Add a --download-only flag to "opam install"

When this flag is enabled, sources are fetched but no other actions
are executed. This is useful to fill the download cache for a future
request in advance, for instance.

src/client/opamClient.ml

@@ -981,7 +981,8 @@ let filter_unpinned_locally t atoms f =
          None))
     atoms
 
-let install_t t ?ask atoms add_to_roots ~deps_only ~assume_built =
+let install_t t ?ask atoms add_to_roots ~deps_only ~download_only
+    ~assume_built =
   log "INSTALL %a" (slog OpamFormula.string_of_atoms) atoms;
   let names = OpamPackage.Name.Set.of_list (List.rev_map fst atoms) in
 
@@ -1116,21 +1117,21 @@ let install_t t ?ask atoms add_to_roots ~deps_only ~assume_built =
       in
       let t, res =
         OpamSolution.apply ?ask t ~requested:names ?add_roots
-          ~assume_built solution in
+          ~download_only ~assume_built solution in
       t, Success res
   in
   OpamSolution.check_solution t solution;
   t
 
 let install t ?autoupdate ?add_to_roots
-    ?(deps_only=false) ?(assume_built=false) names =
+    ?(deps_only=false) ?(download_only=false) ?(assume_built=false) names =
   let atoms = OpamSolution.sanitize_atom_list ~permissive:true t names in
   let autoupdate_atoms = match autoupdate with
     | None -> atoms
     | Some a -> OpamSolution.sanitize_atom_list ~permissive:true t a
   in
   let t = update_dev_packages_t autoupdate_atoms t in
-  install_t t atoms add_to_roots ~deps_only ~assume_built
+  install_t t atoms add_to_roots ~deps_only ~download_only ~assume_built
 
 let check_installed t atoms =
   let available = (Lazy.force t.available_packages) in

src/client/opamClient.mli

@@ -50,14 +50,14 @@ val reinit:
 val install:
   rw switch_state ->
   ?autoupdate:atom list -> ?add_to_roots:bool -> ?deps_only:bool ->
-  ?assume_built:bool -> atom list -> rw switch_state
+  ?download_only:bool -> ?assume_built:bool -> atom list -> rw switch_state
 
 (** Low-level version of [reinstall], bypassing the package name sanitization
     and dev package update, and offering more control *)
 val install_t:
   rw switch_state -> ?ask:bool ->
-  atom list -> bool option -> deps_only:bool -> assume_built:bool ->
-  rw switch_state
+  atom list -> bool option -> deps_only:bool -> download_only:bool ->
+  assume_built:bool -> rw switch_state
 
 (** Check that the given list of packages [atoms] have their dependencies
     satisfied, without calling the solver. *)

src/client/opamCommands.ml

@@ -1245,6 +1245,10 @@ let install =
     mk_flag  ["deps-only"]
       "Install all its dependencies, but don't actually install the package."
   in
+  let download_only =
+    mk_flag ["download-only"]
+      "Fetch the sources of the packages, but don't build or install anything."
+  in
   let restore =
     mk_flag ["restore"]
       "Attempt to restore packages that were marked for installation but have \
@@ -1267,8 +1271,8 @@ let install =
        $(i,PACKAGES) with option $(b,deps-only) enabled."
   in
   let install
-      global_options build_options add_to_roots deps_only restore destdir
-      assume_built check atoms_or_locals =
+      global_options build_options add_to_roots deps_only download_only restore
+      destdir assume_built check atoms_or_locals =
     apply_global_options global_options;
     apply_build_options build_options;
     if atoms_or_locals = [] && not restore then
@@ -1319,7 +1323,8 @@ let install =
           OpamStd.Sys.exit_because `False));
     let st =
       OpamClient.install st atoms
-        ~autoupdate:pure_atoms ?add_to_roots ~deps_only ~assume_built
+        ~autoupdate:pure_atoms ?add_to_roots ~deps_only ~download_only
+        ~assume_built
     in
     match destdir with
     | None -> `Ok ()
@@ -1330,8 +1335,8 @@ let install =
   in
   Term.ret
     Term.(const install $global_options $build_options
-          $add_to_roots $deps_only $restore $destdir $assume_built $check
-          $atom_or_local_list),
+          $add_to_roots $deps_only $download_only $restore $destdir
+          $assume_built $check $atom_or_local_list),
   term_info "install" ~doc ~man
 
