fs.write does not work on external two-byte strings

[joyeecheung]

• Version: master
• Platform: all
• Subsystem: fs

Discovered during #18144

// Flags: --expose_externalize_string

'use strict';

const assert = require('assert');
const fs = require('fs');
const file = 'write-external.txt';
const expected = '中文';
externalizeString(expected);

fs.open(file, 'w', 0o644, function(err, fd) {
  fs.write(fd, expected, 0, 'utf8', function(err, written) {
    fs.closeSync(fd);
    const found = fs.readFileSync(file, 'utf8');
    fs.unlinkSync(file);
    assert.strictEqual(expected, found);  // AssertionError [ERR_ASSERTION]: '中文' strictEqual '-N�e'
  });
});

[targos]

/cc @nodejs/v8

[bnoordhuis]

The surface issue is that fs.write() ignores the encoding argument with externalized strings. The broader issue is that the way node handles external strings is broken and unsafe.

Broken because string_bytes.cc ignores the encoding. Unsafe because the string is collectible by the GC. Storing the pointer like async fs.write() does can result in segfaults and use-after-frees.

I'll see if I can put together a pull request.

[bnoordhuis]

#18216