modules/direnv: Implement pre-cached devshells

That means, that the devshells are only build once and centrally
updated. Ideally, they would be built upon system update, but I haven't
figured out a way to run `nix print-dev-env` in a nix build sandbox yet
(and hope, that it's not necessary anymore once, that functionality
moves to nixpkgs' `mkShell`).

1 files changed, 78 insertions, 0 deletions

diff --git a/modules/by-name/di/direnv/pre-cache.sh b/modules/by-name/di/direnv/pre-cache.sh
new file mode 100755
index 00000000..13c0cd37
--- /dev/null
+++ b/modules/by-name/di/direnv/pre-cache.sh
@@ -0,0 +1,78 @@
+#! /usr/bin/env sh
+
+_pre_cache_log() {
+    printf "pre-cache: %s\n" "$1"
+}
+_pre_cache_preflight() {
+    set -eu
+    _nix_direnv_preflight
+
+    repo="%DEVSHELLS_REPO%"
+    system="x86_64-linux"
+    cache_dir="$XDG_CACHE_HOME/pre_cache"
+    [ -d "$cache_dir" ] || mkdir --parents "$cache_dir"
+
+    repo_ref=$(git -C "$repo" rev-parse HEAD)
+    final_cache="$cache_dir/$repo_ref"
+
+    if ! [ -d "$final_cache" ]; then
+        _pre_cache_log "Cache dir invalidated"
+
+        # There is not yet a final cache directory, so the ref must have changed.
+        # Therefore we delete the whole cache directory, so we invalidate old cached shells.
+        set -u
+        rm --recursive "$cache_dir"
+
+        mkdir --parents "$final_cache"
+    fi
+}
+
+_pre_cache_load_shell() {
+    lang="$1"
+    lang_cache="$final_cache/$lang"
+
+    if ! [ -f "$lang_cache" ]; then
+        tmp_profile="$final_cache/__tmp_profile"
+        profile="$final_cache/__profile"
+
+        # It does not already exists, we need to evaluate it.
+        _nix print-dev-env --profile "$tmp_profile" "$repo#$lang" >"$lang_cache"
+
+        _nix_add_gcroot "$tmp_profile" "$profile"
+        rm "$tmp_profile" "$tmp_profile"*
+    fi
+
+    _pre_cache_log "Loaded dev-shell for $lang."
+    _nix_import_env "$lang_cache"
+}
+
+_pre_cache_build_shells() {
+    installables=""
+
+    for lang in "$@"; do
+        new_part="$repo#devShells.$system.$lang"
+
+        if [ -z "$installables" ]; then
+            installables="$new_part"
+        else
+            installables="$installables $new_part"
+        fi
+    done
+
+    # We want shell-spliting
+    # shellcheck disable=SC2086
+    _nix build --no-link $installables
+
+}
+
+pre_cache_load() {
+    _pre_cache_preflight
+
+    _pre_cache_build_shells "$@"
+
+    for lan in "$@"; do
+        _pre_cache_load_shell "$lan"
+    done
+}
+
+# vim: ft=sh