emacs: make melpaBuild accept recipe content as a string (#334955)

pkgs/applications/editors/emacs/build-support/melpa.nix

@@ -63,11 +63,10 @@ libBuildHelper.extendMkDerivation' genericBuild (finalAttrs:
  /*
  recipe: Optional MELPA recipe.
  Default: a minimally functional recipe
+ This can be a path of a recipe file, a string of the recipe content or an empty string.
+ The default value is used if it is an empty string.
  */
-, recipe ? (writeText "${finalAttrs.pname}-recipe" ''
- (${finalAttrs.ename} :fetcher git :url ""
- ${lib.optionalString (finalAttrs.files != null) ":files ${finalAttrs.files}"})
- '')
+, recipe ? ""
 , preUnpack ? ""
 , postUnpack ? ""
 , meta ? {}
@@ -98,9 +97,21 @@ libBuildHelper.extendMkDerivation' genericBuild (finalAttrs:
 
  preUnpack = ''
  mkdir -p "$NIX_BUILD_TOP/recipes"
- if [ -n "$recipe" ]; then
- cp "$recipe" "$NIX_BUILD_TOP/recipes/$ename"
+ recipeFile="$NIX_BUILD_TOP/recipes/$ename"
+ if [ -r "$recipe" ]; then
+ ln -s "$recipe" "$recipeFile"
+ nixInfoLog "link recipe"
+ elif [ -n "$recipe" ]; then
+ printf "%s" "$recipe" > "$recipeFile"
+ nixInfoLog "write recipe"
+ else
+ cat > "$recipeFile" <<'EOF'
+(${finalAttrs.ename} :fetcher git :url "" ${lib.optionalString (finalAttrs.files != null) ":files ${finalAttrs.files}"})
+EOF
+ nixInfoLog "use default recipe"
  fi
+ nixInfoLog "recipe content:" "$(< $recipeFile)"
+ unset -v recipeFile
 
  ln -s "$packageBuild" "$NIX_BUILD_TOP/package-build"