In machine_setup.sh on macOS, prompt for MPI library path

MPIPreferences.use_system_binary() does not find the MPI library when it
was installed with homebrew on macOS, so prompt for the user to give the
path to the MPI library instead.

machines/shared/add_dependencies_to_project.jl

@@ -1,14 +1,14 @@
 using Pkg, TOML
 
 if abspath(PROGRAM_FILE) == @__FILE__
-    prompt_for_hdf5 = true
+    prompt_for_lib_paths = true
     repo_dir = dirname(Pkg.project().path)
     project_dir = repo_dir
     local_preferences_filename = joinpath(repo_dir, "LocalPreferences.toml")
     local_preferences = TOML.parsefile(local_preferences_filename)
     mk_preferences = local_preferences["moment_kinetics"]
 else
-    prompt_for_hdf5 = false
+    prompt_for_lib_paths = false
 end
 
 machine = mk_preferences["machine"]
@@ -90,6 +90,35 @@ using MPIPreferences
 if "mpi_library_names" ∈ keys(machine_settings) || "mpiexec" ∈ keys(machine_settings)
     MPIPreferences.use_system_binary(library_names=machine_settings["mpi_library_names"],
                                      mpiexec=machine_settings["mpiexec"])
+elseif Sys.isapple()
+    # On macOS, MPIPreferences.use_system_binary() does not automatically find the MPI
+    # library when MPI was installed with homebrew, so prompt the user for the library
+    # path instead.
+    # ?? Could we attempt to auto-detect the MPI library before prompting the user??
+    if prompt_for_lib_paths
+        default_mpi_library_path = get(mk_preferences, "mpi_library_path", "")
+        mpi_library_path = get_input_with_path_completion(
+            "\nEnter the full path to your MPI library (e.g. something like "
+            * "'libmpi.dylib'): [$default_mpi_library_path]")
+        if mpi_library_path == ""
+            mpi_library_path = default_mpi_library_path
+        end
+
+        MPIPreferences.use_system_binary(library_names=mpi_library_path)
+
+        # Just got the value for the setting, now write it to LocalPreferences.toml
+        mk_preferences["mpi_library_path"] = mpi_library_path
+        open(local_preferences_filename, "w") do io
+            TOML.print(io, local_preferences, sorted=true)
+        end
+        # Re-read local_preferences file, so we can modify it again below, keeping the
+        # changes here
+        local_preferences = TOML.parsefile(local_preferences_filename)
+        mk_preferences = local_preferences["moment_kinetics"]
+    else
+        mpi_library_path = mk_preferences["mpi_library_path"]
+        MPIPreferences.use_system_binary(library_names=mpi_library_path)
+    end
 else
     # If settings for MPI library are not given explicitly, then auto-detection by
     # MPIPreferences.use_system_binary() should work.
@@ -122,7 +151,7 @@ elseif machine_settings["hdf5_library_setting"] == "prompt"
         hdf5_dir = local_hdf5_install_dir
         hdf5_lib = joinpath(local_hdf5_install_dir, "libhdf5.so")
         hdf5_lib_hl = joinpath(local_hdf5_install_dir, "libhdf5_hl.so")
-    elseif !prompt_for_hdf5
+    elseif !prompt_for_lib_paths
         hdf5_dir = mk_preferences["hdf5_dir"]
         if hdf5_dir != "default"
             hdf5_lib = joinpath(hdf5_dir, "libhdf5.so")