From 09c474af1cca7c54d3f93ac46d8fbe9549cd4689 Mon Sep 17 00:00:00 2001
From: Vishnu Kaushik <vishnu.kaushik@mongodb.com>
Date: Thu, 11 Feb 2021 22:05:17 +0000
Subject: SERVER-54478 Fix issue where recipient primary failover after forget
 migration can lead to the oplog buffer collection not being dropped on new
 primary

---
 ...nt_migration_recipient_stepdown_after_forget.js | 71 ++++++++++++++++++++++
 .../db/repl/tenant_migration_recipient_service.cpp | 11 ++--
 2 files changed, 77 insertions(+), 5 deletions(-)
 create mode 100644 jstests/replsets/tenant_migration_recipient_stepdown_after_forget.js

diff --git a/jstests/replsets/tenant_migration_recipient_stepdown_after_forget.js b/jstests/replsets/tenant_migration_recipient_stepdown_after_forget.js
new file mode 100644
index 00000000000..53b93f73d57
--- /dev/null
+++ b/jstests/replsets/tenant_migration_recipient_stepdown_after_forget.js
@@ -0,0 +1,71 @@
+/**
+ * Tests whether the new recipient primary properly processes a forgetMigration when the original
+ * primary is made to step down after marking as garbage collectable. The oplog buffer collection
+ * must be dropped.
+ *
+ * @tags: [requires_fcv_49, requires_replication, incompatible_with_windows_tls]
+ */
+
+(function() {
+
+"use strict";
+load("jstests/libs/uuid_util.js");        // For extractUUIDFromObject().
+load("jstests/libs/fail_point_util.js");  // For configureFailPoint().
+load("jstests/libs/parallelTester.js");   // For Thread(), used for async forgetMigration.
+load("jstests/replsets/libs/tenant_migration_test.js");
+load("jstests/replsets/libs/tenant_migration_util.js");
+
+const tenantMigrationTest =
+    new TenantMigrationTest({name: jsTestName(), sharedOptions: {nodes: 2}});
+
+if (!tenantMigrationTest.isFeatureFlagEnabled()) {
+    jsTestLog("Skipping test because the tenant migrations feature flag is disabled");
+    tenantMigrationTest.stop();
+    return;
+}
+
+const kMigrationId = UUID();
+const kTenantId = 'testTenantId';
+const kReadPreference = {
+    mode: "primary"
+};
+const migrationOpts = {
+    migrationIdString: extractUUIDFromObject(kMigrationId),
+    tenantId: kTenantId,
+    readPreference: kReadPreference
+};
+
+tenantMigrationTest.runMigration(
+    migrationOpts, true /* retryOnRetryableErrors */, false /* automaticForgetMigration */);
+
+const fpBeforeDroppingOplogBufferCollection =
+    configureFailPoint(tenantMigrationTest.getRecipientPrimary(),
+                       "fpBeforeDroppingOplogBufferCollection",
+                       {action: "hang"});
+
+jsTestLog("Issuing a forget migration command.");
+const forgetMigrationThread =
+    new Thread(TenantMigrationUtil.forgetMigrationAsync,
+               migrationOpts.migrationIdString,
+               TenantMigrationUtil.createRstArgs(tenantMigrationTest.getDonorRst()),
+               true /* retryOnRetryableErrors */);
+forgetMigrationThread.start();
+
+fpBeforeDroppingOplogBufferCollection.wait();
+
+jsTestLog("Step up a new recipient primary.");
+assert.commandWorked(tenantMigrationTest.getRecipientRst().getSecondaries()[0].adminCommand(
+    {replSetStepUp: ReplSetTest.kForeverSecs, force: true}));
+
+fpBeforeDroppingOplogBufferCollection.off();
+
+jsTestLog("Waiting for forget migration to complete.");
+assert.commandWorked(forgetMigrationThread.returnData());
+
+const configDBCollections =
+    tenantMigrationTest.getRecipientPrimary().getDB('config').getCollectionNames();
+assert(!configDBCollections.includes('repl.migration.oplog_' + migrationOpts.migrationIdString),
+       configDBCollections);
+
+tenantMigrationTest.stop();
+})();
\ No newline at end of file
diff --git a/src/mongo/db/repl/tenant_migration_recipient_service.cpp b/src/mongo/db/repl/tenant_migration_recipient_service.cpp
index d0780d91cd2..bf964c2e834 100644
--- a/src/mongo/db/repl/tenant_migration_recipient_service.cpp
+++ b/src/mongo/db/repl/tenant_migration_recipient_service.cpp
@@ -100,6 +100,7 @@ MONGO_FAIL_POINT_DEFINE(hangBeforeTaskCompletion);
 MONGO_FAIL_POINT_DEFINE(fpAfterReceivingRecipientForgetMigration);
 MONGO_FAIL_POINT_DEFINE(hangAfterCreatingRSM);
 MONGO_FAIL_POINT_DEFINE(skipRetriesWhenConnectingToDonorHost);
+MONGO_FAIL_POINT_DEFINE(fpBeforeDroppingOplogBufferCollection);
 
 namespace {
 // We never restart just the oplog fetcher.  If a failure occurs, we restart the whole state machine
@@ -1377,15 +1378,14 @@ SemiFuture<void> TenantMigrationRecipientService::Instance::run(
 
             pauseAfterRunTenantMigrationRecipientInstance.pauseWhileSet();
 
-            uassert(ErrorCodes::TenantMigrationForgotten,
-                    str::stream() << "Migration " << getMigrationUUID()
-                                  << " already marked for garbage collect",
-                    !_stateDoc.getExpireAt());
-
             return _initializeStateDoc(lk);
         })
         .then([this, self = shared_from_this()] {
             _stateDocPersistedPromise.emplaceValue();
+            uassert(ErrorCodes::TenantMigrationForgotten,
+                    str::stream() << "Migration " << getMigrationUUID()
+                                  << " already marked for garbage collect",
+                    !_stateDoc.getExpireAt());
             _stopOrHangOnFailPoint(&fpAfterPersistingTenantMigrationRecipientInstanceStateDoc);
             return _createAndConnectClients();
         })
@@ -1604,6 +1604,7 @@ SemiFuture<void> TenantMigrationRecipientService::Instance::run(
         })
         .then([this, self = shared_from_this()] { return _markStateDocAsGarbageCollectable(); })
         .then([this, self = shared_from_this()] {
+            _stopOrHangOnFailPoint(&fpBeforeDroppingOplogBufferCollection);
             auto opCtx = cc().makeOperationContext();
             auto storageInterface = StorageInterface::get(opCtx.get());
 
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