testing numprocesses to fix pytest-xdist tests

overlays/test-exclusions.nix

@@ -4,9 +4,10 @@
 # nixpkgs binary cache cannot be used and upstream test suites run locally.
 # The jeeves builder uses /tmp/nix-builds so filesystem tests run on tmpfs
 # instead of ZFS with normalization=formD and utf8only=on; those tests remain
-# enabled. The remaining exclusions cover a UDP readiness race and
+# enabled. The remaining workarounds cover UDP readiness and nested-worker
-# architecture-dependent floating-point differences whose risk we accept for
+# scheduling races, plus architecture-dependent floating-point differences
-# our workloads. Keep these exceptions visible until their causes are fixed.
+# whose risk we accept for our workloads. Keep these exceptions visible until
+# their causes are fixed.
 _final: prev: {
   gnutls = prev.gnutls.overrideAttrs (old: {
     # This test uses a fixed four-second sleep instead of checking UDP
@@ -18,6 +19,14 @@ _final: prev: {
 
   pythonPackagesExtensions = prev.pythonPackagesExtensions ++ [
     (_pythonFinal: pythonPrev: {
+      pytest-xdist = pythonPrev.pytest-xdist.overridePythonAttrs (old: {
+        # Its acceptance tests start nested worker pools. Limit the outer pool
+        # so deliberately crashing inner workers do not race under heavy load.
+        preCheck = builtins.replaceStrings [ "--numprocesses=$NIX_BUILD_CORES" ] [ "--numprocesses=8" ] (
+          old.preCheck or ""
+        );
+      });
+
       scipy = pythonPrev.scipy.overridePythonAttrs (old: {
         # x86-64-v3 FFT implementations produce rounding differences outside
         # these tests' strict tolerances. We accept the numerical-precision