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-rw-r--r--nix/readTree/README.md92
-rw-r--r--nix/readTree/default.nix284
-rw-r--r--nix/readTree/tests/.skip-subtree1
-rw-r--r--nix/readTree/tests/default.nix129
-rw-r--r--nix/readTree/tests/test-example/third_party/default.nix5
-rw-r--r--nix/readTree/tests/test-example/third_party/rustpkgs/aho-corasick.nix1
-rw-r--r--nix/readTree/tests/test-example/third_party/rustpkgs/serde.nix1
-rw-r--r--nix/readTree/tests/test-example/tools/cheddar/default.nix1
-rw-r--r--nix/readTree/tests/test-example/tools/roquefort.nix1
-rw-r--r--nix/readTree/tests/test-marker/directory-marked/default.nix3
-rw-r--r--nix/readTree/tests/test-marker/directory-marked/nested/default.nix3
-rw-r--r--nix/readTree/tests/test-marker/file-children/one.nix3
-rw-r--r--nix/readTree/tests/test-marker/file-children/two.nix3
-rw-r--r--nix/readTree/tests/test-tree-traversal/default-nix/can-be-drv/default.nix7
-rw-r--r--nix/readTree/tests/test-tree-traversal/default-nix/can-be-drv/subdir/a.nix3
-rw-r--r--nix/readTree/tests/test-tree-traversal/default-nix/default.nix5
-rw-r--r--nix/readTree/tests/test-tree-traversal/default-nix/no-merge/default.nix3
-rw-r--r--nix/readTree/tests/test-tree-traversal/default-nix/no-merge/subdir/a.nix1
-rw-r--r--nix/readTree/tests/test-tree-traversal/default-nix/sibling.nix1
-rw-r--r--nix/readTree/tests/test-tree-traversal/default-nix/subdir/a.nix3
-rw-r--r--nix/readTree/tests/test-tree-traversal/no-skip-subtree/a/default.nix3
-rw-r--r--nix/readTree/tests/test-tree-traversal/no-skip-subtree/b/c.nix3
-rw-r--r--nix/readTree/tests/test-tree-traversal/no-skip-subtree/default.nix5
-rw-r--r--nix/readTree/tests/test-tree-traversal/skip-subtree/.skip-subtree1
-rw-r--r--nix/readTree/tests/test-tree-traversal/skip-subtree/a/default.nix3
-rw-r--r--nix/readTree/tests/test-tree-traversal/skip-subtree/b/c.nix3
-rw-r--r--nix/readTree/tests/test-tree-traversal/skip-subtree/default.nix5
-rw-r--r--nix/readTree/tests/test-wrong-no-dots/no-dots-in-function.nix3
-rw-r--r--nix/readTree/tests/test-wrong-not-a-function/not-a-function.nix1
29 files changed, 577 insertions, 0 deletions
diff --git a/nix/readTree/README.md b/nix/readTree/README.md
new file mode 100644
index 000000000000..f8bbe2255e5e
--- /dev/null
+++ b/nix/readTree/README.md
@@ -0,0 +1,92 @@
+readTree
+========
+
+This is a Nix program that builds up an attribute set tree for a large
+repository based on the filesystem layout.
+
+It is in fact the tool that lays out the attribute set of this repository.
+
+As an example, consider a root (`.`) of a repository and a layout such as:
+
+```
+.
+├── third_party
+│   ├── default.nix
+│   └── rustpkgs
+│       ├── aho-corasick.nix
+│       └── serde.nix
+└── tools
+    ├── cheddar
+    │   └── default.nix
+    └── roquefort.nix
+```
+
+When `readTree` is called on that tree, it will construct an attribute set with
+this shape:
+
+```nix
+{
+    tools = {
+        cheddar = ...;
+        roquefort = ...;
+    };
+
+    third_party = {
+        # the `default.nix` of this folder might have had arbitrary other
+        # attributes here, such as this:
+        favouriteColour = "orange";
+
+        rustpkgs = {
+            aho-corasick = ...;
+            serde = ...;
+        };
+    };
+}
+```
+
+Every imported Nix file that yields an attribute set will have a `__readTree =
+true;` attribute merged into it.
