I want a git-like pre-commit hook as specified in man githooks

Quote for reference:

   pre-commit
       This hook is invoked by git commit, and can be bypassed with the --no-verify option. It takes
       no parameters, and is invoked before obtaining the proposed commit log message and making a
       commit. Exiting with a non-zero status from this script causes the git commit command to
       abort before creating a commit.

What have I done so far:

• Recompiled fossil with tcl and th1 support as suggested here https://www.fossil-scm.org/fossil/doc/trunk/www/build.wiki

  My fossil buildscript has these now:

  configure  ...\ 
    --with-th1-hooks \
    --with-tcl=1 \
    --with-tcl-private-stubs=1
  

• Enabled tcl and th1-hooks settings in my sandbox fossil repo by checking boxes via admin web interface in Admin -> Settings
• Used https://fossil-users.fossil-scm.narkive.com/k181QuAG/hooks-in-fossil as reference and set up the th1 hook in Admin -> Settings -> th1-setup

  It looks like this:

  proc command_notify {} {
  if {$::cmd_name eq "commit"} {
  tclInvoke exec ./pre-commit &
  }
  }
  

• Wrote the pre-commit script, its content is

  #!/bin/bash
  
  echo "this is a pre-commit hook"
  

  and made it executable.

Now, when I do fossil commit, I see something like this:

New_Version: 4508082730dd86173c71e960953b9cacd7fe89539791fcbe67ff6715f7564c7f
this is a pre-commit hook

My assumption is that my bash hook gets executed after the commit actually happens, because, as specified here https://www.tcl.tk/man/tcl8.5/tutorial/Tcl26.html it is run in background and "There is no connection between that program and the Tcl script, both can run on independently."

This is not what I want, so I've tried to modify the th1-setup script by removing the '&'. This way it doesn't seem to work at all, because I don't see the echoed line from my bash script anymore when I'm committing. Unfortunately, the same tcl doc page mentioned above states "NOTE: add information on how to wait for the program to finish?" so I'm stuck.

This leaves me with the following TODO list:

• Modify the th1-setup script so my pre-commit bash script hook gets executed in blocking mode rather than in background
• Process the return code of pre-commit script inside the th1-setup script
• If the return code is not 0, then abort the fossil commit

So what are my options then? Should I just write a shell script wrapper for the fossil binary, do all the pre-commit hook logic there and then just call fossil to perform the commit if everything is fine?