SERVER-1280 Allow balancer to be turned off

jstests/slow/sharding_balance3.js

@@ -0,0 +1,71 @@
+// sharding_balance3.js
+
+s = new ShardingTest( "slow_sharding_balance3" , 2 , 2 , 1 , { chunksize : 1 } );
+
+s.adminCommand( { enablesharding : "test" } );
+s.adminCommand( { shardcollection : "test.foo" , key : { _id : 1 } } );
+assert.eq( 1 , s.config.chunks.count()  , "setup1" );
+
+s.config.settings.find().forEach( printjson );
+
+db = s.getDB( "test" );
+
+bigString = ""
+while ( bigString.length < 10000 )
+    bigString += "asdasdasdasdadasdasdasdasdasdasdasdasda";
+
+inserted = 0;
+num = 0;
+while ( inserted < ( 20 * 1024 * 1024 ) ){
+    db.foo.insert( { _id : num++ , s : bigString } );
+    inserted += bigString.length;
+}
+
+db.getLastError();
+assert.lt( 20 , s.config.chunks.count()  , "setup2" );
+
+function dist(){
+    var x = {}
+    s.config.chunks.find( { ns : "test.foo" } ).forEach(
+        function(z){
+            if ( x[z.shard] )
+                x[z.shard]++
+            else
+                x[z.shard] = 1;
+        }
+    );
+    return x;
+}
+
+function diff(){
+    var x = dist();
+    printjson( x )
+    return Math.max( x.shard0 , x.shard1 ) - Math.min( x.shard0 , x.shard1 );
+}
+
+assert.lt( 20 , diff() );
+
+// Wait for balancer to kick in.
+var initialDiff = diff();
+var maxRetries = 3;
+while ( diff() == initialDiff ){
+    sleep( 5000 );
+    assert.lt( 0, maxRetries--, "Balancer did not kick in.");
+}
+
+print("* A");
+print( "disabling the balancer" );
+s.config.settings.update( { _id : "balancer" }, { $set : { stopped : true } } );
+s.config.settings.find().forEach( printjson );
+print("* B");
+
+
+print( diff() )
+
+var currDiff = diff();
+assert.repeat( function(){
+    var d = diff();
+    return d != currDiff;
+} , "balance with stopped flag should not have happened" , 1000 * 30 , 5000 );
+
+s.stop()

s/balance.cpp

@@ -42,8 +42,17 @@ namespace mongo {
         log(2) << "balancer: " << x << endl;
         
         if ( ! x.isEmpty() ){
+
             if ( x["who"].String() == _myid ){
                 log(2) << "balancer: i'm the current balancer" << endl;
+
+                // If need be, we can stop the balancer by creating a 'stopped : true' field in its 
+                // entry.
+                if ( x["stopped"].type() && x["stopped"].Bool() ){
+                    log() << "balancer: stopped flag true" << endl;
+                    return false;
+                }
+
                 return true;
             }
             
@@ -70,11 +79,18 @@ namespace mongo {
         
         BSONObjBuilder updateQuery;
         updateQuery.append( "_id" , "balancer" );
-        if ( x["x"].type() )
+        if ( x["x"].type() ){
             updateQuery.append( x["x"] );
-        else
+
+            // Carry on the stopped flag, if it existed.
+            if ( ! x["stopped"].type() ){
+                updateQuery.append( "stopped" , x["stopped"].Bool() );
+            }
+
+        } else {
             updateQuery.append( "x" , BSON( "$exists" << false ) );
-        
+        }
+
         conn.update( ShardNS::settings , 
                      updateQuery.obj() ,
                      BSON( "$set" << BSON( "who" << _myid << "x" << incarnation ) ) ,