+
+## Traversal logic
+
+`readTree` will follow any subdirectories of a tree and import all Nix files,
+with some exceptions:
+
+* 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).
+
+Traversal is lazy, `readTree` will only build up the tree as requested. This
+currently has the downside that directories with no importable files end up in
+the tree as empty nodes (`{}`).
+
+## Import structure
+
+`readTree` is called with an argument set containing a few parameters:
+
+* `path`: Initial path at which to start the traversal.
+* `args`: Arguments to pass to all imports.
+* `filter`: (optional) A function to filter the argument set on each
+  import based on the location in the tree. This can be used to, for
+  example, implement a "visibility" system inside of a tree.
+* `scopedArgs`: (optional) An argument set that is passed to all
+  imported files via `builtins.scopedImport`. This will forcefully
+  override the given values in the import scope, use with care!
+
+The package headers in this repository follow the form `{ pkgs, ... }:` where
+`pkgs` is a fixed-point of the entire package tree (see the `default.nix` at the
+root of the depot).
+
+In theory `readTree` can pass arguments of different shapes, but I have found
+this to be a good solution for the most part.
+
+Note that `readTree` does not currently make functions overridable, though it is
+feasible that it could do that in the future.
diff --git a/nix/readTree/default.nix b/nix/readTree/default.nix
new file mode 100644
index 000000000000..ba3363d8d69c
--- /dev/null
+++ b/nix/readTree/default.nix
@@ -0,0 +1,284 @@
+# Copyright (c) 2019 Vincent Ambo
+# Copyright (c) 2020-2021 The TVL Authors
+# SPDX-License-Identifier: MIT
+#
+# Provides a function to automatically read a a filesystem structure
+# into a Nix attribute set.
+#
+# Called with an attribute set taking the following arguments:
+#
+#   path: Path to a directory from which to start reading the tree.
+#
+#   args: Argument set to pass to each imported file.
+#
+#   filter: Function to filter `args` based on the tree location. This should
+#           be a function of the form `args -> location -> args`, where the
+#           location is a list of strings representing the path components of
+#           the current readTree target. Optional.
+{ ... }:
+
+let
+  inherit (builtins)
+    attrNames
+    concatMap
+    concatStringsSep
+    elem
+    elemAt
+    filter
+    hasAttr
+    head
+    isAttrs
+    listToAttrs
+    map
+    match
+    readDir
+    substring;
+
+  argsWithPath = args: parts:
+    let meta.locatedAt = parts;
+    in meta // (if isAttrs args then args else args meta);
+
+  readDirVisible = path:
+    let
+      children = readDir path;
+      isVisible = f: f == ".skip-subtree" || (substring 0 1 f) != ".";
+      names = filter isVisible (attrNames children);
+    in
+    listToAttrs (map
+      (name: {
+        inherit name;
+        value = children.${name};
+      })
+      names);
+
+  # Create a mark containing the location of this attribute and
+  # a list of all child attribute names added by readTree.
+  marker = parts: children: {
+    __readTree = parts;
+    __readTreeChildren = builtins.attrNames children;
+  };
+
+  # Create a label from a target's tree location.
+  mkLabel = target:
+    let label = concatStringsSep "/" target.__readTree;
+    in if target ? __subtarget
+    then "${label}:${target.__subtarget}"
+    else label;
+
+  # Merge two attribute sets, but place attributes in `passthru` via
+  # `overrideAttrs` for derivation targets that support it.
