lib.fileset.toSource: Evaluate fileset even for empty directories

lib/fileset/default.nix

@@ -92,6 +92,7 @@ The only way to change which files get added to the store is by changing the `fi
       fileset = _coerce "lib.fileset.toSource: `fileset`" filesetPath;
       rootFilesystemRoot = (splitRoot root).root;
       filesetFilesystemRoot = (splitRoot fileset._internalBase).root;
+      filter = _toSourceFilter fileset;
     in
     if ! isPath root then
       if isStringLike root then
@@ -123,9 +124,10 @@ The only way to change which files get added to the store is by changing the `fi
             - Set `root` to ${toString fileset._internalBase} or any directory higher up. This changes the layout of the resulting store path.
             - Set `fileset` to a file set that cannot contain files outside the `root` ${toString root}. This could change the files included in the result.''
     else
+      builtins.seq filter
       cleanSourceWith {
         name = "source";
         src = root;
-        filter = _toSourceFilter fileset;
+        inherit filter;
       };
 }

lib/fileset/internal.nix

@@ -294,6 +294,7 @@ rec {
     in
     # Special case because the code below assumes that the _internalBase is always included in the result
     # which shouldn't be done when we have no files at all in the base
+    # This also forces the tree before returning the filter, leads to earlier error messages
     if tree == null then
       empty
     else

lib/fileset/tests.sh

@@ -246,6 +246,9 @@ expectFailure 'toSource { root = ./a; fileset = ./a; }' 'lib.fileset.toSource: `
 \s*- If you want to import the file into the store _with_ a containing directory, set `root` to the containing directory, such as '"$work"', and set `fileset` to the file path.'
 rm -rf *
 
+# The fileset argument should be evaluated, even if the directory is empty
+expectFailure 'toSource { root = ./.; fileset = abort "This should be evaluated"; }' 'evaluation aborted with the following error message: '\''This should be evaluated'\'
+
 # Only paths under `root` should be able to influence the result
 mkdir a
 expectFailure 'toSource { root = ./a; fileset = ./.; }' 'lib.fileset.toSource: `fileset` could contain files in '"$work"', which is not under the `root` '"$work"'/a. Potential solutions: