Behavior is not consistent across macOS/linux

[samuela]

I have a bit of code:

class SudoSpawnError extends Error {
  public err: any;
  public stdout?: string;
  public stderr?: string;

  constructor(err: any, stdout?: string, stderr?: string) {
    super(
      `Error occurred in spawn.\n` +
        `  err: ${err}\n` +
        `  stdout: ${stdout}\n` +
        `  stderr: ${stderr}\n`
    );

    this.err = err;
    this.stdout = stdout;
    this.stderr = stderr;
  }
}

async function sudoExec(commands: string[]) {
  return new Promise<{ stdout: string; stderr: string }>((resolve, reject) => {
    sudo.exec(
      `sh ./my-script.sh`,
      some_options,

      // There's no @types/sudo-prompt typings yet, so these are best-effort.
      (err: any, stdout?: string, stderr?: string) => {
        if (
          err === undefined &&
          stdout !== undefined &&
          stderr !== undefined &&
          stdout.includes("success!")
        ) {
          resolve({ stdout, stderr });
        } else {
          reject(new SudoSpawnError(err, stdout, stderr));
        }
      }
    );
  });
}

that tries to run a script that should eventually print "success!".

The issue is that the error handling is quite different between linux and macOS. If the script returns with a non-zero exit code, the linux version includes a useful (not undefined) stderr:

{ Error: Error occurred in spawn.
  err: Error: User did not grant permission.
  stdout: 
  stderr: umount: /home/darthvader/nu/skainswo: target is busy.


    at sudo.exec (/home/darthvader/nu/skainswo/nuvemfs/packages/cli/src/util.ts:108:18)
    at /opt/nu-links/cli-node_modules/node_modules/sudo-prompt/index.js:114:11
    at ChildProcess.exithandler (child_process.js:282:5)
    at emitTwo (events.js:126:13)
    at ChildProcess.emit (events.js:214:7)
    at maybeClose (internal/child_process.js:925:16)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:209:5)
  err: Error: User did not grant permission.
    at /opt/nu-links/cli-node_modules/node_modules/sudo-prompt/index.js:111:23
    at ChildProcess.exithandler (child_process.js:282:5)
    at emitTwo (events.js:126:13)
    at ChildProcess.emit (events.js:214:7)
    at maybeClose (internal/child_process.js:925:16)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:209:5),
  stdout: '',
  stderr: 'umount: /home/darthvader/nu/skainswo: target is busy.\n' }

but on macOS:

{ Error: Error occurred in spawn.
  err: Error: Command failed: bash /var/folders/dk/6wr_v2z91hdfz1y2q405wf740000gn/T/tmp-53671eyCwS3TKWlXD.tmp
umount(/Users/skainswo/nu/skainswo): Resource busy -- try 'diskutil unmount'

  stdout: undefined
  stderr: undefined

    at sudo.exec (/Users/skainswo/nu/skainswo/nuvemfs/packages/cli/src/util.ts:110:18)
    at /opt/nu-links/cli-node_modules/node_modules/sudo-prompt/index.js:163:31
    at ChildProcess.exithandler (child_process.js:288:7)
    at ChildProcess.emit (events.js:197:13)
    at maybeClose (internal/child_process.js:978:16)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:265:5)
  err:
   { Error: Command failed: bash /var/folders/dk/6wr_v2z91hdfz1y2q405wf740000gn/T/tmp-53671eyCwS3TKWlXD.tmp
   umount(/Users/skainswo/nu/skainswo): Resource busy -- try 'diskutil unmount'
   
       at /opt/nu-links/cli-node_modules/node_modules/sudo-prompt/index.js:306:27
       at FSReqCallback.readFileAfterClose [as oncomplete] (internal/fs/read_file_context.js:54:3) code: 1 },
  stdout: undefined,
  stderr: undefined }

Why is stderr given as undefined? It seems like the API should be consistent across platforms.