+  merge = a: b:
+    if a ? overrideAttrs
+    then
+      a.overrideAttrs
+        (prev: {
+          passthru = (prev.passthru or { }) // b;
+        })
+    else a // b;
+
+  # Import a file and enforce our calling convention
+  importFile = args: scopedArgs: path: parts: filter:
+    let
+      importedFile =
+        if scopedArgs != { }
+        then builtins.scopedImport scopedArgs path
+        else import path;
+      pathType = builtins.typeOf importedFile;
+    in
+    if pathType != "lambda"
+    then builtins.throw "readTree: trying to import ${toString path}, but it’s a ${pathType}, you need to make it a function like { depot, pkgs, ... }"
+    else importedFile (filter parts (argsWithPath args parts));
+
+  nixFileName = file:
+    let res = match "(.*)\\.nix" file;
+    in if res == null then null else head res;
+
+  readTree = { args, initPath, rootDir, parts, argsFilter, scopedArgs }:
+    let
+      dir = readDirVisible initPath;
+      joinChild = c: initPath + ("/" + c);
+
+      self =
+        if rootDir
+        then { __readTree = [ ]; }
+        else importFile args scopedArgs initPath parts argsFilter;
+
+      # Import subdirectories of the current one, unless the special
+      # `.skip-subtree` file exists which makes readTree ignore the
+      # children.
+      #
+      # This file can optionally contain information on why the tree
+      # should be ignored, but its content is not inspected by
+      # readTree
+      filterDir = f: dir."${f}" == "directory";
+      children = if hasAttr ".skip-subtree" dir then [ ] else
+      map
+        (c: {
+          name = c;
+          value = readTree {
+            inherit argsFilter scopedArgs;
+            args = args;
+            initPath = (joinChild c);
+            rootDir = false;
+            parts = (parts ++ [ c ]);
+          };
+        })
+        (filter filterDir (attrNames dir));
+
+      # Import Nix files
+      nixFiles =
+        if hasAttr ".skip-subtree" dir then [ ]
+        else filter (f: f != null) (map nixFileName (attrNames dir));
+      nixChildren = map
+        (c:
+          let
+            p = joinChild (c + ".nix");
+            childParts = parts ++ [ c ];
+            imported = importFile args scopedArgs p childParts argsFilter;
+          in
+          {
+            name = c;
+            value =
+              if isAttrs imported
+              then merge imported (marker childParts { })
+              else imported;
+          })
+        nixFiles;
+
+      nodeValue = if dir ? "default.nix" then self else { };
+
+      allChildren = listToAttrs (
+        if dir ? "default.nix"
+        then children
+        else nixChildren ++ children
+      );
+
+    in
+    if isAttrs nodeValue
+    then merge nodeValue (allChildren // (marker parts allChildren))
+    else nodeValue;
+
+  # Helper function to fetch subtargets from a target. This is a
+  # temporary helper to warn on the use of the `meta.targets`
+  # attribute, which is deprecated in favour of `meta.ci.targets`.
+  subtargets = node:
+    let targets = (node.meta.targets or [ ]) ++ (node.meta.ci.targets or [ ]);
+    in if node ? meta.targets then
+      builtins.trace ''
+        Warning: The meta.targets attribute is deprecated.
+
+        Please move the subtargets of //${mkLabel node} to the
+        meta.ci.targets attribute.
+        
+      ''
+        targets else targets;
+
+  # Function which can be used to find all readTree targets within an
+  # attribute set.
+  #
+  # This function will gather physical targets, that is targets which
+  # correspond directly to a location in the repository, as well as
+  # subtargets (specified in the meta.ci.targets attribute of a node).
+  #
+  # This can be used to discover targets for inclusion in CI
+  # pipelines.
+  #
+  # Called with the arguments:
+  #
+  #   eligible: Function to determine whether the given derivation
+  #             should be included in the build.
+  gather = eligible: node:
+    if node ? __readTree then
+    # Include the node itself if it is eligible.
