Add tips on debugging errors in a chroot

README.md

@@ -41,6 +41,29 @@ Troubleshooting
 - Actual logs appear in tty 4, you can switch to it by pressing Alt+F4
 - You can manually launch commands from another tty (e.g. the 2nd) but the output will appear in tty4...
 - To actually enter the real system (chroot ?) you'll need to run `in-target`
+- see also .. next section
+
+### Debugging errors in a chroot 
+
+The .deb install can crash for stupid reasons during ISO install because it's in fact ran inside a chroot.
+
+To easily reproduce and investigate the issue, it's easier to manually create a chroot (for example inside an ynh-dev LXC):
+
+```bash
+apt install cdebootstrap -y
+cd /root/
+mkdir mychroot
+cdebootstrap --arch=amd64 buster ./mychroot/ http://deb.debian.org/debian/
+mount -t proc none ./mychroot/proc
+mount -o bind /dev ./mychroot/dev
+chroot ./mychroot/ bash
+```
+
+- Running `systemctl` should display "Running in chroot, ignoring request." (compared to some error about PID1 if you didnt mount proc and dev)
+- Then run the install, for example using the install script `curl https://install.yunohost.org | bash` (you'll need to install curl first)
+- The install will crash (otherwise you wouldn't be reading this ? ;))
+- You can `nano /var/lib/dpkg/info/yunohost.postinst` and edit that script ... for example adding `-x` to the shebang (= very first line) to have the debug detail when the script will run. Possibly you may want to also tweak the init regen conf hook call (for example also adding `-x`  to the bash call there)
+- Re-run the install with `apt install yunohost`
 
 ### Errors related to mysql-password debconf