@@ -38,6 +38,7 @@
 #include <boost/shared_ptr.hpp>
 
 #include "mongo/client/dbclient_rs.h"
+#include "mongo/platform/atomic_word.h"
 #include "mongo/s/chunk.h"
 #include "mongo/s/shard.h"
 #include "mongo/s/shard_key_pattern.h"
@@ -198,8 +199,9 @@ public:
     void getAllShards(std::set<Shard>& shards) const;
 
     void getAllShardedCollections(std::set<std::string>& namespaces) const;
-
 protected:
+    typedef AtomicUInt64::WordType Counter;
+
     /**
         lockless
     */
@@ -207,8 +209,14 @@ protected:
 
     bool _dropShardedCollections(int& num, std::set<Shard>& allServers, std::string& errmsg);
 
-    bool _load();
-    bool _reload();
+    /**
+     * Returns true if it is successful at loading the DBConfig, false if the database is not found,
+     * and throws on all other errors.
+     * Also returns true without reloading if reloadIteration is not equal to the _reloadCount.
+     * This is to avoid multiple threads attempting to reload do duplicate work.
+     */
+    bool _loadIfNeeded(Counter reloadIteration);
+
     void _save(bool db = true, bool coll = true);
 
     std::string _name;  // e.g. "alleyinsider"
@@ -224,6 +232,11 @@ protected:
 
     mutable mongo::mutex _lock;  // TODO: change to r/w lock ??
     mutable mongo::mutex _hitConfigServerLock;
+
+    // Increments every time this performs a full reload. Since a full reload can take a very
+    // long time for very large clusters, this can be used to minimize duplicate work when multiple
+    // threads tries to perform full reload at roughly the same time.
+    AtomicUInt64 _reloadCount;  // (S)
 };
 
 class ConfigServer : public DBConfig {