SSH - npm seems to break ssh non-pty shells - node.js

Question

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', 'user#host']);
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?
Update may 29th
Actually, I was able to investigate this issue further. I started editing npm-cli.js and commented out everything, then step-by-step uncommented the lines, to see what is going to trigger the above behaviour.
At first, it seemed like the set-blocking module included via npmlog was causing the issue, but after also commenting out the actual code of set-blocking in index.js:4 (which is only stream._handle.setBlocking(blocking)), the bad behaviour still occurs, which confused the fuck out of me. Experimenting further revealed that actually the single piece of process.stdout is causing the whole issue.
To verify, I did the following:
Comment-out lines 22-94 in npm-cli.js. Executing npm install now will essentially do nothing. Also, running my example program above will not have the error.
Add the following code to npm-cli.js:19: if (process.stdout) ;. This also essentially does nothing, but it leads to the error occur again if executing the above test program.
Do the opposite test and change if (process.stdout) ; to if (process) ; - the test program will now run again without error.
So basically the above Node.js testprogram which demonstrates the error can be changed so that instead of npm install a simple oneline script is invoked, which results in the same error:
const spawn = require('child_process').spawn;
const bat = spawn('ssh', ['user#host']);
setTimeout(() => {
bat.stdin.write('node test.js\n'); // this breaks it
setTimeout(() => bat.stdin.write('pwd -P\n'), 2000);
}, 2000);
With test.js containing:
if (process.stdout) ; // only this line
I dont know how the process global object is constructed, or what is going on here, but something in Node.js' process/process.stdout object is somehow breaking SSH connections. I therefore think this is not an actual issue of npm, only indirectly.
Could anyone help to clarify?