+      (if eligible node then [ node ] else [ ])
+      # Include eligible children of the node
+      ++ concatMap (gather eligible) (map (attr: node."${attr}") node.__readTreeChildren)
+      # Include specified sub-targets of the node
+      ++ filter eligible (map
+        (k: (node."${k}" or { }) // {
+          # Keep the same tree location, but explicitly mark this
+          # node as a subtarget.
+          __readTree = node.__readTree;
+          __readTreeChildren = [ ];
+          __subtarget = k;
+        })
+        (subtargets node))
+    else [ ];
+
+  # Determine whether a given value is a derivation.
+  # Copied from nixpkgs/lib for cases where lib is not available yet.
+  isDerivation = x: isAttrs x && x ? type && x.type == "derivation";
+in
+{
+  inherit gather mkLabel;
+
+  __functor = _:
+    { path
+    , args
+    , filter ? (_parts: x: x)
+    , scopedArgs ? { }
+    }:
+    readTree {
+      inherit args scopedArgs;
+      argsFilter = filter;
+      initPath = path;
+      rootDir = true;
+      parts = [ ];
+    };
+
+  # In addition to readTree itself, some functionality is exposed that
+  # is useful for users of readTree.
+
+  # Create a readTree filter disallowing access to the specified
+  # top-level folder in the repository, except for specific exceptions
+  # specified by their (full) paths.
+  #
+  # Called with the arguments:
+  #
+  #   folder: Name of the restricted top-level folder (e.g. 'experimental')
+  #
+  #   exceptions: List of readTree parts (e.g. [ [ "services" "some-app" ] ]),
+  #               which should be able to access the restricted folder.
+  #
+  #   reason: Textual explanation for the restriction (included in errors)
+  restrictFolder = { folder, exceptions ? [ ], reason }: parts: args:
+    if (elemAt parts 0) == folder || elem parts exceptions
+    then args
+    else args // {
+      depot = args.depot // {
+        "${folder}" = throw ''
+          Access to targets under //${folder} is not permitted from
+          other repository paths. Specific exceptions are configured
+          at the top-level.
+
+          ${reason}
+          At location: ${builtins.concatStringsSep "." parts}
+        '';
+      };
+    };
+
+  # This definition of fix is identical to <nixpkgs>.lib.fix, but is
+  # provided here for cases where readTree is used before nixpkgs can
+  # be imported.
+  #
+  # It is often required to create the args attribute set.
+  fix = f: let x = f x; in x;
+
+  # Takes an attribute set and adds a meta.ci.targets attribute to it
+  # which contains all direct children of the attribute set which are
+  # derivations.
+  #
+  # Type: attrs -> attrs
+  drvTargets = attrs:
+    attrs // {
+      # preserve .meta from original attrs
+      meta = (attrs.meta or { }) // {
+        # preserve .meta.ci (except .targets) from original attrs
+        ci = (attrs.meta.ci or { }) // {
+          targets = builtins.filter
+            (x: isDerivation attrs."${x}")
+            (builtins.attrNames attrs);
+        };
+      };
+    };
+}
diff --git a/nix/readTree/tests/.skip-subtree b/nix/readTree/tests/.skip-subtree
new file mode 100644
index 000000000000..46952a4ca635
--- /dev/null
+++ b/nix/readTree/tests/.skip-subtree
@@ -0,0 +1 @@
+These tests call their own readTree, so the toplevel one shouldn’t bother
diff --git a/nix/readTree/tests/default.nix b/nix/readTree/tests/default.nix
