SERVER-16186: newCollectionsUsePowerOf2Sizes should warn users it no longer works

Author: markbenvenuto

jstests/mmap_v1/use_power_of_2_a.js

@@ -30,8 +30,4 @@ assert.eq(db.adminCommand({getParameter:1,
           newCollectionsUsePowerOf2Sizes: true}).newCollectionsUsePowerOf2Sizes, true);
 
 test(1);
-assert.commandWorked(db.adminCommand({setParameter:1, newCollectionsUsePowerOf2Sizes: false}));
-test(0);
 
-// reset the server to default value
-assert.commandWorked(db.adminCommand({setParameter:1, newCollectionsUsePowerOf2Sizes: true}));

jstests/tool/dumprestoreWithNoOptions.js

@@ -23,6 +23,14 @@ dbname2 = "NOT_"+dbname;
 
 db.dropDatabase();
 
+// MMapV1 always sets newcollectionsusepowerof2sizes, WT does not
+defaultFlags = { "flags" : 1 }
+var ss = db.serverStatus();
+
+if (ss.storageEngine.name != "mmapv1") {
+    defaultFlags = {};
+}
+
 var options = { capped: true, size: 4096, autoIndexId: true };
 db.createCollection('capped', options);
 assert.eq( 1, db.capped.getIndexes().length, "auto index not created" );
@@ -45,7 +53,7 @@ t.runTool( "restore" , "--dir" , t.ext , "--noOptionsRestore");
 
 assert.eq( 1, db.capped.count() , "wrong number of docs restored to capped" );
 assert(true !== db.capped.stats().capped, "restore options were not ignored");
-assert.eq( {}, db.capped.exists().options,
+assert.eq( defaultFlags, db.capped.exists().options,
        "restore options not ignored: " + tojson( db.capped.exists() ) );
 
 // Dump/restore single DB
@@ -74,7 +82,7 @@ db = db.getSiblingDB(dbname2);
 
 assert.eq( 1, db.capped.count() , "wrong number of docs restored to capped" );
 assert(true !== db.capped.stats().capped, "restore options were not ignored");
-assert.eq( {}, db.capped.exists().options, 
+assert.eq( defaultFlags, db.capped.exists().options, 
           "restore options not ignored: " + tojson( db.capped.exists() ) );
 
 // Dump/restore single collection
@@ -106,7 +114,7 @@ db = db.getSiblingDB(dbname);
 
 assert.eq( 1, db.capped.count() , "wrong number of docs restored to capped" );
 assert( true !== db.capped.stats().capped, "restore options were not ignored" );
-assert.eq( {}, db.capped.exists().options, 
+assert.eq( defaultFlags, db.capped.exists().options, 
           "restore options not ignored: " + tojson( db.capped.exists() ) );
 
 t.stop();

src/mongo/db/storage/mmap_v1/mmap_v1_database_catalog_entry.cpp

@@ -58,7 +58,37 @@
 
 namespace mongo {
 
-    MONGO_EXPORT_SERVER_PARAMETER(newCollectionsUsePowerOf2Sizes, bool, true);
+    namespace {
+        bool newCollectionsUsePowerOf2SizesFlag = true; // Unused, needed for server parameter.
+
+        /**
+         * Declaration for the "newCollectionsUsePowerOf2Sizes" server parameter,
+         * which is now deprecated in 2.8.
+         * Note that:
+         * - setting to true performs a no-op.
+         * - setting to false will fail.
+         */
+        class NewCollectionsUsePowerOf2SizesParameter : public ExportedServerParameter<bool> {
+        public:
+            NewCollectionsUsePowerOf2SizesParameter() :
+                ExportedServerParameter<bool>(ServerParameterSet::getGlobal(),
+                "newCollectionsUsePowerOf2Sizes",
+                &newCollectionsUsePowerOf2SizesFlag,
+                true,
+                true) {}
+
+            virtual Status validate(const bool& potentialNewValue) {
+                if (!potentialNewValue) {
+                    return Status(ErrorCodes::BadValue,
+                        "newCollectionsUsePowerOf2Sizes cannot be set to false. "
+                        "Use noPadding instead during createCollection.");
+                }
+
+                return Status::OK();
+            }
+
+        } exportedNewCollectionsUsePowerOf2SizesParameter;
+    }
 
     /**
      * Registers the insertion of a new entry in the _collections cache with the RecoveryUnit,
@@ -618,8 +648,10 @@ namespace mongo {
             if ( options.flagsSet ) {
                 md.setUserFlag( txn, options.flags );
             }
-            else if ( newCollectionsUsePowerOf2Sizes ) {
-                md.setUserFlag( txn, NamespaceDetails::Flag_UsePowerOf2Sizes );
+            else {
+                // For compatibility with previous versions if the user sets no flags,
+                // we set Flag_UsePowerOf2Sizes in case the user downgrades
+                md.setUserFlag(txn, NamespaceDetails::Flag_UsePowerOf2Sizes);
             }
         }
         else if ( options.cappedMaxDocs > 0 ) {