Issue with `realpath` on symlink with Node 15.14.0

[julien-f]

const mock = require('mock-fs')
mock({
  'file.txt': 'file content',
  'symlink': mock.symlink({ path: 'file.txt' })
})

require('fs').realpathSync('symlink')

Result:

node:fs:1820
        const dev = BigIntPrototypeToString(stats[0], 32);
                    ^

TypeError: BigInt.prototype.toString requires that 'this' be a BigInt
    at Number.toString (<anonymous>)
    at Object.realpathSync (node:fs:1820:21)
    at Object.<anonymous> (/tmp/index.js:7:15)
    at Module._compile (node:internal/modules/cjs/loader:1092:14)
    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1121:10)
    at Module.load (node:internal/modules/cjs/loader:972:32)
    at Function.Module._load (node:internal/modules/cjs/loader:813:14)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:76:12)
    at node:internal/main/run_main_module:17:47

[3cp]

That line is in nodejs source code.

https://github.com/nodejs/node/blob/87805375645f7af36fce2e59289559f746a7131d/lib/fs.js#L1837

It seems nodejs v15 has a breaking change, the numbers in stats need to be BigInt now. Probably mock-fs needs to add some conditional code to make sure to work with both nodejs <15 and nodejs >=15.

[3cp]

The nodejs v15 doc still says all the fields can be either number or bigint. Maybe internally it only uses bigint.

https://nodejs.org/dist/latest-v15.x/docs/api/fs.html#fs_stats_dev

stats.dev#

<number> | <bigint>
The numeric identifier of the device containing the file.

[3cp]

In nodejs v14, the code was

const dev = stats.dev.toString(32);
// and
const dev = stats[0].toString(32);

So it was working against both BigInt and number. Now nodejs v15 assumes the stats field is a BigInt.

[3cp]

BigInt support was added to Nodejs v10.4.0. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt

[3cp]

It's a missing implementation in mock-fs: (from nodejs doc)

A object provides information about a file. Objects returned from fs.stat(), fs.lstat() and fs.fstat() and their synchronous counterparts are of this type. If bigint in the options passed to those methods is true, the numeric values will be bigint instead of number, and the object will contain additional nanosecond-precision properties suffixed with Ns.

Nodejs v15 expects bigint probably because it internally uses stat API with bigint:true.

That bigint option for stat() APIs was added in nodejs v10.5.0.

v10.5.0 | Accepts an additional options object to specify whether the numeric values returned should be bigint.

[julien-f]

Thank you for this step by step investigation report, it's a model for open source development :+1:

[3cp]

I will try to implement that bigint option for stat API when I got more time.

[julien-f]

No hurry on my side :slightly_smiling_face:

[3cp]

Something unexpected. After implemented the bigint option, nodejs v16 passed on Windows, but still failed on Linux/macOS. It seems nodejs runs different code path for Win and Posix.

Update: I got some missing part.