SERVER-46961 Test sync source in quiesce mode

jstests/replsets/sync_source_enters_quiesce_mode.js

@@ -0,0 +1,52 @@
+/**
+ * Tests that reading from an existing sync source continues uninterrupted when the sync source
+ * enters quiesce mode.
+ *
+ * @tags: [requires_fcv_46]
+ */
+(function() {
+"use strict";
+
+load("jstests/libs/fail_point_util.js");
+
+// Set the oplog fetcher batch size to 1, in order to test fetching multiple batches while the sync
+// source is in quiesce mode.
+const rst = new ReplSetTest(
+    {nodes: 3, useBridge: true, nodeOptions: {setParameter: "bgSyncOplogFetcherBatchSize=1"}});
+rst.startSet();
+rst.initiateWithHighElectionTimeout();
+
+const primary = rst.getPrimary();
+assert.eq(primary, rst.nodes[0]);
+
+const syncSource = rst.nodes[1];
+const syncingNode = rst.nodes[2];
+
+jsTestLog("Ensure syncingNode is syncing from syncSource.");
+syncingNode.disconnect(primary);
+assert.commandWorked(primary.getDB("test").c.insert({a: 1}, {writeConcern: {w: 3}}));
+
+jsTestLog("Ensure syncingNode is behind syncSource.");
+// Do not use stopServerReplication(), since this can cause the node to change sync source.
+let hangOplogQueryFailPoint =
+    configureFailPoint(syncSource, "planExecutorHangBeforeShouldWaitForInserts");
+hangOplogQueryFailPoint.wait();
+assert.commandWorked(primary.getDB("test").c.insert([{a: 2}, {a: 3}, {a: 4}]),
+                     {writeConcern: {w: 2}});
+
+jsTestLog("Transition syncSource to quiesce mode.");
+let quiesceModeFailPoint = configureFailPoint(syncSource, "hangDuringQuiesceMode");
+// We must skip validation due to the failpoint that hangs awaitData queries.
+rst.stop(syncSource, null /*signal*/, {skipValidation: true}, {forRestart: true, waitpid: false});
+quiesceModeFailPoint.wait();
+
+jsTestLog("Check that syncing continues uninterrupted.");
+hangOplogQueryFailPoint.off();
+rst.awaitReplication();
+
+jsTestLog("Finish test.");
+syncingNode.reconnect(primary);
+quiesceModeFailPoint.off();
+rst.restart(syncSource);
+rst.stopSet();
+})();