Use OCaml implementation of exec -a

Signed-off-by: Marek Kubica <marek@tarides.com>

Author: Leonidas-from-XIV

test/blackbox-tests/test-cases/pkg/different-dune-in-path.t

@@ -25,12 +25,26 @@ Make a project that depends on the test packages:
   > (lang dune 3.13)
   > (package
   >  (name x)
-  >  (allow_empty)
   >  (depends foo bar))
   > EOF
 
   $ cat > dune <<EOF
-  > (data_only_dirs bin)
+  > (executable
+  >  (name exec_a)
+  >  (public_name exec-a)
+  >  (libraries unix))
+  > EOF
+
+Small helper binary to implement `exec -a` in a portable way
+
+  $ cat > exec_a.ml <<EOF
+  > let () =
+  >   let desired_argv0 = Sys.argv.(1) in
+  >   let prog = Sys.argv.(2) in
+  >   let args = Array.length Sys.argv in
+  >   let new_args = Array.init (args - 2) (fun i -> Sys.argv.(i+2)) in
+  >   new_args.(0) <- desired_argv0;
+  >   Unix.execvp prog new_args
   > EOF
 
 Make lockfiles for the packages.
@@ -65,13 +79,25 @@ Make lockfiles for the packages.
 Test that the project can be built normally.
   $ build_pkg foo
 
+
 Make a fake dune exe:
-  $ mkdir bin
-  $ cat > bin/dune <<EOF
+
+  $ mkdir .bin
+  $ cat > .bin/dune <<EOF
   > #!/bin/sh
   > echo "Fake dune! (args: \$@)"
   > EOF
-  $ chmod a+x bin/dune
+  $ chmod a+x .bin/dune
+
+  $ dune build @install
+  $ mkdir .temp
+  $ dune install --prefix $PWD/.temp
+  $ cp .temp/bin/exec-a $PWD/.bin/
+
+Show that the binary works like `exec -a` would work
+
+  $ .bin/exec-a "desired-argv0" sh -c 'echo $0'
+  desired-argv0
 
   $ dune clean
 
@@ -88,16 +114,16 @@ Remember the digests, to not to have to call nested Dunes:
 
 Call Dune with an absolute PATH as argv[0]:
 
-  $ PATH=$PWD/bin:$PATH $DUNE build $pkg_root/$foo_digest/target/
-  $ PATH=$PWD/bin:$PATH $DUNE build $pkg_root/$bar_digest/target/
+  $ PATH=$PWD/.bin:$PATH $DUNE build $pkg_root/$foo_digest/target/
+  $ PATH=$PWD/.bin:$PATH $DUNE build $pkg_root/$bar_digest/target/
   Fake dune! (args: build -p bar @install)
 
 Call Dune with argv[0] set to a relative PATH. Make sure "dune" in PATH refers
 to the fake dune:
 
-  $ (PATH=$PWD/bin:$PATH exec -a "dune" sh -c "which dune")
-  $TESTCASE_ROOT/bin/dune
+  $ (PATH=$PWD/.bin:$PATH $PWD/.bin/exec-a "dune" sh -c "which dune")
+  $TESTCASE_ROOT/.bin/dune
 
 Make sure that fake dune is not picked up when dune is called with argv[0] = "dune":
 
-  $ (PATH=$PWD/bin:$PATH exec -a "dune" $DUNE build $pkg_root/$foo_digest/target/)
+  $ (PATH=$PWD/bin:$PATH $PWD/.bin/exec-a "dune" $DUNE build $pkg_root/$foo_digest/target/)