new file mode 100644
index 000000000000..fcca141714a8
--- /dev/null
+++ b/nix/readTree/tests/default.nix
@@ -0,0 +1,129 @@
+{ depot, lib, ... }:
+
+let
+  inherit (depot.nix.runTestsuite)
+    runTestsuite
+    it
+    assertEq
+    assertThrows
+    ;
+
+  tree-ex = depot.nix.readTree {
+    path = ./test-example;
+    args = { };
+  };
+
+  example = it "corresponds to the README example" [
+    (assertEq "third_party attrset"
+      (lib.isAttrs tree-ex.third_party
+        && (! lib.isDerivation tree-ex.third_party))
+      true)
+    (assertEq "third_party attrset other attribute"
+      tree-ex.third_party.favouriteColour
+      "orange")
+    (assertEq "rustpkgs attrset aho-corasick"
+      tree-ex.third_party.rustpkgs.aho-corasick
+      "aho-corasick")
+    (assertEq "rustpkgs attrset serde"
+      tree-ex.third_party.rustpkgs.serde
+      "serde")
+    (assertEq "tools cheddear"
+      "cheddar"
+      tree-ex.tools.cheddar)
+    (assertEq "tools roquefort"
+      tree-ex.tools.roquefort
+      "roquefort")
+  ];
+
+  tree-tl = depot.nix.readTree {
+    path = ./test-tree-traversal;
+    args = { };
+  };
+
+  traversal-logic = it "corresponds to the traversal logic in the README" [
+    (assertEq "skip subtree default.nix is read"
+      tree-tl.skip-subtree.but
+      "the default.nix is still read")
+    (assertEq "skip subtree a/default.nix is skipped"
+      (tree-tl.skip-subtree ? a)
+      false)
+    (assertEq "skip subtree b/c.nix is skipped"
+      (tree-tl.skip-subtree ? b)
+      false)
+    (assertEq "skip subtree a/default.nix would be read without .skip-subtree"
+      (tree-tl.no-skip-subtree.a)
+      "am I subtree yet?")
+    (assertEq "skip subtree b/c.nix would be read without .skip-subtree"
+      (tree-tl.no-skip-subtree.b.c)
+      "cool")
+
+    (assertEq "default.nix attrset is merged with siblings"
+      tree-tl.default-nix.no
+      "siblings should be read")
+    (assertEq "default.nix means sibling isn’t read"
+      (tree-tl.default-nix ? sibling)
+      false)
+    (assertEq "default.nix means subdirs are still read and merged into default.nix"
+      (tree-tl.default-nix.subdir.a)
+      "but I’m picked up")
+
+    (assertEq "default.nix can be not an attrset"
+      tree-tl.default-nix.no-merge
+      "I’m not merged with any children")
+    (assertEq "default.nix is not an attrset -> children are not merged"
+      (tree-tl.default-nix.no-merge ? subdir)
+      false)
+
+    (assertEq "default.nix can contain a derivation"
+      (lib.isDerivation tree-tl.default-nix.can-be-drv)
+      true)
+    (assertEq "Even if default.nix is a derivation, children are traversed and merged"
+      tree-tl.default-nix.can-be-drv.subdir.a
+      "Picked up through the drv")
+    (assertEq "default.nix drv is not changed by readTree"
+      tree-tl.default-nix.can-be-drv
+      (import ./test-tree-traversal/default-nix/can-be-drv/default.nix { }))
+  ];
+
+  # these each call readTree themselves because the throws have to happen inside assertThrows
+  wrong = it "cannot read these files and will complain" [
+    (assertThrows "this file is not a function"
+      (depot.nix.readTree {
+        path = ./test-wrong-not-a-function;
+        args = { };
+      }).not-a-function)
+    # can’t test for that, assertThrows can’t catch this error
