buildstream: run the pytest suite as a NixOS VM test

buildbox-casd finds `buildbox-fuse` on PATH and defaults to real
FUSE-based staging, which needs `/dev/fuse` and a working mount path
that the Nix build sandbox doesn't provide, hence the
`BUILDBOX_STAGER=copy-or-link` workaround forcing the hardlink/copy
fallback stager, and the `/usr/bin/env sh` shebang patch for the dummy
buildbox-casd scripts in tests/internals/cascache.py.

Add nixosTests.buildstream and point passthru.tests.pytest at it
instead: so the suite exercises the actual FUSE-backed staging path.

The main derivation no longer builds the pytest/test-fixture inputs at
all (they're only needed by the VM test), keeping just
`versionCheckHook` for the existing install-check.

`bst` is a buildPythonApplication, so its own site-packages only
contains buildstream's own files, not the propagated runtime deps
(protobuf, grpcio, click, ...) that only reach the wrapped `bst`
script's own PYTHONPATH; `toPythonModule` reinterprets it as a regular
library so `withPackages` pulls in its full dependency closure for
`pytest` to import from.

The default `diskSize = "auto"` only sizes the VM's root filesystem to
fit the system closure, with no scratch space; the suite builds real
CAS/cache artifacts under pytest's basetemp for hundreds of
parametrized cases, easily exceeding that. Bumped disk/memory sizing
to comfortably cover a full run.

Verified this run passes: `nix-build -E 'let pkgs = import ./. {}; in
pkgs.nixosTests.buildstream.test'` completes successfully.

Assisted-by: Claude <noreply@anthropic.com>

nixos/tests/all-tests.nix

@@ -380,6 +380,7 @@ in
   budgie = runTest ./budgie.nix;
   buildbot = runTest ./buildbot.nix;
   buildkite-agents = runTest ./buildkite-agents.nix;
+  buildstream = runTest ./buildstream.nix;
   bulwark = runTest ./bulwark.nix;
   c2fmzq = runTest ./c2fmzq.nix;
   caddy = runTest ./caddy.nix;

nixos/tests/buildstream.nix

@@ -0,0 +1,71 @@
+{ pkgs, ... }:
+let
+  bst = pkgs.buildstream;
+
+  # `bst` is a buildPythonApplication; its own site-packages only contains
+  # buildstream's own files, not the propagated runtime deps (protobuf,
+  # grpcio, click, ...) that only reach the wrapped `bst` script's PYTHONPATH.
+  # `toPythonModule` reinterprets it as a regular library so its full
+  # dependency closure gets merged in by `withPackages` below.
+  bstModule = pkgs.python3.pkgs.toPythonModule bst;
+
+  # Test fixture plugin package used by test_source_mirror_plugin[pip]; upstream
+  # normally installs this via tox before running the pip-origin plugin loading test.
+  samplePlugins = pkgs.python3Packages.buildPythonPackage {
+    pname = "sample-plugins";
+    version = "1.2.3";
+    pyproject = true;
+    build-system = [ pkgs.python3Packages.setuptools ];
+    src = "${bst.src}/tests/plugins/sample-plugins";
+    dontCheck = true;
+  };
+in
+{
+  name = "buildstream";
+
+  meta.maintainers = with pkgs.lib.maintainers; [ shymega ];
+
+  nodes.machine =
+    { pkgs, ... }:
+    {
+      # The default "auto" diskSize only sizes the root filesystem to fit
+      # the system closure, with no scratch space; the suite builds real
+      # CAS/cache artifacts under pytest's basetemp (`./tmp`, per
+      # setup.cfg) for hundreds of parametrized cases, which exhausts that
+      # in a few minutes (`OSError: could not create numbered dir`).
+      virtualisation.diskSize = 16 * 1024;
+      virtualisation.memorySize = 2048;
+
+      environment.systemPackages = [
+        (pkgs.python3.withPackages (
+          ps: with ps; [
+            bstModule
+            pexpect
+            pyftpdlib
+            pytest
+            pytest-datafiles
+            pytest-env
+            pytest-timeout
+            pytest-xdist
+            samplePlugins
+          ]
+        ))
+        bst
+        pkgs.buildbox
+        pkgs.gitMinimal
+      ];
+    };
+
+  testScript = ''
+    machine.succeed("cp -r ${bst.src} /tmp/src && chmod -R u+w /tmp/src")
+
+    # The dummy buildbox-casd scripts spawned by tests/internals/cascache.py use
+    # an `/usr/bin/env sh` shebang, which doesn't exist here either.
+    machine.succeed(
+        "sed -i 's|#!/usr/bin/env sh|#!${pkgs.runtimeShell}|'"
+        + " /tmp/src/tests/internals/cascache.py"
+    )
+
+    machine.succeed("cd /tmp/src && HOME=/root pytest")
+  '';
+}

