ci_set_matrix: fall open when no selected family builds anywhere

family_list maps a family to the toolchains that build it, and seven hw/bsp
families are in neither: cxd56, efm32, espressif, f1c100s, pic32mz, py32f0,
same7x. Scoping to one of them intersected to nothing, so every toolchain key was
[], every cmake leg skipped on `if: inputs.build-args != '[]'`, code-metrics took
its no-metrics branch, and the PR went green from a build job that ran no
compiler. The only signal was a stderr line nothing greps for.

Not a coverage regression - master gave the same diff no compile coverage either,
since none of the other families compiles same7x's board.h. What is new is that
the gap used to be masked by the full matrix and is now the whole answer, and
that green now means "ran no compiler" rather than "compiled 64 families".

A selection whose families ALL miss is now unusable rather than empty: it prints
UNSCOPED, which build.yml and .circleci/config.yml already grep to drop the build
extras with it, and emits the full matrix. The two neighbouring cases keep their
own answers - an explicit families: [] is still a legitimate nothing-selected, and
a partial miss still scopes to the families that do build, noting the rest.

The contract test pinned an exact count of fall-open markers, which this would
have broken; it now pins the invariant (every message that emits the full matrix
carries the marker) and was checked to still fail when a marker is removed.

Also corrects the drift guard's note about espressif: hil-build-esp builds its
boards by name, but that job is gated on repository_owner, so on a fork an
espressif-only PR builds nowhere.

3 files changed, 56 insertions, 13 deletions

diff --git a/.github/scripts/ci_set_matrix.py b/.github/scripts/ci_set_matrix.py
index ee3609bed..79f466893 100755
--- a/.github/scripts/ci_set_matrix.py
+++ b/.github/scripts/ci_set_matrix.py
@@ -127,12 +127,22 @@ def set_matrix_json(select=None):
         if sel_fams is not None:
             fams = [f for f in fams if f in sel_fams]
         matrix[toolchain] = fams
-    if sel_fams is not None:
-        # a family this file does not list builds on no toolchain, so the selection maps
-        # to an empty matrix and every leg skips - which looks exactly like a working
-        # scoped run. Say so: hw/bsp holds several families CI has never built
-        # (efm32, py32f0, ...) and espressif, whose boards are built by hil-build-esp
+    if sel_fams:
+        # a family this file does not list builds on no toolchain, so it contributes no
+        # leg. hw/bsp holds several CI has never built (efm32, py32f0, same7x, ...) plus
+        # espressif, whose boards hil-build-esp builds by name.
         unbuilt = sorted(f for f in sel_fams if f not in family_list)
+        if unbuilt and not any(matrix.values()):
+            # NONE of the selected families is buildable here, so every leg would skip
+            # and the PR would go green from a build job that ran no compiler. That is
+            # an unusable selection, not "nothing selected": say UNSCOPED - which
+            # build.yml and .circleci/config.yml both grep for - and emit the full
+            # matrix. An explicit families: [] is still a legitimate nothing-selected,
+            # and a PARTIAL miss still scopes to the families that do build.
+            print(f'ci_set_matrix: UNSCOPED - no selected family is built by any '
+                  f'toolchain here ({", ".join(unbuilt)}), emitting the full matrix',
+                  file=sys.stderr)
+            return set_matrix_json(None)
         if unbuilt:
             print(f'ci_set_matrix: selected families built by no toolchain here: '
                   f'{", ".join(unbuilt)}', file=sys.stderr)
diff --git a/test/hil/test/test_ci_metrics.py b/test/hil/test/test_ci_metrics.py
index 6c236e827..a76b6e3a0 100644
--- a/test/hil/test/test_ci_metrics.py
+++ b/test/hil/test/test_ci_metrics.py
@@ -435,8 +435,17 @@ class TestWorkflowSelectionHandOff(unittest.TestCase):
         self.assertIn('BUILD_SELECT_FILE', self.build)
         scripts = os.path.join(os.path.dirname(CIRCLECI), '.github', 'scripts')
         matrix = open(os.path.join(scripts, 'ci_set_matrix.py')).read()
-        self.assertEqual(matrix.count('ci_set_matrix: UNSCOPED'), 2,
-                         'every fall-open path must print the marker build.yml greps for')
+        # count-independent: pin the INVARIANT, not the number of fall-open paths -
+        # every message that emits the full matrix must carry the marker, and a purely
+        # informational note (a partial family miss) must not claim to have done so.
+        # Adjacent string literals are joined first, since these messages wrap.
+        import re as _re
+        flat = _re.sub(r"['\"]\s*\n\s*f?['\"]", '', matrix)
+        hits = [m.start() for m in _re.finditer('emitting the full ', flat)]
+        self.assertGreaterEqual(len(hits), 2, 'fall-open messages not found')
+        for i in hits:
+            self.assertIn('UNSCOPED', flat[max(0, i - 200):i],
+                          'a fall-open path without the marker build.yml greps for')
 
