Fix use-after-free when generating sourcemap types

We triggered a UAF because we call handleSourcemapUpdate twice,
once per typechecker (autocomplete and non-autocomplete).

However, the call to instanceTypes.clear() was moved into the
handleSourcemapUpdate function, meaning it would clear the
types created after the first call, causing a UAF.

We fix this by moving the arena clear call to before update is called

Fixes #657

CHANGELOG.md

@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed use-after-free when generating sourcemap types
+
 ## [1.30.0] - 2024-06-23
 
 ### Deprecated

src/platform/roblox/RobloxSourcemap.cpp

@@ -243,6 +243,8 @@ bool RobloxPlatform::updateSourceMap()
         workspaceFolder->frontend.clear();
         updateSourceNodeMap(sourceMapContents.value());
 
+        // Recreate instance types
+        instanceTypes.clear(); // NOTE: used across BOTH instances of handleSourcemapUpdate, don't clear in between!
         auto config = workspaceFolder->client->getConfiguration(workspaceFolder->rootUri);
         bool expressiveTypes = config.diagnostics.strictDatamodelTypes;
 
@@ -323,9 +325,6 @@ void RobloxPlatform::handleSourcemapUpdate(Luau::Frontend& frontend, const Luau:
         }
     }
 
-    // Recreate instance types
-    instanceTypes.clear();
-
     // Create a type for the root source node
     getSourcemapType(globals, instanceTypes, rootSourceNode);