ocamldebug support

[ejgallego]

This is an effort to centralize the discussion for a plan to improve ocamldebug support in Dune (c.f. #3818)

Some comments:

• interface: one possibility would be to add dune debug $byte_file, that would take care of doing the right ocamldebug call, other is to expose variables so the call can be written easily as a rule
• ocamldebug call: here we need something similar to what dune top and dune utop do, in particular:
  • all the cma files and stubs for $byte_file must be up to date
  • the corresponding -I options have to be passed to ocamldebug so cmi and cma files are built
  • additionally, printers must be loaded; we could use the attributes à la utop, or have an extra field in the (executable ...) stanza as to declare the printers library. The printers could be automatically loaded, but it would be a good idea to actually generate a concrete file to be used with the source command

I think that if we agree on the above, doing a basic rule should be pretty quick.

[ghost]

Regarding the name, dune debug feels a bit too generic. I'd expect dune debug to be about debugging the build. Perhaps dune ocamldebug or dune ocaml debug.

[ghost]

BTW, as a programmer what I would really like is being able to write Dune_ocamldebug.break () somewhere in my code and have the execution magically pop-up a debugger halted at this program point.

[ejgallego]

BTW, as a programmer what I would really like is being able to write Dune_ocamldebug.break () somewhere in my code and have the execution magically pop-up a debugger halted at this program point.

That's very interesting, does OCaml support that already? I'm not familiar enough with the internals of the BC interpreter.

[ghost]

I don't think so. It would probably require some change to ocamldebug

[rgrinberg]

'd expect dune debug to be about debugging the build. Perhaps dune ocamldebug or dune ocaml debug.

I'd strongly prefer $ dune ocaml debug. Since we can now nest command groups, we should really look for logical ways to group commands.

[progman1]

what will happen with a local .ocamldebug?
is there an implicit dependency such that it copies to the build directory?

moreover, could the default executable of the debugger be over-rideable with an
exec from the workspace %{exe:...} ?
and once the custom debugger is no longer part of the workspace be able to
override the default name of ocamldebugger?

[Alizter]

@richardlford perhaps this is relevant to your recent work?

[richardlford]

My PR, #7750, addresses this issue.