Revert "SERVER-51333 setFeatureCompatibilityVersion fails when downgrading from FCV 4.4 to FCV 4.2 with long collection names present"

This reverts commit d1f0d546f21ff900d3a0b083006bf6a4b61657a8.

2 files changed, 24 insertions, 30 deletions

diff --git a/jstests/multiVersion/long_collection_names.js b/jstests/multiVersion/long_collection_names.js
index b4e81634dc6..d72b56b3dcf 100644
--- a/jstests/multiVersion/long_collection_names.js
+++ b/jstests/multiVersion/long_collection_names.js
@@ -67,29 +67,44 @@ conn = MongoRunner.runMongod(restartOpts42);
 assert.eq(null, conn, 'mongod was able to start with version ' + tojson(restartOpts42));
 
 /**
- * Cannot downgrade to FCV 4.2 on a 4.4 binary when long collection names are present.
+ * Restart with the 4.4 binary to set the FCV to 4.2.
  */
 let restartOpts44 = Object.extend(mongodOptions44, {restart: true});
 conn = MongoRunner.runMongod(restartOpts44);
 assert.neq(null, conn, 'mongod was unable to start with version ' + tojson(restartOpts44));
 
 testDb = conn.getDB(dbName);
-assert.commandFailedWithCode(testDb.adminCommand({setFeatureCompatibilityVersion: lastStableFCV}),
-                             ErrorCodes.InvalidNamespace);
+assert.commandWorked(testDb.adminCommand({setFeatureCompatibilityVersion: lastStableFCV}));
+MongoRunner.stopMongod(conn);
 
 /**
- * FCV can be set to 4.2 after removing the long collection name. However, we cannot create any new
- * collections with long names in FCV 4.2.
+ * Restart with the 4.2 binary while in FCV 4.2 with long collection names present. This shouldn't
+ * crash the server.
  */
+conn = MongoRunner.runMongod(restartOpts42);
+assert.neq(null, conn, 'mongod was unable to start with version ' + tojson(restartOpts42));
+
 testDb = conn.getDB(dbName);
-assert.eq(true, testDb.getCollection(longCollName).drop());
 
-assert.commandWorked(testDb.adminCommand({setFeatureCompatibilityVersion: lastStableFCV}));
+// Ensure we have the proper collections.
+let collNames = testDb.getCollectionNames();
+
+assert.eq(true, collNames.includes(shortCollName));
+assert.eq(true, collNames.includes(longCollName));
+
+MongoRunner.stopMongod(conn);
+
+/**
+ * Restart with the 4.4 binary while in FCV 4.2. We shouldn't be able to create any collections with
+ * long names.
+ */
+conn = MongoRunner.runMongod(restartOpts44);
+assert.neq(null, conn, 'mongod was unable to start with version ' + tojson(restartOpts44));
+
+testDb = conn.getDB(dbName);
 
 // Creating a long collection name on a 4.4 binary with FCV 4.2 should fail.
 assert.commandFailedWithCode(testDb.createCollection('c'.repeat(8192)), 4862100);
-assert.commandFailedWithCode(testDb.createCollection(longCollName),
-                             ErrorCodes.IncompatibleServerVersion);
 
 // Running rename within the same database or across two databases should fail for long collection
 // names.
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 a4e63703057..50815f25fa4 100644
--- a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
+++ b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
@@ -33,7 +33,6 @@
 
 #include "mongo/db/auth/authorization_session.h"
 #include "mongo/db/catalog/coll_mod.h"
-#include "mongo/db/catalog/collection_catalog_helper.h"
 #include "mongo/db/catalog/database.h"
 #include "mongo/db/catalog/database_holder.h"
 #include "mongo/db/commands.h"
@@ -350,26 +349,6 @@ public:
                     numIndexBuilds == 0U);
             }
 
-            // Cannot downgrade to FCV 4.2 when long collection names are present.
-            const std::vector<std::string> dbNames = CollectionCatalog::get(opCtx).getAllDbNames();
-            for (const auto& dbName : dbNames) {
-                Lock::DBLock dbLock(opCtx, dbName, MODE_IS);
-                catalog::forEachCollectionFromDb(
-                    opCtx, dbName, MODE_IS, [&](const Collection* collection) {
-                        const auto collNss = collection->ns();
-                        uassert(ErrorCodes::InvalidNamespace,
-                                str::stream()
-                                    << "Cannot downgrade the cluster when there are long "
-                                    << "collection names present. FCV 4.2 limit: "
-                                    << NamespaceString::MaxNSCollectionLenFCV42
-                                    << ". Found: " << collNss
-                                    << ", but there may be more. Rename or drop the collection",
-                                collNss.size() <= NamespaceString::MaxNSCollectionLenFCV42);
-                        return true;
-                    });
-            }
-
-
             FeatureCompatibilityVersion::setTargetDowngrade(opCtx);
 
             // Safe reconfig introduces a new "term" field in the config document. If the user tries