     def test_membrowse_upload_sees_the_same_board_as_the_build(self):
         # $EX_ARGS is passed for the BOARD it selects: --one-first picks a board that can
diff --git a/test/hil/test/test_ci_select.py b/test/hil/test/test_ci_select.py
index f5c64ac1c..8f1841531 100644
--- a/test/hil/test/test_ci_select.py
+++ b/test/hil/test/test_ci_select.py
@@ -777,12 +777,15 @@ class TestOrphanInvariant(unittest.TestCase):
         for v in vendors:
             self.assertTrue(ci_select.mcu_families(v + '/x.c', REPO), f'{v}: resolves to no family')
 
-    # hw/bsp families ci_set_matrix's family_list does not map to any toolchain. Before
-    # scoping these were harmless - the matrix was always every family in family_list,
-    # so a PR touching one of them still compiled the other 64. Now the selection
-    # intersects to nothing and every leg skips, so a family landing here by accident is
-    # a silent hole. espressif is deliberate: its boards are built by hil-build-esp,
-    # keyed on board name rather than family.
+    # hw/bsp families ci_set_matrix's family_list does not map to any toolchain. Master
+    # gave a PR touching one of these no compile coverage either - none of the other 64
+    # families compiles same7x's board.h - so this is not new. What IS new is that the
+    # gap used to be masked by a full matrix and is now the whole answer, which is why
+    # ci_set_matrix treats a selection that intersects family_list to NOTHING as
+    # unusable (UNSCOPED -> full matrix) rather than emitting an all-empty one.
+    # espressif is here because hil-build-esp builds its boards by name rather than by
+    # family - though only on hathach/tinyusb: that job is gated on repository_owner,
+    # so on a fork an espressif-only PR builds nowhere.
     UNBUILT_FAMILIES = {'cxd56', 'efm32', 'espressif', 'f1c100s', 'pic32mz', 'py32f0',
                         'same7x'}
 
@@ -1505,6 +1508,27 @@ class TestCiSetMatrix(unittest.TestCase):
         self.assertEqual(json.loads(r.stdout), base)
         self.assertIn('ci_set_matrix: UNSCOPED', r.stderr)   # build.yml greps this
 
+    def test_families_no_toolchain_builds_falls_open(self):
+        # hw/bsp/same7x is real but in no toolchain's list, so scoping to it emits an
+        # all-empty matrix: every leg skips and the PR goes green from a build job that
+        # ran no compiler. Unusable, not "nothing selected" - and the marker matters,
+        # because that is what build.yml and CircleCI grep to drop the build extras too.
+        base = json.loads(self.run_matrix().stdout)
+        r = self.run_matrix('--select',
+                            json.dumps({'build': {'full': False, 'families': ['same7x']}}))
+        self.assertEqual(r.returncode, 0, r.stderr)
+        self.assertEqual(json.loads(r.stdout), base)
+        self.assertIn('ci_set_matrix: UNSCOPED', r.stderr)
+
+    def test_a_partial_toolchain_miss_still_scopes(self):
+        # one buildable family is real coverage: scope to it and just note the other
+        r = self.run_matrix('--select', json.dumps(
+            {'build': {'full': False, 'families': ['stm32f4', 'same7x']}}))
+        self.assertEqual(r.returncode, 0, r.stderr)
+        self.assertEqual(json.loads(r.stdout)['arm-gcc'], ['stm32f4'])
+        self.assertNotIn('UNSCOPED', r.stderr)
+        self.assertIn('same7x', r.stderr)
+
     def test_explicit_empty_families_selects_nothing(self):
         # an explicit [] IS a legitimate answer (a diff that builds nothing)
         r = self.run_matrix('--select',