tests: Stop spawned MPI ranks regenerating the reference norms

tests/test_mpi.py

@@ -1,3 +1,4 @@
+import os
 from functools import cached_property
 
 import numpy as np
@@ -3453,10 +3454,16 @@ def gen_serial_norms(shape, so):
     """
     Computes the norms of the outputs in serial mode to compare with
     """
+    fname = f"norms{len(shape)}.npy"
     day = np.datetime64('today')
     try:
-        l = np.load(f"norms{len(shape)}.npy", allow_pickle=True)
-        assert l[-1] == day
+        l = np.load(fname, allow_pickle=True)
+        # The ranks spawned by the `parallel` marker import this module too, but
+        # must only ever read the norms generated by the parent pytest process.
+        # If the day changed since then, regenerating here would run the "serial"
+        # solver under MPI, with all ranks racing on the same file
+        spawned = int(os.environ.get('DEVITO_PYTEST_FLAG', 0))
+        assert spawned or l[-1] == day
     except:
         tn = 500.  # Final time
         nrec = 130  # Number of receivers
@@ -3475,7 +3482,12 @@ def gen_serial_norms(shape, so):
         Ev = norm(v)
         Esrca = norm(srca)
 
-        np.save(f"norms{len(shape)}.npy", (Eu, Erec, Ev, Esrca, day), allow_pickle=True)
+        # Write to a private file and rename it into place, so that a concurrent
+        # reader (e.g., another xdist worker) never sees a partial file
+        tmpname = f"{fname}.{os.getpid()}.tmp"
+        with open(tmpname, 'wb') as f:
+            np.save(f, (Eu, Erec, Ev, Esrca, day), allow_pickle=True)
+        os.replace(tmpname, fname)
 
 
 class TestIsotropicAcoustic: