Get undetected txi2p-tahoe test dependency into the test environment

default.nix

@@ -28,7 +28,7 @@ in
 # and those cases are handled below.  The version can only be extracted if
 # `setup.py update_version` has been run (this is not at all ideal but it
 # seems difficult to fix) - so for now just be sure to run that first.
-mach-nix.buildPythonPackage {
+mach-nix.buildPythonPackage rec {
   # Define the location of the Tahoe-LAFS source to be packaged.  Clean up all
   # as many of the non-source files (eg the `.git` directory, `~` backup
   # files, nix's own `result` symlink, etc) as possible to avoid needing to
@@ -86,4 +86,8 @@ mach-nix.buildPythonPackage {
     # in its source distribution.
     click-default-group.patches = [];
   };
+
+  passthru.meta.mach-nix = {
+    inherit providers _;
+  };
 }

tests.nix

@@ -16,9 +16,15 @@ let
   tahoe-lafs = import ./. (args // { extras = [ "test" ]; });
 
   # Put it into a Python environment.
-  python-env = pkgs.${pythonVersion}.withPackages (ps: [
-    tahoe-lafs
-  ]);
+  python-env = mach-nix.mkPython {
+    inherit (tahoe-lafs.meta.mach-nix) providers _;
+    packagesExtra = [ tahoe-lafs ];
+    requirements = ''
+    # txi2p-tahoe is another dependency with an environment marker that
+    # mach-nix doesn't automatically pick up.
+    txi2p-tahoe
+    '';
+  };
 in
 # Make a derivation that runs the unit test suite.
 pkgs.runCommand "tahoe-lafs-tests" { } ''