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

5 files changed, 45 insertions, 32 deletions

diff --git a/jstests/hooks/run_validate_collections.js b/jstests/hooks/run_validate_collections.js
index b94c3317c0e..c3ad9af2c72 100644
--- a/jstests/hooks/run_validate_collections.js
+++ b/jstests/hooks/run_validate_collections.js
@@ -49,6 +49,8 @@ const requiredFCV = jsTest.options().forceValidationWithFeatureCompatibilityVers
 let originalFCV;
 let originalTransactionLifetimeLimitSeconds;
 
+let skipFCV = false;
+
 if (requiredFCV) {
     // Running the setFeatureCompatibilityVersion command may implicitly involve running a
     // multi-statement transaction. We temporarily raise the transactionLifetimeLimitSeconds to be
@@ -72,13 +74,18 @@ if (requiredFCV) {
     }
 
     // Now that we are certain that an upgrade or downgrade of the FCV is not in progress, ensure
-    // the 'requiredFCV' is set.
-    assert.commandWorked(adminDB.runCommand({setFeatureCompatibilityVersion: requiredFCV}));
+    // the 'requiredFCV' is set. If we're trying to set the FCV to 4.2 while having long collection
+    // name present, we'll get the 'InvalidNamespace' error.
+    const fcvRes = adminDB.runCommand({setFeatureCompatibilityVersion: requiredFCV});
+    if (!fcvRes.ok) {
+        assert.commandFailedWithCode(fcvRes, ErrorCodes.InvalidNamespace);
+        skipFCV = true;
+    }
 }
 
-new CollectionValidator().validateNodes(hostList);
+new CollectionValidator().validateNodes(hostList, skipFCV);
 
-if (originalFCV && originalFCV.version !== requiredFCV) {
+if (originalFCV && originalFCV.version !== requiredFCV && !skipFCV) {
     assert.commandWorked(adminDB.runCommand({setFeatureCompatibilityVersion: originalFCV.version}));
 }
 
diff --git a/jstests/hooks/validate_collections.js b/jstests/hooks/validate_collections.js
index cf7f1be9707..01275869617 100644
--- a/jstests/hooks/validate_collections.js
+++ b/jstests/hooks/validate_collections.js
@@ -84,7 +84,7 @@ function CollectionValidator() {
     };
 
     // Run a separate thread to validate collections on each server in parallel.
-    const validateCollectionsThread = function(validatorFunc, host) {
+    const validateCollectionsThread = function(validatorFunc, host, skipFCV) {
         try {
             print('Running validate() on ' + host);
             const conn = new Mongo(host);
@@ -98,7 +98,7 @@ function CollectionValidator() {
             }
 
             const requiredFCV = jsTest.options().forceValidationWithFeatureCompatibilityVersion;
-            if (requiredFCV) {
+            if (requiredFCV && !skipFCV) {
                 // Make sure this node has the desired FCV as it may take time for the updates to
                 // replicate to the nodes that weren't part of the w=majority.
                 assert.soonNoExcept(() => {
@@ -122,7 +122,7 @@ function CollectionValidator() {
         }
     };
 
-    this.validateNodes = function(hostList) {
+    this.validateNodes = function(hostList, skipFCV) {
         // We run the scoped threads in a try/finally block in case any thread throws an exception,
         // in which case we want to still join all the threads.
         let threads = [];
@@ -130,7 +130,7 @@ function CollectionValidator() {
         try {
             hostList.forEach(host => {
                 const thread =
-                    new Thread(validateCollectionsThread, this.validateCollections, host);
+                    new Thread(validateCollectionsThread, this.validateCollections, host, skipFCV);
                 threads.push(thread);
                 thread.start();
             });
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/jstests/core/create_collection.js b/jstests/noPassthroughWithMongod/create_collection.js
index 2c139708e2e..2c139708e2e 100644
--- a/jstests/core/create_collection.js
+++ b/jstests/noPassthroughWithMongod/create_collection.js
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