flatpak-worker: Add script to automatically clear the flatpak-builder cache if it gets too large

roles/flatpak-worker/default.nix

@@ -30,8 +30,23 @@ let
     pkgs.buildbot-worker
   ]);
 
+  flatpakScripts = with pkgs; stdenv.mkDerivation {
+    name = "flatpak-scripts";
+    src = ./utils;
+
+    propagatedBuildInputs = [ bash coreutils ];
+
+    unpackPhase = "true";
+    installPhase = ''
+      mkdir $out $out/bin
+      install -m755 $src/clean_cache.sh $out/bin
+      patchShebangs $out/bin
+    '';
+  };
+
   flatpakEnvPackages = with pkgs; [
     bash
+    flatpakScripts
     flatpak
     git
   ];

roles/flatpak-worker/utils/clean_cache.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+
+DIRECTORY_SIZE=$(du -sb .flatpak-builder | cut -f1)
+MAX_SIZE=10737418240 # 10GiB
+
+if [[ $DIRECTORY_SIZE -eq "" ]]; then
+    echo "Failed to calculate cache size, bailing out"
+    exit 0
+fi
+
+if [[ $DIRECTORY_SIZE -gt $MAX_SIZE ]]; then
+    echo "Cache is too large ($DIRECTORY_SIZE bytes), clearing"
+    rm -rf .flatpak-builder
+else
+    echo "Cache is still within a reasonable size ($DIRECTORY_SIZE bytes)"
+fi