SERVER-13376 Make sync7.js less sensitive to delays

(cherry picked from commit 06c40ab0bb2a36c709fb4d3b91961e91fb08b1f1)

2 files changed, 20 insertions, 9 deletions

diff --git a/jstests/sharding/sync7.js b/jstests/sharding/sync7.js
index 87eeaa0b36b..8c1430ea3c0 100644
--- a/jstests/sharding/sync7.js
+++ b/jstests/sharding/sync7.js
@@ -3,11 +3,11 @@
 s = new ShardingTest( "moveDistLock", 3, 0, undefined, { sync : true } );
 
 s._connections[0].getDB( "admin" ).runCommand( { _skewClockCommand : 1, skew : 15000 } )
-s._connections[1].getDB( "admin" ).runCommand( { _skewClockCommand : 1, skew : -16000 } )
+s._connections[1].getDB( "admin" ).runCommand( { _skewClockCommand : 1, skew : -32000 } )
 
 // We need to start another mongos after skewing the clock, since the first mongos will have already
 // tested the config servers (via the balancer) before we manually skewed them
-otherMongos = startMongos( { port : 30020, v : 0, configdb : s._configDB } );
+otherMongos = startMongos( { port : 30020, v : 2, configdb : s._configDB } );
 
 // Initialize DB data
 initDB = function(name) {
diff --git a/src/mongo/client/distlock.h b/src/mongo/client/distlock.h
index 183826afcce..774d94a0df8 100644
--- a/src/mongo/client/distlock.h
+++ b/src/mongo/client/distlock.h
@@ -57,12 +57,19 @@ namespace mongo {
     };
 
     /**
-     * The distributed lock is a configdb backed way of synchronizing system-wide tasks. A task must be identified by a
-     * unique name across the system (e.g., "balancer"). A lock is taken by writing a document in the configdb's locks
-     * collection with that name.
+     * The distributed lock is a configdb backed way of synchronizing system-wide tasks. A task
+     * must be identified by a unique name across the system (e.g., "balancer"). A lock is taken
+     * by writing a document in the configdb's locks collection with that name.
      *
-     * To be maintained, each taken lock needs to be revalidated ("pinged") within a pre-established amount of time. This
-     * class does this maintenance automatically once a DistributedLock object was constructed.
+     * To be maintained, each taken lock needs to be revalidated ("pinged") within a
+     * pre-established amount of time. This class does this maintenance automatically once a
+     * DistributedLock object was constructed. The ping procedure records the local time to
+     * the ping document, but that time is untrusted and is only used as a point of reference
+     * of whether the ping was refreshed or not. Ultimately, the clock a configdb is the source
+     * of truth when determining whether a ping is still fresh or not. This is achieved by
+     * (1) remembering the ping document time along with config server time when unable to
+     * take a lock, and (2) ensuring all config servers report similar times and have similar
+     * time rates (the difference in times must start and stay small).
      */
     class DistributedLock {
     public:
@@ -147,9 +154,13 @@ namespace mongo {
         const ConnectionString& getRemoteConnection();
 
         /**
-         * Check the skew between a cluster of servers
+         * Checks the skew among a cluster of servers and returns true if the min and max clock
+         * times among the servers are within maxClockSkew.
          */
-        static bool checkSkew( const ConnectionString& cluster, unsigned skewChecks = NUM_LOCK_SKEW_CHECKS, unsigned long long maxClockSkew = MAX_LOCK_CLOCK_SKEW, unsigned long long maxNetSkew = MAX_LOCK_NET_SKEW );
+        static bool checkSkew( const ConnectionString& cluster,
+                               unsigned skewChecks = NUM_LOCK_SKEW_CHECKS,
+                               unsigned long long maxClockSkew = MAX_LOCK_CLOCK_SKEW,
+                               unsigned long long maxNetSkew = MAX_LOCK_NET_SKEW );
 
         /**
          * Get the remote time from a server or cluster