+    # (assertThrows "this file is a function but doesn’t have dots"
+    #   (depot.nix.readTree {} ./test-wrong-no-dots).no-dots-in-function)
+  ];
+
+  read-markers = depot.nix.readTree {
+    path = ./test-marker;
+    args = { };
+  };
+
+  assertMarkerByPath = path:
+    assertEq "${lib.concatStringsSep "." path} is marked correctly"
+      (lib.getAttrFromPath path read-markers).__readTree
+      path;
+
+  markers = it "marks nodes correctly" [
+    (assertMarkerByPath [ "directory-marked" ])
+    (assertMarkerByPath [ "directory-marked" "nested" ])
+    (assertMarkerByPath [ "file-children" "one" ])
+    (assertMarkerByPath [ "file-children" "two" ])
+    (assertEq "nix file children are marked correctly"
+      read-markers.file-children.__readTreeChildren [ "one" "two" ])
+    (assertEq "directory children are marked correctly"
+      read-markers.directory-marked.__readTreeChildren [ "nested" ])
+    (assertEq "absence of children is marked"
+      read-markers.directory-marked.nested.__readTreeChildren [ ])
+  ];
+
+in
+runTestsuite "readTree" [
+  example
+  traversal-logic
+  wrong
+  markers
+]
diff --git a/nix/readTree/tests/test-example/third_party/default.nix b/nix/readTree/tests/test-example/third_party/default.nix
new file mode 100644
index 000000000000..27d87242189d
--- /dev/null
+++ b/nix/readTree/tests/test-example/third_party/default.nix
@@ -0,0 +1,5 @@
+{ ... }:
+
+{
+  favouriteColour = "orange";
+}
diff --git a/nix/readTree/tests/test-example/third_party/rustpkgs/aho-corasick.nix b/nix/readTree/tests/test-example/third_party/rustpkgs/aho-corasick.nix
new file mode 100644
index 000000000000..f964fa583c43
--- /dev/null
+++ b/nix/readTree/tests/test-example/third_party/rustpkgs/aho-corasick.nix
@@ -0,0 +1 @@
+{ ... }: "aho-corasick"
diff --git a/nix/readTree/tests/test-example/third_party/rustpkgs/serde.nix b/nix/readTree/tests/test-example/third_party/rustpkgs/serde.nix
new file mode 100644
index 000000000000..54b3c0503f12
--- /dev/null
+++ b/nix/readTree/tests/test-example/third_party/rustpkgs/serde.nix
@@ -0,0 +1 @@
+{ ... }: "serde"
diff --git a/nix/readTree/tests/test-example/tools/cheddar/default.nix b/nix/readTree/tests/test-example/tools/cheddar/default.nix
new file mode 100644
index 000000000000..a1919442d86d
--- /dev/null
+++ b/nix/readTree/tests/test-example/tools/cheddar/default.nix
@@ -0,0 +1 @@
+{ ... }: "cheddar"
diff --git a/nix/readTree/tests/test-example/tools/roquefort.nix b/nix/readTree/tests/test-example/tools/roquefort.nix
new file mode 100644
index 000000000000..03c7492bb6b1
--- /dev/null
+++ b/nix/readTree/tests/test-example/tools/roquefort.nix
@@ -0,0 +1 @@
+{ ... }: "roquefort"
diff --git a/nix/readTree/tests/test-marker/directory-marked/default.nix b/nix/readTree/tests/test-marker/directory-marked/default.nix
new file mode 100644
index 000000000000..5bd3e36b5397
--- /dev/null
+++ b/nix/readTree/tests/test-marker/directory-marked/default.nix
@@ -0,0 +1,3 @@
+{ ... }:
+
+{ }
diff --git a/nix/readTree/tests/test-marker/directory-marked/nested/default.nix b/nix/readTree/tests/test-marker/directory-marked/nested/default.nix
new file mode 100644
index 000000000000..5bd3e36b5397
--- /dev/null
+++ b/nix/readTree/tests/test-marker/directory-marked/nested/default.nix
@@ -0,0 +1,3 @@
+{ ... }:
+
+{ }
diff --git a/nix/readTree/tests/test-marker/file-children/one.nix b/nix/readTree/tests/test-marker/file-children/one.nix
