PM2 is not restarting the simplest crashing app possible

What's going wrong?

PM2 by default should restart my app from what I know. However it doesn't restart the app if error is thrown in top level code, like:

file.js

throw new Error("boom");

How could we reproduce this issue?

Create file as above.
Run pm2 start file.js
Use pm2 list to see that app is not restarting and it's "online"

Supporting information

I first discovered this on my Raspberry, i thought it may be some issue with the OS, Node version or something, but I tested it on my local PC and it's the same. Even old version (4.4.1) is behaving the same, so this must be a long living bug I guess.

I tried changing the code to:

setTimeout(() => {
    throw new Error("g");
}, 100);

and then it works as expected

$ pm2 report

// ON RASPBERRY:
--- PM2 report ----------------------------------------------------------------
Date                 : Mon Aug 30 2021 19:44:23 GMT+0200 (czas środkowoeuropejski letni)
===============================================================================
--- Daemon -------------------------------------------------
pm2d version         : 5.1.1
node version         : 15.9.0
node path            : /home/dzek/.yarn/bin/pm2
argv                 : /home/dzek/.local/node-v15.9.0-linux-armv6l/bin/node,/home/dzek/.config/yarn/global/node_modules/pm2/lib/Daemon.js
argv0                : node
user                 : dzek
uid                  : 1001
gid                  : 1001
uptime               : 27min
===============================================================================
--- CLI ----------------------------------------------------
local pm2            : 5.1.1
node version         : 15.9.0
node path            : /home/dzek/.yarn/bin/pm2
argv                 : /home/dzek/.local/node-v15.9.0-linux-armv6l/bin/node,/home/dzek/.yarn/bin/pm2,report
argv0                : node
user                 : dzek
uid                  : 1001
gid                  : 1001
===============================================================================
--- System info --------------------------------------------
arch                 : arm
platform             : linux
type                 : Linux
cpus                 : ARMv6-compatible processor rev 7 (v6l)
cpus nb              : 1
freemem              : 120193024
totalmem             : 450981888
home                 : /home/dzek
===============================================================================
--- PM2 list -----------------------------------------------
┌────┬────────────────────┬──────────┬──────┬───────────┬──────────┬──────────┐
│ id │ name               │ mode     │ ↺    │ status    │ cpu      │ memory   │
├────┼────────────────────┼──────────┼──────┼───────────┼──────────┼──────────┤
│ 0  │ index              │ fork     │ 0    │ online    │ 2.9%     │ 40.5mb   │
└────┴────────────────────┴──────────┴──────┴───────────┴──────────┴──────────┘
===============================================================================
--- Daemon logs --------------------------------------------
/home/dzek/.pm2/pm2.log last 20 lines:
PM2        | 2021-08-30T19:16:33: PM2 log: ===============================================================================
PM2        | 2021-08-30T19:16:33: PM2 log: --- New PM2 Daemon started ----------------------------------------------------
PM2        | 2021-08-30T19:16:33: PM2 log: Time                 : Mon Aug 30 2021 19:16:33 GMT+0200 (czas środkowoeuropejski letni)
PM2        | 2021-08-30T19:16:33: PM2 log: PM2 version          : 5.1.1
PM2        | 2021-08-30T19:16:33: PM2 log: Node.js version      : 15.9.0
PM2        | 2021-08-30T19:16:33: PM2 log: Current arch         : arm
PM2        | 2021-08-30T19:16:33: PM2 log: PM2 home             : /home/dzek/.pm2
PM2        | 2021-08-30T19:16:33: PM2 log: PM2 PID file         : /home/dzek/.pm2/pm2.pid
PM2        | 2021-08-30T19:16:33: PM2 log: RPC socket file      : /home/dzek/.pm2/rpc.sock
PM2        | 2021-08-30T19:16:33: PM2 log: BUS socket file      : /home/dzek/.pm2/pub.sock
PM2        | 2021-08-30T19:16:33: PM2 log: Application log path : /home/dzek/.pm2/logs
PM2        | 2021-08-30T19:16:33: PM2 log: Worker Interval      : 30000
PM2        | 2021-08-30T19:16:33: PM2 log: Process dump file    : /home/dzek/.pm2/dump.pm2
PM2        | 2021-08-30T19:16:33: PM2 log: Concurrent actions   : 2
PM2        | 2021-08-30T19:16:33: PM2 log: SIGTERM timeout      : 1600
PM2        | 2021-08-30T19:16:33: PM2 log: ===============================================================================
PM2        | 2021-08-30T19:16:34: PM2 log: App [index:0] starting in -fork mode-
PM2        | 2021-08-30T19:16:34: PM2 log: App [index:0] online

