SERVER-38933 currentOp is not resilient to drop shard

buildscripts/resmokeconfig/suites/sharding_continuous_config_stepdown.yml

@@ -25,6 +25,7 @@ selector:
   - jstests/sharding/basic_merge.js
   - jstests/sharding/count1.js
   - jstests/sharding/count2.js
+  - jstests/sharding/current_op_with_drop_shard.js
   - jstests/sharding/cursor1.js
   - jstests/sharding/diffservers1.js
   - jstests/sharding/disable_autosplit.js

jstests/sharding/current_op_with_drop_shard.js

@@ -0,0 +1,25 @@
+// Tests that currentOp is resilient to drop shard.
+(function() {
+    'use strict';
+
+    const st = new ShardingTest({shards: 2, rs: {nodes: 1}});
+
+    // We need the balancer to remove a shard.
+    st.startBalancer();
+
+    const mongosDB = st.s.getDB(jsTestName());
+    const shardName = st.shard0.shardName;
+
+    var res = st.s.adminCommand({removeShard: shardName});
+    assert.commandWorked(res);
+    assert.eq('started', res.state);
+    assert.soon(function() {
+        res = st.s.adminCommand({removeShard: shardName});
+        assert.commandWorked(res);
+        return ('completed' === res.state);
+    }, "removeShard never completed for shard " + shardName);
+
+    assert.commandWorked(mongosDB.currentOp());
+
+    st.stop();
+})();

src/mongo/db/pipeline/mongos_process_interface.cpp

@@ -441,6 +441,9 @@ MongoSInterface::DispatchShardPipelineResults MongoSInterface::dispatchShardPipe
         if (!shardRegistry->reload(opCtx)) {
             shardRegistry->reload(opCtx);
         }
+        // Rebuild the set of shards as the shard registry might have changed.
+        shardIds = getTargetedShards(
+            opCtx, mustRunOnAll, executionNsRoutingInfo, shardQuery, aggRequest.getCollation());
     }
 
     // Explain does not produce a cursor, so instead we scatter-gather commands to the shards.