SSH - npm seems to break ssh non-pty shells

I am developing a library/tool which lets the user execute arbitrary commands via SSH. Overall, this lib/tool will serve as software deployment tool, which needs access to remote machines to execute several commands (cd, mkdir, git ..., npm install, scp, etc).

While it basically works to execute remote commands via SSH, it seems that every time the command npm install is executed, the SSH connection gets terminated. I cannot tell what is causing this, but this very simple Node.js script can demonstrate it:

const spawn = require('child_process').spawn;
const bat = spawn('ssh', ['-T', '-oRequestTTY=no', '-oBatchMode=yes', 'fial@dev-obelisk-1']);

bat.stdout.on('data', (data) => console.log('STDOUT: ' + data.toString()));
bat.stderr.on('data', (data) => console.log('STDERR: ' + data.toString()));
bat.on('exit', (code) => console.log(`Child exited with code ${code}`));

setTimeout(() => {
    bat.stdin.write('pwd -P\n');
    bat.stdin.write('cd someDir\n');
    bat.stdin.write('npm install\n');

    setTimeout(() => bat.stdin.write('pwd -P\n'), 2000);
}, 2000);

This will break/terminate the forked SSH process after npm install, so the delayed pwd -P will also fail. Removing the npm install command will make the SSH process stay intact until the app is terminated by the user.

I have actually faced this problem when I was working with the c library libssh, which had the very same issue, although I failed to notice that the npm install command seems to actually trigger the problem. See this related post: Channel in libssh gets closed for no obvious reason

What I found out is:
1. I am using a non-pty SSH shell
2. The libsshpacket-level debug output shows a packet 98 sent by the server just before the connection is closed
3. According to the RFC, packet 98 is SSH_MSG_CHANNEL_REQUEST, which can also be used to request a pty

So my asumption is, that I am working on a non-pty shell over SSH, and that something in the npm programm directly or indirectly leads to a server-side request for a pty, which cannot be handled by the non-pty shell I am working on, and thus, the SSH connection is closed on protocol-level.

Now my question is, what could be causing this, and is there a way to get rid of this problem?

via user826955