GPUs follow the engine's own CUDA_VISIBLE_DEVICES

Its entries are the cards present, and a local child's allocation is
named through them, so under =2,3 card 0 is device 2. Remote workers
keep mapping against their own mask.

backend/tests/flow/test_resources.py

@@ -33,6 +33,7 @@ def test_the_cards_are_counted_from_the_devices_the_driver_makes(monkeypatch):
     """
     from fluksio.flow import resources
 
+    monkeypatch.delenv("CUDA_VISIBLE_DEVICES", raising=False)
     monkeypatch.setattr(
         resources.glob,
         "glob",
@@ -150,12 +151,37 @@ def test_the_share_becomes_the_thread_limit():
     assert "CUDA_VISIBLE_DEVICES" not in env
 
 
-def test_a_gpu_node_is_told_which_card_is_its():
+def test_a_gpu_node_is_told_which_card_is_its(monkeypatch):
+    monkeypatch.delenv("CUDA_VISIBLE_DEVICES", raising=False)
     env = derive_env(Resources(cpus=1, gpus=2), Allocation(cpus=1, gpus=(1, 3)))
 
     assert env["CUDA_VISIBLE_DEVICES"] == "1,3"
 
 
+@pytest.mark.parametrize(
+    ("mask", "cards", "named"),
+    [(None, 2, "0"), ("2,3", 2, "2"), ("GPU-a1b2, MIG-c3d4", 2, "GPU-a1b2")],
+)
+def test_the_engines_own_mask_is_the_cards_it_has(monkeypatch, mask, cards, named):
+    """Under ``CUDA_VISIBLE_DEVICES=2,3`` card 0 is device 2, not device 0.
+
+    A child's mask replaces the engine's, so an unmapped 0 would be a card
+    the engine was never given. A remote worker maps against its own mask.
+    """
+    from fluksio.flow import resources
+
+    monkeypatch.setattr(resources.glob, "glob", lambda pattern: ["/dev/nvidia0"] * 2)
+    if mask is None:
+        monkeypatch.delenv("CUDA_VISIBLE_DEVICES", raising=False)
+    else:
+        monkeypatch.setenv("CUDA_VISIBLE_DEVICES", mask)
+    wanted, allocation = Resources(cpus=1, gpus=1), Allocation(cpus=1, gpus=(0,))
+
+    assert machine_gpus() == cards
+    assert derive_env(wanted, allocation)["CUDA_VISIBLE_DEVICES"] == named
+    assert derive_env(wanted, allocation, local=False)["CUDA_VISIBLE_DEVICES"] == "0"
+
+
 def test_what_the_node_asked_for_wins():
     """A declaration outranks what the allocation would imply — it is deliberate."""
     wanted = Resources(