v8.2.0: TypeError: stat.isSocket is not a function

I was investigating balena-cli issue balena-io/balena-cli/issues/1306 and my preliminary findings suggest that:

balena-cli v10.16.0 works OK with balena-preload v8.1.4

balena-cli v10.17.1 and later (also v11.2.1) fail with balena-preload v8.2.0, with the error:

./balena-cli-11.2.1/balena preload test.img -a test-rpi -c current
Building Docker preloader image. [                        ] 0%
TypeError: stat.isSocket is not a function
at onstat (/snapshot/balena-cli/node_modules/balena-preload/node_modules/tar-fs/index.js:103:14)
at /snapshot/balena-cli/node_modules/balena-preload/node_modules/tar-fs/index.js:32:39
at zalgoSafe (pkg/prelude/bootstrap.js:261:10)
at pkg/prelude/bootstrap.js:901:9
at pkg/prelude/bootstrap.js:337:5
at pkg/prelude/bootstrap.js:315:14
at FSReqWrap.wrapper [as oncomplete] (fs.js:603:17)

Interestingly, I've only reproduced the failure on Linux (Docker image debian:9.9), not macOS, so it looks like a system-dependent issue.

The offending pull request may be: https://github.com/balena-io/balena-preload/pull/206/files

I patched balena-cli/node_modules/balena-preload/node_modules/tar-fs/index.js for debugging, and added these console.log calls:

console.log(`[preload/tar-fs] onstat() filename="${filename}" stat=${require('util').inspect(stat)}`);
console.log(`
  isFile()="${stat.isFile()}"
  isDirectory()="${stat.isDirectory()}"
  isSymbolicLink()="${stat.isSymbolicLink()}"
  isFIFO()="${stat.isFIFO()}"`
);
if (stat.isSocket()) return onnextentry() // tar does not support sockets...

The result was:

[preload/tar-fs] onstat() filename="Dockerfile" stat={ mode: 33188,
  size: 361,
  blksize: 4096,
  blocks: 0,
  dev: 0,
  gid: 20,
  ino: 0,
  nlink: 0,
  rdev: 0,
  uid: 500,
  atime: 2019-06-16T13:37:18.530Z,
  mtime: 2019-06-16T13:36:44.320Z,
  ctime: 2019-06-16T13:36:44.320Z,
  birthtime: 2019-06-16T13:36:44.320Z,
  atimeMs: 1560692238530,
  mtimeMs: 1560692204320,
  ctimeMs: 1560692204320,
  birthtimeMs: 1560692204320,
  isFile: [Function],
  isDirectory: [Function],
  isSymbolicLink: [Function],
  isFIFO: [Function] }

      isFile()="true"
      isDirectory()="false"
      isSymbolicLink()="false"
      isFIFO()="false"
TypeError: stat.isSocket is not a function
    at onstat (/snapshot/balena-cli/node_modules/balena-preload/node_modules/tar-fs/index.js:114:14)
    at /snapshot/balena-cli/node_modules/balena-preload/node_modules/tar-fs/index.js:32:39
    at zalgoSafe (pkg/prelude/bootstrap.js:261:10)
    at pkg/prelude/bootstrap.js:901:9
    at pkg/prelude/bootstrap.js:337:5
    at pkg/prelude/bootstrap.js:315:14
    at FSReqWrap.wrapper [as oncomplete] (fs.js:658:17)

I have further checked these tar-fs lines:

var xfs = opts.fs || fs
var statNext = statAll(xfs, opts.dereference ? xfs.stat : xfs.lstat, cwd, ignore, opts.entries, opts.sort)

And found that opts.fs and opts.dereference are undefined, meaning it should be using the standard fs.lstat() function. I tried setting opts.dereference=true, so it used fs.stat() instead of fs.lstat(), and it made no difference (still results in the same TypeError).

So a seemingly standard fs.lstat('Dockerfile') call returns an object that does not have the isSocket() function, apparently in violation of the Node.js documented API. But we can't say it's a Node.js bug, because:

It works OK with a balena-cli npm installation on Linux and macOS.
~It works OK~ with a balena-cli standalone (pkg) installation on macOS. Edit/correction: it fails on macOS too.
It does not work with a balena-cli standalone (pkg) installation on Linux.

~So it's the combination of pkg and Linux that causes it to fail.~ So it's down to pkg (the tool that generates the standalone package). I have also investigated the version of Node.js used by pkg on Linux. It seems to use pre-selected minor/patch releases for a given major Node release, namely v10.4.1 for Node 10 and v8.11.3 for Node 8. pkg caches the fetched Node in the homedir:

/root/.pkg-cache/v2.5/fetched-v10.4.1-linux-x64
/root/.pkg-cache/v2.5/fetched-v8.11.3-linux-x64

I am not sure how / whether pkg modifies those fetched versions of Node, but I have tested with both of those (Node 8 and Node 10) and both equally result in that same TypeError.

Although I am none the wiser on the root cause of the problem, I have found that the following very simple tar-fs patch avoids the TypeError and allows balena preload to work:

    // if (stat.isSocket()) return onnextentry()
    if (stat.isSocket && stat.isSocket()) return onnextentry()

Given that neither tar-fs nor balena-preload use pkg but balena-cli does, a patch fix would perhaps belong to balena-cli rather than balena-preload or upstream. I am planning to create a balena-cli PR with that patch, so we can move on.

balena-io-modules / balena-preload

v8.2.0: TypeError: stat.isSocket is not a function #208