SERVER-47006 Implement a downgrade floor between 4.4 and 4.2

2 files changed, 29 insertions, 15 deletions

diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp
index 4044e5481e5..a393905d2fc 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp
@@ -135,9 +135,8 @@ bool WiredTigerFileVersion::shouldDowngrade(bool readOnly,
     if (!serverGlobalParams.featureCompatibility.isVersionInitialized()) {
         // If the FCV document hasn't been read, trust the WT compatibility. MongoD will
         // downgrade to the same compatibility it discovered on startup.
-        return _startupVersion == StartupVersion::IS_42 ||
-            _startupVersion == StartupVersion::IS_40 || _startupVersion == StartupVersion::IS_36 ||
-            _startupVersion == StartupVersion::IS_34;
+        return _startupVersion == StartupVersion::IS_44_FCV_42 ||
+            _startupVersion == StartupVersion::IS_42;
     }
 
     if (serverGlobalParams.featureCompatibility.getVersion() !=
@@ -166,22 +165,18 @@ bool WiredTigerFileVersion::shouldDowngrade(bool readOnly,
 
 std::string WiredTigerFileVersion::getDowngradeString() {
     if (!serverGlobalParams.featureCompatibility.isVersionInitialized()) {
-        invariant(_startupVersion != StartupVersion::IS_44);
+        invariant(_startupVersion != StartupVersion::IS_44_FCV_44);
 
         switch (_startupVersion) {
-            case StartupVersion::IS_34:
-                return "compatibility=(release=2.9)";
-            case StartupVersion::IS_36:
-                return "compatibility=(release=3.0)";
-            case StartupVersion::IS_40:
-                return "compatibility=(release=3.1)";
+            case StartupVersion::IS_44_FCV_42:
+                return "compatibility=(release=3.3)";
             case StartupVersion::IS_42:
-                return "compatibility=(release=3.2)";
+                return "compatibility=(release=3.3)";
             default:
                 MONGO_UNREACHABLE;
         }
     }
-    return "compatibility=(release=3.2)";
+    return "compatibility=(release=3.3)";
 }
 
 using std::set;
@@ -1017,12 +1012,28 @@ void WiredTigerKVEngine::appendGlobalStats(BSONObjBuilder& b) {
 }
 
 void WiredTigerKVEngine::_openWiredTiger(const std::string& path, const std::string& wtOpenConfig) {
-    std::string configStr = wtOpenConfig + ",compatibility=(require_min=\"3.1.0\")";
-
+    // MongoDB 4.4 will always run in compatibility version 10.0.
+    std::string configStr = wtOpenConfig + ",compatibility=(require_min=\"10.0.0\")";
     auto wtEventHandler = _eventHandler.getWtEventHandler();
 
     int ret = wiredtiger_open(path.c_str(), wtEventHandler, configStr.c_str(), &_conn);
     if (!ret) {
+        _fileVersion = {WiredTigerFileVersion::StartupVersion::IS_44_FCV_44};
+        return;
+    }
+
+    // MongoDB 4.4 doing clean shutdown in FCV 4.2 will use compatibility version 3.3.
+    configStr = wtOpenConfig + ",compatibility=(require_min=\"3.3.0\")";
+    ret = wiredtiger_open(path.c_str(), wtEventHandler, configStr.c_str(), &_conn);
+    if (!ret) {
+        _fileVersion = {WiredTigerFileVersion::StartupVersion::IS_44_FCV_42};
+        return;
+    }
+
+    // MongoDB 4.2 uses compatibility version 3.2.
+    configStr = wtOpenConfig + ",compatibility=(require_min=\"3.2.0\")";
+    ret = wiredtiger_open(path.c_str(), wtEventHandler, configStr.c_str(), &_conn);
+    if (!ret) {
         _fileVersion = {WiredTigerFileVersion::StartupVersion::IS_42};
         return;
     }
diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.h b/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.h
index 35d34b32d9e..9ae3015e8cd 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.h
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.h
@@ -59,7 +59,10 @@ class WiredTigerEngineRuntimeConfigParameter;
 class WiredTigerMaxCacheOverflowSizeGBParameter;
 
 struct WiredTigerFileVersion {
-    enum class StartupVersion { IS_34, IS_36, IS_40, IS_42, IS_44 };
+    // MongoDB 4.4+ will not open on datafiles left behind by 4.2.5 and earlier. MongoDB 4.4
+    // shutting down in FCV 4.2 will leave data files that 4.2.6+ will understand
+    // (IS_44_FCV_42). MongoDB 4.2.x always writes out IS_42.
+    enum class StartupVersion { IS_42, IS_44_FCV_42, IS_44_FCV_44 };
 
     StartupVersion _startupVersion;
     bool shouldDowngrade(bool readOnly, bool repairMode, bool hasRecoveryTimestamp);