SERVER-49527 recoverFromOplogAsStandalone does not relax index constraints

(cherry picked from commit e0cde43310)

jstests/replsets/standalone_replication_recovery_relaxes_index_constaints.js

@@ -0,0 +1,61 @@
+/*
+ * Tests that 'recoverFromOplogAsStandalone' relaxes index constraints. This test is
+ * non-deterministic. If there were a bug, it would still succeed with low probability, but should
+ * never fail without a bug.
+ *
+ * This test only makes sense for storage engines that support recover to stable timestamp.
+ * @tags: [requires_wiredtiger, requires_persistence, requires_journaling, requires_replication,
+ * requires_majority_read_concern]
+ */
+
+(function() {
+"use strict";
+load("jstests/replsets/rslib.js");
+load("jstests/libs/write_concern_util.js");
+
+const name = jsTestName();
+const dbName = name;
+const collName = 'coll';
+const logLevel = tojson({storage: {recovery: 2}, replication: 3});
+
+const rst = new ReplSetTest({
+    nodes: 1,
+});
+
+function getColl(conn) {
+    return conn.getDB(dbName)[collName];
+}
+
+jsTestLog("Initiating as a replica set.");
+rst.startSet();
+rst.initiate();
+let node = rst.getPrimary();
+
+assert.commandWorked(getColl(node).insert({_id: 1}, {writeConcern: {w: 1, j: 1}}));
+assert.commandWorked(getColl(node).createIndex({x: 1}, {unique: true}));
+
+jsTestLog("Running inserts and removes");
+const start = (new Date()).getTime();
+const waitTimeMillis = 5 * 1000;
+const baseNum = 10;
+let iter = 2;
+Random.setRandomSeed();
+while (((new Date()).getTime() - start) < waitTimeMillis) {
+    iter++;
+    const uniqueKey = Math.floor(Random.rand() * baseNum);
+    assert.commandWorked(getColl(node).insert({_id: iter, x: uniqueKey}));
+    assert.commandWorked(getColl(node).remove({_id: iter}));
+}
+
+jsTestLog("Kill the node");
+rst.stop(node, 9, {allowedExitCode: MongoRunner.EXIT_SIGKILL});
+
+jsTestLog("Restart the node with 'recoverFromOplogAsStandalone'");
+node = rst.restart(node, {
+    noReplSet: true,
+    setParameter: {recoverFromOplogAsStandalone: true, logComponentVerbosity: logLevel}
+});
+reconnect(node);
+
+rst.stopSet();
+})();

src/mongo/db/repl/replication_coordinator_impl.cpp

@@ -2361,6 +2361,9 @@ bool ReplicationCoordinatorImpl::isInPrimaryOrSecondaryState_UNSAFE() const {
 
 bool ReplicationCoordinatorImpl::shouldRelaxIndexConstraints(OperationContext* opCtx,
                                                              const NamespaceString& ns) {
+    if (ReplSettings::shouldRecoverFromOplogAsStandalone()) {
+        return true;
+    }
     return !canAcceptWritesFor(opCtx, ns);
 }