Comma: use the NIX_INDEX_DATABASE env var if possible.

comma-wrapper.nix

@@ -1,15 +1,36 @@
 { runCommand
 , makeWrapper
 , comma
+, nix-index-unwrapped
 , nix-index-database
 }:
+let
+  commaOverridden = comma.override { inherit nix-index-unwrapped; };
+in
 runCommand "comma-with-db-${comma.version}"
 {
   nativeBuildInputs = [ makeWrapper ];
   meta.mainProgram = "comma";
 } ''
+  # Function that does the same as nixpkgs.lib.versionAtLeast,
+  # return true if the first string denotes a version equal to or newer than
+  # the second one.
+  function versionAtLeast() {
+    printf '%s\n' "$2" "$1" | sort --version-sort --check=quiet
+  }
+
   mkdir -p $out/share/cache/nix-index
   ln -s ${nix-index-database} $out/share/cache/nix-index/files
-  makeWrapper ${comma}/bin/, $out/bin/, \
-    --set XDG_CACHE_HOME $out/share/cache
+
+  version="$(${nix-index-unwrapped}/bin/nix-index --version | cut -d' ' -f2)"
+
+  for cmd in "," "comma"; do
+    if versionAtLeast "''${version}" "0.1.6"; then
+      makeWrapper ${commaOverridden}/bin/''${cmd} $out/bin/''${cmd} \
+        --set NIX_INDEX_DATABASE $out/share/cache/nix-index
+    else
+      makeWrapper ${commaOverridden}/bin/''${cmd} $out/bin/''${cmd} \
+        --set XDG_CACHE_HOME $out/share/cache
+    fi
+  done
 ''