Fix performance regression when calling opam install --deps-only on a…

…n already installed package

master_changes.md

@@ -24,6 +24,7 @@ users)
 ## Actions
 
 ## Install
+  * Fix performance regression when calling opam install --deps-only on an already installed package [#5908 @kit-ty-kate - fix #5817]
 
 ## Remove
 

src/client/opamClient.ml

@@ -1471,17 +1471,20 @@ let install_t t ?ask ?(ignore_conflicts=false) ?(depext_only=false)
           nvs (t, deps_of_packages))
       dname_map (t, OpamPackage.Set.empty)
   in
+  let pkg_skip, pkg_new =
+    get_installed_atoms t atoms in
   let atoms, deps_atoms =
     if deps_only then
       [],
       List.map (fun (n, c) -> OpamPackage.Name.Map.find n dname_map, c) atoms
     else
       atoms, []
   in
-  let pkg_skip, pkg_new =
-    get_installed_atoms t atoms in
   let pkg_reinstall =
     if assume_built then OpamPackage.Set.of_list pkg_skip
+    else if deps_only then OpamPackage.Set.empty
+    (* NOTE: As we only install dependency packages, there are no intersections
+       between t.reinstall and pkg_skip *)
     else Lazy.force t.reinstall %% OpamPackage.Set.of_list pkg_skip
   in
   (* Add the packages to the list of package roots and display a
@@ -1544,8 +1547,7 @@ let install_t t ?ask ?(ignore_conflicts=false) ?(depext_only=false)
   OpamSolution.check_availability t available_packages atoms;
 
   if pkg_new = [] && OpamPackage.Set.is_empty pkg_reinstall &&
-     formula = OpamFormula.Empty &&
-     deps_atoms = []
+     formula = OpamFormula.Empty
   then t else
   let t, atoms =
     if assume_built then