new file mode 100644
index 000000000000..5bd3e36b5397
--- /dev/null
+++ b/nix/readTree/tests/test-marker/file-children/one.nix
@@ -0,0 +1,3 @@
+{ ... }:
+
+{ }
diff --git a/nix/readTree/tests/test-marker/file-children/two.nix b/nix/readTree/tests/test-marker/file-children/two.nix
new file mode 100644
index 000000000000..5bd3e36b5397
--- /dev/null
+++ b/nix/readTree/tests/test-marker/file-children/two.nix
@@ -0,0 +1,3 @@
+{ ... }:
+
+{ }
diff --git a/nix/readTree/tests/test-tree-traversal/default-nix/can-be-drv/default.nix b/nix/readTree/tests/test-tree-traversal/default-nix/can-be-drv/default.nix
new file mode 100644
index 000000000000..95d13d3c2750
--- /dev/null
+++ b/nix/readTree/tests/test-tree-traversal/default-nix/can-be-drv/default.nix
@@ -0,0 +1,7 @@
+{ ... }:
+derivation {
+  name = "im-a-drv";
+  system = builtins.currentSystem;
+  builder = "/bin/sh";
+  args = [ "-c" ''echo "" > $out'' ];
+}
diff --git a/nix/readTree/tests/test-tree-traversal/default-nix/can-be-drv/subdir/a.nix b/nix/readTree/tests/test-tree-traversal/default-nix/can-be-drv/subdir/a.nix
new file mode 100644
index 000000000000..2ee2d648f042
--- /dev/null
+++ b/nix/readTree/tests/test-tree-traversal/default-nix/can-be-drv/subdir/a.nix
@@ -0,0 +1,3 @@
+{ ... }:
+
+"Picked up through the drv"
diff --git a/nix/readTree/tests/test-tree-traversal/default-nix/default.nix b/nix/readTree/tests/test-tree-traversal/default-nix/default.nix
new file mode 100644
index 000000000000..6003b5398305
--- /dev/null
+++ b/nix/readTree/tests/test-tree-traversal/default-nix/default.nix
@@ -0,0 +1,5 @@
+{ ... }:
+
+{
+  no = "siblings should be read";
+}
diff --git a/nix/readTree/tests/test-tree-traversal/default-nix/no-merge/default.nix b/nix/readTree/tests/test-tree-traversal/default-nix/no-merge/default.nix
new file mode 100644
index 000000000000..c469533fbe5b
--- /dev/null
+++ b/nix/readTree/tests/test-tree-traversal/default-nix/no-merge/default.nix
@@ -0,0 +1,3 @@
+{ ... }:
+
+"I’m not merged with any children"
diff --git a/nix/readTree/tests/test-tree-traversal/default-nix/no-merge/subdir/a.nix b/nix/readTree/tests/test-tree-traversal/default-nix/no-merge/subdir/a.nix
new file mode 100644
index 000000000000..2008c2d2411f
--- /dev/null
+++ b/nix/readTree/tests/test-tree-traversal/default-nix/no-merge/subdir/a.nix
@@ -0,0 +1 @@
+"not accessible since parent default.nix is not an attrset"
diff --git a/nix/readTree/tests/test-tree-traversal/default-nix/sibling.nix b/nix/readTree/tests/test-tree-traversal/default-nix/sibling.nix
new file mode 100644
index 000000000000..8c57f2c16137
--- /dev/null
+++ b/nix/readTree/tests/test-tree-traversal/default-nix/sibling.nix
@@ -0,0 +1 @@
+"I’m left alone"
diff --git a/nix/readTree/tests/test-tree-traversal/default-nix/subdir/a.nix b/nix/readTree/tests/test-tree-traversal/default-nix/subdir/a.nix
new file mode 100644
index 000000000000..cf0ac2c8f3cf
--- /dev/null
+++ b/nix/readTree/tests/test-tree-traversal/default-nix/subdir/a.nix
@@ -0,0 +1,3 @@
+{ ... }:
+
+"but I’m picked up"
diff --git a/nix/readTree/tests/test-tree-traversal/no-skip-subtree/a/default.nix b/nix/readTree/tests/test-tree-traversal/no-skip-subtree/a/default.nix
new file mode 100644
index 000000000000..a586ce534ce2
--- /dev/null
+++ b/nix/readTree/tests/test-tree-traversal/no-skip-subtree/a/default.nix
@@ -0,0 +1,3 @@
+{ ... }:
+
+"am I subtree yet?"
