feat(nix/writeTree): init

Add //nix/writeTree, a function to make a derivation to build a
directory structure from a Nix attribute set.

Co-authored-by: sterni <sternenseemann@systemli.org>
Change-Id: I9c0fc91611a55a20ad33de6f2b27abde4b6abd21
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10963
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Autosubmit: aspen <root@gws.fyi>
Reviewed-by: aspen <root@gws.fyi>

nix/writeTree/default.nix

@@ -0,0 +1,51 @@
+{ depot, lib, pkgs, ... }:
+let
+  inherit (lib) fix pipe mapAttrsToList isAttrs concatLines isString;
+
+  inherit (depot.nix.utils) isDirectory isRegularFile;
+
+  esc = s: lib.escapeShellArg /* ensure paths import into store */ "${s}";
+
+  writeTreeAtPath = path: tree:
+    ''
+      mkdir -p "$out/"${esc path}
+    ''
+    + pipe tree [
+      (mapAttrsToList (k: v:
+        # TODO(sterni): a more discoverable isPathLike would fit into //nix/utils
+        # ATTN: This check has the flaw that it accepts paths without context
+        # that would not be available in the sandbox!
+        if lib.types.path.check v then
+          if isRegularFile v then
+            "cp --reflink=auto ${esc v} \"$out/\"${esc path}/${esc k}"
+          else if isDirectory v then ''
+            mkdir -p "$out/"${esc path}
+            cp -r --reflink=auto ${esc v} "$out/"${esc path}/${esc k}
+          ''
+          else
+            throw "invalid path type (expected file or directory)"
+        else if isAttrs v then
+          writeTreeAtPath "${path}/${k}" v
+        else if isString v then
+          "cp --reflink=auto ${esc v} \"$out/\"${esc path}/${esc k}"
+        else
+          throw "invalid type (expected file, directory, or attrs)"))
+      concatLines
+    ];
+
+  /* Create a directory tree specified by a Nix attribute set structure.
+
+     Each value in `tree` should either be a file, a directory, or another tree
+     attribute set. Those paths will be written to a directory tree
+     corresponding to the structure of the attribute set.
+
+     Type: string -> attrSet -> derivation
+  */
+  writeTree = name: tree:
+    pkgs.runCommandLocal name { } (writeTreeAtPath "" tree);
+in
+
+# __functor trick so readTree can add the tests attribute
+{
+  __functor = _: writeTree;
+}