 (* REMOVE *)

src/client/opamSolution.ml

@@ -262,7 +262,8 @@ end
 
 (* Process the atomic actions in a graph in parallel, respecting graph order,
    and report to user. Takes a graph of atomic actions *)
-let parallel_apply t ~requested ?add_roots ~assume_built action_graph =
+let parallel_apply t ~requested ?add_roots ~assume_built ~download_only
+    action_graph =
   log "parallel_apply";
 
   let remove_action_packages =
@@ -336,14 +337,29 @@ let parallel_apply t ~requested ?add_roots ~assume_built action_graph =
 
   (* 1/ process the package actions (fetch, build, installations and removals) *)
 
-  let action_graph = (* Add build actions *)
+  let action_graph = (* Add build and fetch actions *)
     let noop_remove nv =
       OpamAction.noop_remove_package t nv in
     PackageActionGraph.explicit
       ~noop_remove
       ~sources_needed:(fun p -> OpamPackage.Set.mem p sources_needed)
       action_graph
   in
+  let action_graph =
+    if download_only then
+      (* remove actions other than fetches *)
+      let g = PackageActionGraph.copy action_graph in
+      PackageActionGraph.iter_vertex (fun v ->
+          match v with
+          | `Fetch _ -> ()
+          | `Install _ | `Reinstall _ | `Change _
+          | `Remove _ | `Build _ ->
+            PackageActionGraph.remove_vertex g v
+        ) action_graph;
+      g
+    else action_graph
+  in
+
   (match OpamSolverConfig.(!r.cudf_file) with
    | None -> ()
    | Some f ->
@@ -826,7 +842,8 @@ let run_hook_job t name ?(local=[]) w =
     Done true
 
 (* Apply a solution *)
-let apply ?ask t ~requested ?add_roots ?(assume_built=false) solution =
+let apply ?ask t ~requested ?add_roots ?(assume_built=false)
+    ?(download_only=false) solution =
   log "apply";
   if OpamSolver.solution_is_empty solution then
     (* The current state satisfies the request contraints *)
@@ -914,7 +931,8 @@ let apply ?ask t ~requested ?add_roots ?(assume_built=false) solution =
         OpamStd.Sys.exit_because `Configuration_error;
       let t0 = t in
       let t, r =
-        parallel_apply t ~requested ?add_roots ~assume_built action_graph
+        parallel_apply t ~requested ?add_roots ~assume_built ~download_only
+          action_graph
       in
       let success = match r with | OK _ -> true | _ -> false in
       let post_session =
@@ -955,7 +973,7 @@ let resolve t action ~orphans ?reinstall ~requested request =
   r
 
 let resolve_and_apply ?ask t action ~orphans ?reinstall ~requested ?add_roots
-    ?(assume_built=false) request =
+    ?(assume_built=false) ?download_only request =
   match resolve t action ~orphans ?reinstall ~requested request with
   | Conflicts cs ->
     log "conflict!";
@@ -964,5 +982,7 @@ let resolve_and_apply ?ask t action ~orphans ?reinstall ~requested ?add_roots
          (OpamSwitchState.unavailable_reason t) cs);
     t, Conflicts cs
   | Success solution ->
-    let t, res = apply ?ask t ~requested ?add_roots ~assume_built solution in
+    let t, res =
+      apply ?ask t ~requested ?add_roots ~assume_built ?download_only solution
+    in
     t, Success res

src/client/opamSolution.mli

@@ -34,6 +34,7 @@ val apply:
   requested:OpamPackage.Name.Set.t ->
   ?add_roots:OpamPackage.Name.Set.t ->
   ?assume_built:bool ->
+  ?download_only:bool ->
   OpamSolver.solution ->
   rw switch_state * solution_result
 
@@ -50,6 +51,7 @@ val resolve_and_apply:
   requested:OpamPackage.Name.Set.t ->
   ?add_roots:OpamPackage.Name.Set.t ->
   ?assume_built:bool ->
+  ?download_only:bool ->
   atom request ->
   rw switch_state * (solution_result, OpamCudf.conflict) result