ci: install tox from a hash-pinned requirements file

.github/dependabot.yml

@@ -19,3 +19,11 @@ updates:
       day: 'friday'
       time: '05:00'
       timezone: 'Etc/UTC'
+
+  - package-ecosystem: 'pip'
+    directory: '/.github/tox'
+    schedule:
+      interval: 'weekly'
+      day: 'friday'
+      time: '05:00'
+      timezone: 'Etc/UTC'

.github/tox/requirements.in

@@ -0,0 +1 @@
+tox==4.56.4

.github/tox/requirements.txt

@@ -0,0 +1,64 @@
+#
+# This file is autogenerated by pip-compile with Python 3.13
+# by the following command:
+#
+#    pip-compile --generate-hashes --output-file=requirements.txt --strip-extras requirements.in
+#
+cachetools==7.1.4 \
+    --hash=sha256:323dc4127934744db5b54eb4924482d7edafbf9554e820d1531c2e08c0e4ef54 \
+    --hash=sha256:437f55a4e0c1b01a4f3077cc470e6991d47430970e36fbcb77e2be0df4fc1cd6
+    # via tox
+colorama==0.4.6 \
+    --hash=sha256:08695f5cb7ed6e0531a20572697297273c47b8cae5a63ffc6d6ed5c201be6e44 \
+    --hash=sha256:4f1d9991f5acc0ca119f9d443620b77f9d6b33703e51011c16baf57afb285fc6
+    # via tox
+distlib==0.4.3 \
+    --hash=sha256:4b0ce306c966eb73bc3a7b6abad017c556dadd92c44701562cd528ac7fde4d5b \
+    --hash=sha256:f152097224a0ae24be5a0f6bae1b9359af82133bce63f98a95f86cae1aede9ed
+    # via virtualenv
+filelock==3.29.7 \
+    --hash=sha256:5b481979797ae69e72f0b389d89a80bdd585c260c5b3f1fb9c0a5ba9bb3f195d \
+    --hash=sha256:987db6f789a3a2a59f55081801b2b3697cb97e2a736b5f1a9e99b559285fbc51
+    # via
+    #   python-discovery
+    #   tox
+    #   virtualenv
+packaging==26.2 \
+    --hash=sha256:5fc45236b9446107ff2415ce77c807cee2862cb6fac22b8a73826d0693b0980e \
+    --hash=sha256:ff452ff5a3e828ce110190feff1178bb1f2ea2281fa2075aadb987c2fb221661
+    # via
+    #   pyproject-api
+    #   tox
+platformdirs==4.10.0 \
+    --hash=sha256:31e761a6a0ca04faf7353ea759bdba55652be214725111e5aac52dfa29d4bef7 \
+    --hash=sha256:fb516cdb12eb0d857d0cd85a7c57cea4d060bee4578d6cf5a14dfdf8cbf8784a
+    # via
+    #   python-discovery
+    #   tox
+    #   virtualenv
+pluggy==1.6.0 \
+    --hash=sha256:7dcc130b76258d33b90f61b658791dede3486c3e6bfb003ee5c9bfb396dd22f3 \
+    --hash=sha256:e920276dd6813095e9377c0bc5566d94c932c33b27a3e3945d8389c374dd4746
+    # via tox
+pyproject-api==1.10.1 \
+    --hash=sha256:c2b2726bd7aa9217b6c50b621fef5b2ae5def4d55b779c9e0694c15e0a8517ba \
+    --hash=sha256:fa9e6f66c35b5017e909825d8f2b5d5482ea699d7be809d21c03bd1f7317f36a
+    # via tox
+python-discovery==1.4.4 \
+    --hash=sha256:5cad33982d412c1f3ffb8f9ca4ea292c9680bca3942451d30b69c37fce53a4a3 \
+    --hash=sha256:abebe9120b43453b68c908acfb1e72a19d1a959ed2cb620ad38fc57d08056dbe
+    # via
+    #   tox
+    #   virtualenv
+tomli-w==1.2.0 \
+    --hash=sha256:188306098d013b691fcadc011abd66727d3c414c571bb01b1a174ba8c983cf90 \
+    --hash=sha256:2dd14fac5a47c27be9cd4c976af5a12d87fb1f0b4512f81d69cce3b35ae25021
+    # via tox
+tox==4.56.4 \
+    --hash=sha256:53bac88382b9638a5ce40fbaddd6076e1c8f638751af7bf8e759392c953df2f0 \
+    --hash=sha256:d49e371119ebfafb15054ad7ccff3d03027ccb65195fae3ac656b431b5524055
+    # via -r requirements.in
+virtualenv==21.6.0 \
+    --hash=sha256:bce9d097950fef9d81129b333babfb7767072850c2f1acce0ec536708401bfd1 \
+    --hash=sha256:e18a4d750f2b64dea73e72ffde3922f3c52365fabdc8157ebd3da20d031c4734
+    # via tox

.github/workflows/lf-unit-tests.yml

@@ -34,15 +34,29 @@ jobs:
       - name: 'Checkout repository'
         uses: 'actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0' # v7.0.0
 
-      - name: 'Set up Python'
+      # The interpreter tox builds the test env for. Its absolute path is
+      # handed to tox via `--discover` below, so this step does not rely on
+      # the executable staying on PATH once the next step runs.
+      - name: 'Set up the target Python ${{ matrix.python-version }}'
+        id: 'target-python'
         uses: 'actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1' # v6.3.0
         with:
           python-version: '${{ matrix.python-version }}'
 
+      # The interpreter tox itself runs under. Pinned to one version so that
+      # a single hash-pinned requirements file covers the whole matrix:
+      # tox cannot be resolved for 3.9 and 3.10+ at once, because virtualenv
+      # requires `filelock >= 3.24.2` from 3.10 on and no filelock that new
+      # supports 3.9. Listed last, so `python` and `pip` refer to it.
+      - name: 'Set up the Python that runs tox'
+        uses: 'actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1' # v6.3.0
+        with:
+          python-version: '3.13'
+
       - name: 'Install tox'
-        run: 'pip install tox'
+        run: 'pip install --require-hashes --requirement .github/tox/requirements.txt'
 
       - name: 'Run tox for this Python (all matching ansible-core envs)'
         # `-f pyXYZ` selects every tox env carrying this Python's factor,
         # e.g. py311 -> py311-ansible215/216/217/218.
-        run: 'tox -f py$(echo "${{ matrix.python-version }}" | tr -d ".")'
+        run: 'tox --discover "${{ steps.target-python.outputs.python-path }}" -f "py$(echo "${{ matrix.python-version }}" | tr -d ".")"'