diff --git a/nix/readTree/tests/test-tree-traversal/no-skip-subtree/b/c.nix b/nix/readTree/tests/test-tree-traversal/no-skip-subtree/b/c.nix
new file mode 100644
index 000000000000..06216c417be9
--- /dev/null
+++ b/nix/readTree/tests/test-tree-traversal/no-skip-subtree/b/c.nix
@@ -0,0 +1,3 @@
+{ ... }:
+
+"cool"
diff --git a/nix/readTree/tests/test-tree-traversal/no-skip-subtree/default.nix b/nix/readTree/tests/test-tree-traversal/no-skip-subtree/default.nix
new file mode 100644
index 000000000000..3d9f241cddb5
--- /dev/null
+++ b/nix/readTree/tests/test-tree-traversal/no-skip-subtree/default.nix
@@ -0,0 +1,5 @@
+{ ... }:
+
+{
+  but = "the default.nix is still read";
+}
diff --git a/nix/readTree/tests/test-tree-traversal/skip-subtree/.skip-subtree b/nix/readTree/tests/test-tree-traversal/skip-subtree/.skip-subtree
new file mode 100644
index 000000000000..87271ba5e101
--- /dev/null
+++ b/nix/readTree/tests/test-tree-traversal/skip-subtree/.skip-subtree
@@ -0,0 +1 @@
+this file makes subdirs be skipped, I hope
diff --git a/nix/readTree/tests/test-tree-traversal/skip-subtree/a/default.nix b/nix/readTree/tests/test-tree-traversal/skip-subtree/a/default.nix
new file mode 100644
index 000000000000..a586ce534ce2
--- /dev/null
+++ b/nix/readTree/tests/test-tree-traversal/skip-subtree/a/default.nix
@@ -0,0 +1,3 @@
+{ ... }:
+
+"am I subtree yet?"
diff --git a/nix/readTree/tests/test-tree-traversal/skip-subtree/b/c.nix b/nix/readTree/tests/test-tree-traversal/skip-subtree/b/c.nix
new file mode 100644
index 000000000000..06216c417be9
--- /dev/null
+++ b/nix/readTree/tests/test-tree-traversal/skip-subtree/b/c.nix
@@ -0,0 +1,3 @@
+{ ... }:
+
+"cool"
diff --git a/nix/readTree/tests/test-tree-traversal/skip-subtree/default.nix b/nix/readTree/tests/test-tree-traversal/skip-subtree/default.nix
new file mode 100644
index 000000000000..3d9f241cddb5
--- /dev/null
+++ b/nix/readTree/tests/test-tree-traversal/skip-subtree/default.nix
@@ -0,0 +1,5 @@
+{ ... }:
+
+{
+  but = "the default.nix is still read";
+}
diff --git a/nix/readTree/tests/test-wrong-no-dots/no-dots-in-function.nix b/nix/readTree/tests/test-wrong-no-dots/no-dots-in-function.nix
new file mode 100644
index 000000000000..4681253af831
--- /dev/null
+++ b/nix/readTree/tests/test-wrong-no-dots/no-dots-in-function.nix
@@ -0,0 +1,3 @@
+{}:
+
+"This is a function, but readTree wants to pass a bunch of arguments, and not having dots means we depend on exactly which arguments."
diff --git a/nix/readTree/tests/test-wrong-not-a-function/not-a-function.nix b/nix/readTree/tests/test-wrong-not-a-function/not-a-function.nix
new file mode 100644
index 000000000000..f46ee2a35565
--- /dev/null
+++ b/nix/readTree/tests/test-wrong-not-a-function/not-a-function.nix
@@ -0,0 +1 @@
+"This file needs to be a function, otherwise readTree doesn’t like it!"