$ pm2 report
// ON PC, with the "test fix" applied before generating report

--- PM2 report ----------------------------------------------------------------
Date                 : Mon Aug 30 2021 19:43:31 GMT+0200 (Central European Summer Time)
===============================================================================
--- Daemon -------------------------------------------------
pm2d version         : 4.4.1
node version         : 15.2.0
node path            : /home/dzek/.yarn/bin/pm2
argv                 : /usr/bin/node,/home/dzek/.config/yarn/global/node_modules/pm2/lib/Daemon.js
argv0                : node
user                 : dzek
uid                  : 1000
gid                  : 1000
uptime               : 4min
===============================================================================
--- CLI ----------------------------------------------------
local pm2            : 4.4.1
node version         : 15.2.0
node path            : /home/dzek/.yarn/bin/pm2
argv                 : /usr/bin/node,/home/dzek/.yarn/bin/pm2,report
argv0                : node
user                 : dzek
uid                  : 1000
gid                  : 1000
===============================================================================
--- System info --------------------------------------------
arch                 : x64
platform             : linux
type                 : Linux
cpus                 : Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz
cpus nb              : 8
freemem              : 1936064512
totalmem             : 12561264640
home                 : /home/dzek
===============================================================================
--- PM2 list -----------------------------------------------
┌─────┬──────────┬─────────────┬─────────┬─────────┬──────────┬────────┬──────┬───────────┬──────────┬──────────┬──────────┬──────────┐
│ id  │ name     │ namespace   │ version │ mode    │ pid      │ uptime │ ↺    │ status    │ cpu      │ mem      │ user     │ watching │
├─────┼──────────┼─────────────┼─────────┼─────────┼──────────┼────────┼──────┼───────────┼──────────┼──────────┼──────────┼──────────┤
│ 0   │ index    │ default     │ 0.0.2   │ fork    │ 14599    │ 4m     │ 0    │ online    │ 0.2%     │ 50.7mb   │ dzek     │ disabled │
│ 1   │ test     │ default     │ 0.0.2   │ fork    │ 0        │ 0      │ 16   │ errored   │ 0%       │ 0b       │ dzek     │ disabled │
└─────┴──────────┴─────────────┴─────────┴─────────┴──────────┴────────┴──────┴───────────┴──────────┴──────────┴──────────┴──────────┘
===============================================================================
--- Daemon logs --------------------------------------------
/home/dzek/.pm2/pm2.log last 20 lines:
PM2        | 2021-08-30T19:42:37: PM2 log: App [test:1] exited with code [1] via signal [SIGINT]
PM2        | 2021-08-30T19:42:37: PM2 log: App [test:1] starting in -fork mode-
PM2        | 2021-08-30T19:42:37: PM2 log: App [test:1] online
PM2        | 2021-08-30T19:42:37: PM2 log: App [test:1] exited with code [1] via signal [SIGINT]
PM2        | 2021-08-30T19:42:37: PM2 log: App [test:1] starting in -fork mode-
PM2        | 2021-08-30T19:42:37: PM2 log: App [test:1] online
PM2        | 2021-08-30T19:42:37: PM2 log: App [test:1] exited with code [1] via signal [SIGINT]
PM2        | 2021-08-30T19:42:37: PM2 log: App [test:1] starting in -fork mode-
PM2        | 2021-08-30T19:42:37: PM2 log: App [test:1] online
PM2        | 2021-08-30T19:42:38: PM2 log: App [test:1] exited with code [1] via signal [SIGINT]
PM2        | 2021-08-30T19:42:38: PM2 log: App [test:1] starting in -fork mode-
PM2        | 2021-08-30T19:42:38: PM2 log: App [test:1] online
PM2        | 2021-08-30T19:42:38: PM2 log: App [test:1] exited with code [1] via signal [SIGINT]
PM2        | 2021-08-30T19:42:38: PM2 log: App [test:1] starting in -fork mode-
PM2        | 2021-08-30T19:42:38: PM2 log: App [test:1] online
PM2        | 2021-08-30T19:42:38: PM2 log: App [test:1] exited with code [1] via signal [SIGINT]
PM2        | 2021-08-30T19:42:38: PM2 log: App [test:1] starting in -fork mode-
PM2        | 2021-08-30T19:42:38: PM2 log: App [test:1] online
PM2        | 2021-08-30T19:42:39: PM2 log: App [test:1] exited with code [1] via signal [SIGINT]
PM2        | 2021-08-30T19:42:39: PM2 log: Script /Projects/o-0/services/doorbell/app/dist/test.js had too many unstable restarts (16). Stopped. "errored"

Start by reproducing the issue with the shown file.js, then run pm2 start file.js and pm2 list. Compare the top-level throw with the setTimeout case and trace the process lifecycle handling. Done means a top-level crash is detected and restarted consistently, with the resulting status and restart count behaving as expected.