refactor(read-tree): Keep traversing even if a default.nix is found r/97

This makes it possible for people to drop a default.nix into folders
along the way that add additional things into the attribute set at
that level.

These default.nix files are imported and merged with the rest of the
traversal from that point on. In theory nothing stops a user from
putting a derivation into one of them, but the effects of merging that
derivation's underlying attribute set with random other things from
the traversal are undefined.

This feature is being introduced for a slight revamp of the thirdParty
layout.

1 files changed, 5 insertions, 4 deletions

diff --git a/read-tree.nix b/read-tree.nix
index d742c69ea4..6f900a0667 100644
--- a/read-tree.nix
+++ b/read-tree.nix
@@ -31,7 +31,7 @@ let
     in if res == null then null else head res;
 
   filterNixFiles = dir:
-    let files = filter (e: isFile e.value) dir;
+    let files = filter (e: isFile e.value && e.name != "default.nix") dir;
         nixFiles = map (f: {
           # Name and value are intentionally flipped to get the
           # correct attribute set structure back out
@@ -70,9 +70,10 @@ let
     in listToAttrs (imported ++ dirs);
 
   importOr = path: dir: f:
-    if dir ? "default.nix"
-      then import path (argsWithPath args (pathParts path))
-      else f path (attrsToList dir);
+    let contents = f path (attrsToList dir);
+    in if dir ? "default.nix"
+      then import path (argsWithPath args (pathParts path)) // contents
+      else contents;
 
   readTree = path: importOr path (readDir path) traverse;
 in readTree initPath