ffi: refresh cached string buffers on every call

Native code can mutate temporary string storage during an FFI call.
Rewrite cached buffers on every conversion so a later call with the
same JavaScript string receives a fresh copy of its UTF-8 bytes.

Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
Assisted-by: codex:gpt-5.6-sol

Author: trivikr

lib/internal/ffi/fast-api.js

@@ -134,9 +134,6 @@ function getStringConversionPointer(state, value, index) {
   const size = value.length * 3 + 1;
   const buffers = state.buffers[state.depth - 1];
   let entry = buffers[index];
-  if (entry !== undefined && entry.string === value) {
-    return entry.pointer;
-  }
   if (StringPrototypeIncludes(value, '\0')) {
     throwFFIArgError(`Argument ${index} must not contain null bytes`);
   }
@@ -146,15 +143,13 @@ function getStringConversionPointer(state, value, index) {
       __proto__: null,
       buffer,
       pointer: getRawPointer(buffer),
-      string: undefined,
     };
     buffers[index] = entry;
   }
 
   const buffer = entry.buffer;
   const written = buffer.write(value, 0, size - 1, 'utf8');
   buffer[written] = 0;
-  entry.string = value;
   return entry.pointer;
 }
 

test/ffi/fixture_library/ffi_test_library.c

@@ -108,6 +108,10 @@ FFI_EXPORT uint8_t string_equals_hello(const char* str) {
   return str && strcmp(str, "hello") == 0;
 }
 
+FFI_EXPORT char* overwrite_string(char* str, int32_t value, uint64_t length) {
+  return memset(str, value, (size_t)length);
+}
+
 FFI_EXPORT char* string_concat(const char* a, const char* b) {
   if (!a || !b) {
     // NOLINTNEXTLINE (readability/null_usage)

test/ffi/test-ffi-fast-buffer.js

@@ -96,6 +96,24 @@ test('fast FFI string buffers survive reentrant callbacks', {
   }
 });
 
+test('fast FFI refreshes cached temporary string buffers', () => {
+  const lib = new ffi.DynamicLibrary(libraryPath);
+  const overwriteString = lib.getFunction('overwrite_string', {
+    arguments: ['string', 'i32', 'u64'],
+    return: 'pointer',
+  });
+
+  try {
+    const mutated = overwriteString('hello', 0x79, 1n);
+    assert.strictEqual(ffi.toString(mutated), 'yello');
+
+    const refreshed = overwriteString('hello', 0x79, 0n);
+    assert.strictEqual(ffi.toString(refreshed), 'hello');
+  } finally {
+    lib.close();
+  }
+});
+
 test('optimized buffer signatures preserve pointer-like conversions', () => {
   const lib = new ffi.DynamicLibrary(libraryPath);
   const asBuffer = lib.getFunction('pointer_to_usize', {