feat(nix/readTree): implement .skip-tree marker for subtree ignoring r/5862

With this change, readTree gains the ability to notice a `.skip-tree`
marker in addition to the `.skip-subtree` marker.

The behaviour of the new marker will completely ignore the folder that
the marker is located in (i.e. no node will be present for it in the
parent at all).

To make this work, the recursive function in readTree had to be
modified to return a sentinel value (noting that a tree has requested
to be skipped) which is then filtered out when constructing the list
of children. The actual `readTree` function is now a wrapper around
this inner, sentinel-yielding implementation which unwraps the result
set.

For obvious reasons, `.skip-tree` is not allowed at the top-level and
readTree will throw an error if it encounters it there.

Fixes: b/244
Change-Id: Ica731bc1af356e881fd3d31c7109f62ffd2762ea
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8185
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

1 files changed, 8 insertions, 5 deletions

diff --git a/nix/readTree/README.md b/nix/readTree/README.md
index f8bbe2255e..5d430d1cfc 100644
--- a/nix/readTree/README.md
+++ b/nix/readTree/README.md
@@ -52,14 +52,17 @@ true;` attribute merged into it.
 `readTree` will follow any subdirectories of a tree and import all Nix files,
 with some exceptions:
 
+* If a folder contains a `default.nix` file, no *sibling* Nix files will be
+  imported - however children are traversed as normal.
+* If a folder contains a `default.nix` it is loaded and, if it
+  evaluates to a set, *merged* with the children. If it evaluates to
+  anything other than a set, else the children are *not traversed*.
+* A folder can opt out from readTree completely by containing a
+  `.skip-tree` file. The content of the file is not read. These
+  folders will be missing completely from the readTree structure.
 * A folder can declare that its children are off-limit by containing a
   `.skip-subtree` file. Since the content of the file is not checked, it can be
   useful to leave a note for a human in the file.
-* If a folder contains a `default.nix` file, no *sibling* Nix files will be
-  imported - however children are traversed as normal.
-* If a folder contains a `default.nix` it is loaded and, if it evaluates to a
-  set, *merged* with the children. If it evaluates to anything else the children
-  are *not traversed*.
 * The `default.nix` of the top-level folder on which readTree is
   called is **not** read to avoid infinite recursion (as, presumably,
   this file is where readTree itself is called).