SERVER-63255 Fix dbStats output for non-existing database to match existing database output fields

3 files changed, 32 insertions, 9 deletions

diff --git a/jstests/core/dbstats.js b/jstests/core/dbstats.js
index a7cbd9c6bb8..559b6c11774 100644
--- a/jstests/core/dbstats.js
+++ b/jstests/core/dbstats.js
@@ -84,4 +84,17 @@ if (!isMongoS) {
     assert.eq(2, dbStats.collections, tojson(dbStats));  // testColl + system.views
     assert.eq(1, dbStats.views, tojson(dbStats));
 }
+
+// Check that the output for non-existing database and  the output for empty database
+// have the same fields.
+const testEmptyAndNonExistingDB = db.getSiblingDB(jsTestName() + "_non_existing_and_empty");
+testEmptyAndNonExistingDB.dropDatabase();
+
+const statsNonExistingDB = testEmptyAndNonExistingDB.runCommand({dbStats: 1, freeStorage: 1});
+testEmptyAndNonExistingDB.runCommand({create: "test_empty_collection"});
+
+const statsEmptyDB = testEmptyAndNonExistingDB.runCommand({dbStats: 1, freeStorage: 1});
+assert.sameMembers(Object.keys(statsNonExistingDB),
+                   Object.keys(statsEmptyDB),
+                   "dbStats for non-existing and empty dbs should return the same fields");
 })();
diff --git a/src/mongo/db/catalog/database_impl.cpp b/src/mongo/db/catalog/database_impl.cpp
index 5e092f5d5b1..c46c8f76235 100644
--- a/src/mongo/db/catalog/database_impl.cpp
+++ b/src/mongo/db/catalog/database_impl.cpp
@@ -369,6 +369,8 @@ void DatabaseImpl::getStats(OperationContext* opCtx,
             return true;
         });
 
+    // Make sure that the same fields are returned for non-existing dbs
+    // in `DBStats::errmsgRun`
     output->appendNumber("collections", nCollections);
     output->appendNumber("views", nViews);
     output->appendNumber("objects", objects);
diff --git a/src/mongo/db/commands/dbcommands.cpp b/src/mongo/db/commands/dbcommands.cpp
index 4a2caafb16d..2267c619692 100644
--- a/src/mongo/db/commands/dbcommands.cpp
+++ b/src/mongo/db/commands/dbcommands.cpp
@@ -663,22 +663,30 @@ public:
 
         Database* db = autoDb.getDb();
         if (!db) {
-            // TODO: This preserves old behaviour where we used to create an empty database
-            // metadata even when the database is accessed for read. Without this several
-            // unit-tests will fail, which are fairly easy to fix. If backwards compatibility
-            // is not needed for the missing DB case, we can just do the same that's done in
-            // CollectionStats.
+            // This preserves old behavior where we used to create an empty database even when the
+            // database was accessed for a read. Changing this behavior would impact users that have
+            // learned to depend on it, so we continue to support it. Ensure that these fields match
+            // exactly the fields in `DatabaseImpl::getStats`.
+
             result.appendNumber("collections", 0);
             result.appendNumber("views", 0);
             result.appendNumber("objects", 0);
             result.append("avgObjSize", 0);
             result.appendNumber("dataSize", 0);
             result.appendNumber("storageSize", 0);
-            result.appendNumber("totalSize", 0);
-            result.appendNumber("indexes", 0);
-            result.appendNumber("indexSize", 0);
+            if (includeFreeStorage) {
+                result.appendNumber("freeStorageSize", 0);
+                result.appendNumber("indexes", 0);
+                result.appendNumber("indexSize", 0);
+                result.appendNumber("indexFreeStorageSize", 0);
+                result.appendNumber("totalSize", 0);
+                result.appendNumber("totalFreeStorageSize", 0);
+            } else {
+                result.appendNumber("indexes", 0);
+                result.appendNumber("indexSize", 0);
+                result.appendNumber("totalSize", 0);
+            }
             result.appendNumber("scaleFactor", scale);
-            result.appendNumber("fileSize", 0);
             if (!getGlobalServiceContext()->getStorageEngine()->isEphemeral()) {
                 result.appendNumber("fsUsedSize", 0);
                 result.appendNumber("fsTotalSize", 0);