non-termination

To reproduce, create a directory with just the following file:

$ cat myocamlbuild.ml 
open Printf
open Ocamlbuild_plugin

let () = dispatch (function
  | After_rules -> (
      printf "BEFORE RULE\n%!";
      rule "m4" ~dep:"%.m4" ~prod:"%" (fun _ _ -> Nop);
      printf "AFTER RULE\n%!"
    )
  | _ -> ()
)

Now try to build anything:

$ ocamlbuild foo
Finished, 1 target (0 cached) in 00:00:00.
BEFORE RULE
AFTER RULE
(* program hangs ... never finishes here *)

The issue is related to the "%" pattern. A more specific pattern such as ~dep:"%.ml.m4" ~prod:"%.ml" avoids the problem. I guess the "%" pattern is too general and ocamlbuild goes into a loop trying to produce foo from foo.m4, which doesn't exist so it tries to produce that from foo.m4.m4, and so on.

Reproduce the hang with myocamlbuild.ml and the ocamlbuild foo command, then trace handling of the rule pattern "%.m4" to "%". Compare it with the specific "%.ml.m4" to "%.ml" pattern. Done means the broad pattern no longer causes non-termination and the reproducer exits normally.