fix(pkg): do not lock unreachable packages (#10814)

unreachable packages are those that aren't reachable via the
dependencies of local packages

typically, such packages are included when solving via post deps

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

src/dune_pkg/opam_solver.ml

@@ -758,6 +758,53 @@ module Solver_result = struct
     }
 end
 
+let reject_unreachable_packages =
+  let reachable deps_of_package ~roots =
+    let seen = ref Package_name.Set.empty in
+    let rec loop = function
+      | [] -> ()
+      | pkg :: rest ->
+        if Package_name.Set.mem !seen pkg
+        then loop rest
+        else (
+          seen := Package_name.Set.add !seen pkg;
+          let deps =
+            match Package_name.Map.find deps_of_package pkg with
+            | None -> []
+            | Some deps -> deps
+          in
+          loop (List.rev_append deps rest))
+    in
+    loop roots;
+    !seen
+  in
+  fun ~local_packages ~pkgs_by_name ->
+    let roots = Package_name.Map.keys local_packages in
+    let pkgs_by_name =
+      Package_name.Map.merge pkgs_by_name local_packages ~f:(fun name lhs rhs ->
+        match lhs, rhs with
+        | None, None -> assert false
+        | Some _, Some _ ->
+          Code_error.raise
+            "package is both local and returned by solver"
+            [ "name", Package_name.to_dyn name ]
+        | Some (pkg : Lock_dir.Pkg.t), None -> Some (List.map pkg.depends ~f:snd)
+        | None, Some (pkg : Local_package.For_solver.t) ->
+          let deps =
+            List.map pkg.dependencies ~f:(fun (d : Package_dependency.t) -> d.name)
+          in
+          let depopts =
+            List.filter_map pkg.depopts ~f:(fun (d : Package_dependency.t) ->
+              Option.some_if
+                (Package_name.Map.mem local_packages d.name
+                 || Package_name.Map.mem pkgs_by_name d.name)
+                d.name)
+          in
+          Some (deps @ depopts))
+    in
+    reachable pkgs_by_name ~roots
+;;
+
 let solve_lock_dir
   solver_env
   version_preference
@@ -860,6 +907,11 @@ let solve_lock_dir
                       (Package_name.to_string name)
                       (Package_name.to_string dep_name)
                   ]));
+        let reachable = reject_unreachable_packages ~local_packages ~pkgs_by_name in
+        let pkgs_by_name =
+          Package_name.Map.filteri pkgs_by_name ~f:(fun name _ ->
+            Package_name.Set.mem reachable name)
+        in
         Lock_dir.create_latest_version
           pkgs_by_name
           ~local_packages:(Package_name.Map.values local_packages)

test/blackbox-tests/test-cases/pkg/compiler-post-dependencies.t

@@ -44,8 +44,6 @@ lists to prevent circular dependencies at package build time.
 
   $ solve ocaml-base-compiler 
   Solution for dune.lock:
-  - host-system-other.0.0.1
-  - ocaml.0.0.1
   - ocaml-base-compiler.0.0.1
 
 Ensure that packages can be resolved at build time. This checks that

test/blackbox-tests/test-cases/pkg/post-deps-solving.t

@@ -9,15 +9,13 @@ Solving for post dependencies:
   > depends: [ "bar" {post} ]
   > EOF
 
-We don't need bar, but we still include it:
+We don't need bar, so we skip it
 
   $ solve foo
   Solution for dune.lock:
-  - bar.0.0.1
   - foo.0.0.1
 
-  $ cat dune.lock/foo.pkg dune.lock/bar.pkg
-  (version 0.0.1)
+  $ cat dune.lock/foo.pkg
   (version 0.0.1)
 
 Self dependency
@@ -66,11 +64,9 @@ post "cycle":
 
   $ solve foo
   Solution for dune.lock:
-  - bar.0.0.1
   - foo.0.0.1
 
-  $ cat dune.lock/foo.pkg dune.lock/bar.pkg
-  (version 0.0.1)
+  $ cat dune.lock/foo.pkg
   (version 0.0.1)
 
 In depopts: