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

2 files changed, 30 insertions, 24 deletions

diff --git a/jstests/multiVersion/long_collection_names.js b/jstests/multiVersion/long_collection_names.js
index d72b56b3dcf..b4e81634dc6 100644
--- a/jstests/multiVersion/long_collection_names.js
+++ b/jstests/multiVersion/long_collection_names.js
@@ -67,44 +67,29 @@ conn = MongoRunner.runMongod(restartOpts42);
 assert.eq(null, conn, 'mongod was able to start with version ' + tojson(restartOpts42));
 
 /**
- * Restart with the 4.4 binary to set the FCV to 4.2.
+ * Cannot downgrade to FCV 4.2 on a 4.4 binary when long collection names are present.
  */
 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.commandWorked(testDb.adminCommand({setFeatureCompatibilityVersion: lastStableFCV}));
-MongoRunner.stopMongod(conn);
+assert.commandFailedWithCode(testDb.adminCommand({setFeatureCompatibilityVersion: lastStableFCV}),
+                             ErrorCodes.InvalidNamespace);
 
 /**
- * Restart with the 4.2 binary while in FCV 4.2 with long collection names present. This shouldn't
- * crash the server.
+ * 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.
  */
-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());
 
-// 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);
+assert.commandWorked(testDb.adminCommand({setFeatureCompatibilityVersion: lastStableFCV}));
 
 // 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 50815f25fa4..a4e63703057 100644
--- a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
+++ b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
@@ -33,6 +33,7 @@
 
 #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"
@@ -349,6 +350,26 @@ 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