ffi: fix use-after-free in PrepareFunction

src/node_ffi.cc

@@ -144,12 +144,14 @@ Maybe<void*> DynamicLibrary::ResolveSymbol(Environment* env,
 Maybe<DynamicLibrary::PreparedFunction> DynamicLibrary::PrepareFunction(
     Environment* env, const std::string& name, Local<Object> signature) {
   std::shared_ptr<FFIFunction> fn;
-  auto existing = functions_.find(name);
   FunctionSignature parsed;
 
   if (!ParseFunctionSignature(env, name, signature).To(&parsed)) {
     return {};
   }
+  // Look up the cache only after parsing: the signature's getters run user
+  // code that may close the library, which clears `functions_`.
+  auto existing = functions_.find(name);
   auto [return_type, args, return_type_name, arg_type_names] =
       std::move(parsed);
 

test/ffi/test-ffi-dynamic-library.js

@@ -290,6 +290,21 @@ test('closed libraries reject subsequent operations', () => {
   assert.throws(() => lib.getSymbols(), /Library is closed/);
 });
 
+test('closing the library from a signature getter of a cached function', () => {
+  const lib = new ffi.DynamicLibrary(libraryPath);
+  lib.getFunction('add_i32', fixtureSymbols.add_i32);
+
+  assert.throws(() => {
+    lib.getFunction('add_i32', {
+      arguments: ['i32', 'i32'],
+      get return() {
+        lib.close();
+        return 'i32';
+      },
+    });
+  }, { code: 'ERR_FFI_LIBRARY_CLOSED' });
+});
+
 test('optimized fast calls reject calls after the library is closed', () => {
   const { lib, functions } = ffi.dlopen(libraryPath, {
     multiply_f64: fixtureSymbols.multiply_f64,