Ocamlbuild plugin: build dynamic dependencies only if the target has changed - plugins

Question

I'm trying to write an ocamlbuild plugin. The goal is to build Qt apps (without OCaml code for the time being). To handle the final linking phase, I have written the list of objs, mocs, uis and rcs in a .qtobjs file (like a .itarget), and the list of Qt libs in a .qtpackages file. The rule that applies to these files parses these lists and dynamically builds the objects, mocs and so on before linking everything.
The problem is that I would like not to dynamically build the targets that have not changed. See the rule below:
rule "Link a c++ executable that uses Qt."
~deps:["%.qtobjs"; "%.qtpackages"]
~prod:"%.cxxnative"
(fun env build ->
let dir = Pathname.dirname (env "%.qtobjs") in
let output = env "%.cxxnative" in
let objlist = pwd / (env "%.qtobjs") in
let liblist = pwd / (env "%.qtpackages") in
let packages = read_lines liblist in
let lflags = List.map (fun p -> "-l"^p) packages
# find_all "lflags" in
let objs = List.map (fun o -> dir / o) (read_lines objlist) in
(* Here, I would like to forget the targets that have not changed *)
let objs_to_build = List.filter
(fun target ->
(* something to tell if "target" has to be rebuilt *))
objs in
(* This is where I build the dependencies : *)
let success_builds = build
(List.map (fun o -> [o])
objs_to_build) in
let objects = filter_map
(function
| Outcome.Good x when get_extension x = "opp" ->
Some (pwd / "_build" / (update_extension "o" x))
| Outcome.Good _ -> None
(* Because I don't want to link the ui.h file generated by uic *)
| Outcome.Bad exn -> raise exn)
success_builds in
let tags_objs = tags_of_pathname (env "%.qtobjs") ++ "link" in
let tags_packs = tags_of_pathname (env "%.qtpackages") in
Cmd (S [A (find_one "cxx"); T tags_objs; T tags_packs; Command.atomize lflags;
A "-o"; P output; Command.atomize objects]));
I would like not to call "build" when it's not necessary.
NB: a function
newer_than: Pathname.t -> Pathname.t -> bool
could solve my problem.

Answer 1

Problem solved.
For those who are interested, the problem was in the COMPILING rule : because the ".o" rule was already used by ocamlbuild, I registered it to officially produce ".opp" files but the command actually wrote .o files. So when recompiling, ocamlbuild had no clue that the object file was already present.
Three options:
replace the default ".o" rule (I don't even know if it is possible ?),
produce ".opp" files (but it's less standard),
or alter the compiling rule to do Nop if the .o file exists:
if Pathname.exists (env "%.o") then Nop
else Cmd (S [A ("c++");
A "-c";
A "-o"; P (env "%.o"); P (env "%.cpp")]));