pkgs/by-name/bu/buildstream/package.nix

@@ -3,7 +3,7 @@
   python3Packages,
   fetchFromGitHub,
   gitUpdater,
-  runtimeShell,
+  nixosTests,
 
   # buildInputs
   buildbox,
@@ -15,8 +15,6 @@
   installShellFiles,
 
   # tests
-  addBinToPathHook,
-  gitMinimal,
   versionCheckHook,
 
   # Optional features
@@ -73,46 +71,16 @@ python3Packages.buildPythonApplication (finalAttrs: {
     patch
   ];
 
-  # The dummy buildbox-casd scripts spawned by tests/internals/cascache.py use
-  # an `/usr/bin/env sh` shebang, which doesn't exist in the Nix build sandbox.
-  postPatch = ''
-    substituteInPlace tests/internals/cascache.py \
-      --replace-fail '#!/usr/bin/env sh' '#!${runtimeShell}'
-  '';
-
   # /dev/fuse is not available inside the Nix build sandbox, so buildbox-casd's
-  # default FUSE-based staging strategy cannot work here. Force the hardlink/copy
-  # stager instead (this is a real, supported buildbox-casd staging mode, not a
-  # workaround: https://gitlab.com/BuildGrid/buildbox/buildbox/-/blob/main/casd/buildboxcasd_server.cpp).
-  preCheck = ''
-    export BUILDBOX_STAGER=copy-or-link
-  '';
-
+  # default FUSE-based staging strategy cannot work here, and the shebang used
+  # by tests/internals/cascache.py's dummy buildbox-casd scripts
+  # (`/usr/bin/env sh`) doesn't exist there either. The pytest suite is run as
+  # a NixOS VM test instead, where both of those are available; see
+  # `passthru.tests.pytest`.
   pythonImportsCheck = [ "buildstream" ];
 
   nativeCheckInputs = [
-    addBinToPathHook
-    buildbox
-    gitMinimal
-    python3Packages.pexpect
-    python3Packages.pyftpdlib
-    python3Packages.pytest-datafiles
-    python3Packages.pytest-env
-    python3Packages.pytest-timeout
-    python3Packages.pytest-xdist
-    python3Packages.pytestCheckHook
     versionCheckHook
-
-    # Test fixture plugin package used by test_source_mirror_plugin[pip]; upstream
-    # normally installs this via tox before running the pip-origin plugin loading test.
-    (python3Packages.buildPythonPackage {
-      pname = "sample-plugins";
-      version = "1.2.3";
-      pyproject = true;
-      build-system = [ python3Packages.setuptools ];
-      src = "${finalAttrs.src}/tests/plugins/sample-plugins";
-      dontCheck = true;
-    })
   ];
 
   postInstall = ''
@@ -123,8 +91,12 @@ python3Packages.buildPythonApplication (finalAttrs: {
 
   versionCheckProgram = "${placeholder "out"}/bin/bst";
 
-  passthru.updateScript = gitUpdater {
-    ignoredVersions = "dev";
+  passthru = {
+    updateScript = gitUpdater {
+      ignoredVersions = "dev";
+    };
+
+    tests.pytest = nixosTests.buildstream;
   };
 
   meta = {