From c6d2220f9bb1daceb31f097c06c39cd2e917fb9c Mon Sep 17 00:00:00 2001
From: Jordi Serra Torrens <jordi.serra-torrens@mongodb.com>
Date: Tue, 6 Jul 2021 18:24:06 +0000
Subject: [PATCH] repro bf-21787

---
 jstests/sharding/bf-21787                     | 31 +++++++++++++++++++
 ..._feature_compatibility_version_command.cpp |  3 ++
 2 files changed, 34 insertions(+)
 create mode 100644 jstests/sharding/bf-21787

diff --git a/jstests/sharding/bf-21787 b/jstests/sharding/bf-21787
new file mode 100644
index 0000000000..15f0eaefa8
--- /dev/null
+++ b/jstests/sharding/bf-21787
@@ -0,0 +1,31 @@
+(function() {
+'use strict';
+
+load("jstests/libs/fail_point_util.js");
+load('jstests/libs/parallel_shell_helpers.js');
+
+let st = new ShardingTest({config: 3, mongos: 1, shards: 1});
+
+let failpoint = configureFailPoint(st.configRS.getPrimary(), 'hangBeforeUpdateFcvDoc');
+
+jsTest.log("Run setFCV that will wait on the hangBeforeUpdateFcvDoc failpoint")
+let awaitShell = startParallelShell(funWithArgs(function(foo) {
+    assert.commandWorked(db.adminCommand({setFeatureCompatibilityVersion: lastLTSFCV}));
+}, "foo"), st.s.port);
+
+failpoint.wait();
+
+jsTest.log("Stepdown the configRS primary")
+assert.commandWorked(st.configRS.getPrimary().adminCommand({replSetStepDown: 60, force: 1}));
+st.rs0.awaitNodesAgreeOnPrimary();
+
+jsTest.log("Launch FCV on the new primary")
+assert.commandWorked(st.s.adminCommand({setFeatureCompatibilityVersion: lastLTSFCV}));
+st.configRS.awaitReplication();
+
+jsTest.log("Unset the failpoint on the former primary so it continues running setFCV")
+failpoint.off();
+awaitShell();
+
+st.stop();
+})();
diff --git a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
index 70d01d9c44..4a74fc7ec2 100644
--- a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
+++ b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
@@ -91,6 +91,7 @@ MONGO_FAIL_POINT_DEFINE(hangWhileUpgrading);
 MONGO_FAIL_POINT_DEFINE(hangAfterStartingFCVTransition);
 MONGO_FAIL_POINT_DEFINE(failDowngrading);
 MONGO_FAIL_POINT_DEFINE(hangWhileDowngrading);
+MONGO_FAIL_POINT_DEFINE(hangBeforeUpdateFcvDoc);
 
 /**
  * Ensures that only one instance of setFeatureCompatibilityVersion can run at a given time.
@@ -487,6 +488,8 @@ public:
             // downgraded requestedVersion.
             const auto fcvChangeRegion(FeatureCompatibilityVersion::enterFCVChangeRegion(opCtx));
 
+            hangBeforeUpdateFcvDoc.pauseWhileSet();
+
             FeatureCompatibilityVersion::updateFeatureCompatibilityVersionDocument(
                 opCtx,
                 serverGlobalParams.featureCompatibility.getVersion(),
-- 
2.17.1