ffi: getFunction() evaluates signature getters twice, causing native/JavaScript ABI mismatch

[trivikr]

Version

main

Platform

macOS 26.5.2

Subsystem

ffi

What steps will reproduce the bug?

import { DynamicLibrary, suffix } from 'node:ffi';

// Use Node.js's prebuilt FFI test fixture, which exports sum_8_i32().
const libraryPath =
  `../node/test/ffi/fixture_library/build/Release/ffi_test_library.${suffix}`;

const lib = new DynamicLibrary(libraryPath);
let reads = 0;

const signature = {
  return: 'i32',

  get arguments() {
    const args = ++reads === 1 ?
      Array(8).fill('i32') :
      ['i32'];

    console.log(`read ${reads}: returning ${args.length} arguments`);
    console.log(
      new Error().stack.split('\n').slice(2, 4).join('\n'),
    );

    return args;
  },
};

const fn = lib.getFunction('sum_8_i32', signature);

console.log(`JavaScript wrapper arity: ${fn.length}`);
console.log(`native sum_8_i32 result: ${fn(1)}`);

lib.close();

How often does it reproduce? Is there a required condition?

Always

What is the expected behavior? Why is that the expected behavior?

read 1: returning 8 arguments
    at DynamicLibrary.getFunction (node:ffi:105:15)
    at file:///Users/trivikram/workspace/test-repro/repro.js:27:16
JavaScript wrapper arity: 8
node:internal/ffi-shared-buffer:110
  const err = new TypeError(msg);
              ^

TypeError: Invalid argument count: expected 8, got 1
    at throwFFIArgError (node:internal/ffi-shared-buffer:110:15)
    at throwFFIArgCountError (node:internal/ffi-shared-buffer:116:3)
    at sum_8_i32 (node:internal/ffi-shared-buffer:515:7)
    at file:///Users/trivikram/workspace/test-repro/repro.js:30:41
    at ModuleJob.run (node:internal/modules/esm/module_job:569:25)
    at process.processTicksAndRejections (node:internal/process/task_queues:104:5)
    at async node:internal/modules/esm/loader:650:26
    at async asyncRunEntryPointWithESMLoader (node:internal/modules/run_main:101:5) {
  code: 'ERR_INVALID_ARG_VALUE'
}

Node should snapshot the signature once, producing an eight-argument wrapper that rejects the one-argument call.

For clean expected output, call the function with eight arguments:

console.log(
  `native sum_8_i32 result: ${fn(1, 2, 3, 4, 5, 6, 7, 8)}`,
);

which will output the following instead of error

native sum_8_i32 result: 36

What do you see instead?

read 1: returning 8 arguments
    at DynamicLibrary.getFunction (node:ffi:105:15)
    at file:///Users/trivikram/workspace/test-repro/repro.js:27:16
read 2: returning 1 arguments
    at DynamicLibrary.getFunction (node:ffi:106:41)
    at file:///Users/trivikram/workspace/test-repro/repro.js:27:16
JavaScript wrapper arity: 1
native sum_8_i32 result: 1

The signature getter is evaluated twice, giving the native call interface eight arguments but the JavaScript wrapper only one, so an unsafe native call succeeds.

Additional information

No response