build: make MySQLX bridge dependencies deterministic

.github/workflows/CI-lint-groups-json.yml

@@ -25,6 +25,8 @@ jobs:
         run: python3 test/tap/groups/lint_groups_json.py
       - name: Check AI TAP shard split
         run: python3 test/tap/groups/test_ai_group_shards.py
+      - name: Check TAP Makefile dependency graph
+        run: python3 test/tap/groups/test_makefile_dependencies.py
       - name: Check every TAP source is registered in groups.json
         run: python3 test/tap/groups/check_groups.py --source
       - name: Check cluster simulator coverage contract

test/tap/groups/test_makefile_dependencies.py

@@ -0,0 +1,63 @@
+#!/usr/bin/env python3
+"""Regression contracts for TAP Makefile dependency boundaries."""
+
+import subprocess
+import unittest
+from pathlib import Path
+
+
+ROOT = Path(__file__).resolve().parents[3]
+TAP_TESTS_DIR = ROOT / "test/tap/tests"
+MYSQLX_BRIDGE_TARGETS = (
+    "test_mysqlx_plugin_load-t",
+    "test_mysqlx_admin_tables-t",
+)
+
+
+class MakefileDependencyTest(unittest.TestCase):
+    def test_mysqlx_bridge_targets_share_one_unit_submake(self):
+        result = subprocess.run(
+            [
+                "make",
+                "--no-print-directory",
+                "-C",
+                str(TAP_TESTS_DIR),
+                "-n",
+                "-j2",
+                "MAKE=echo",
+                *MYSQLX_BRIDGE_TARGETS,
+            ],
+            cwd=ROOT,
+            text=True,
+            capture_output=True,
+            check=False,
+            timeout=30,
+        )
+
+        self.assertEqual(result.returncode, 0, result.stdout + result.stderr)
+
+        unit_submakes = [
+            line.split()[2:]
+            for line in result.stdout.splitlines()
+            if line.startswith("-C unit ")
+        ]
+        self.assertEqual(
+            unit_submakes,
+            [list(MYSQLX_BRIDGE_TARGETS)],
+            result.stdout,
+        )
+
+        symlinks = [
+            line
+            for line in result.stdout.splitlines()
+            if line.startswith("ln -fs unit/")
+        ]
+        self.assertCountEqual(
+            symlinks,
+            [f"ln -fs unit/{target} {target}" for target in MYSQLX_BRIDGE_TARGETS],
+            result.stdout,
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()

test/tap/tests/Makefile

@@ -293,15 +293,17 @@ test_tokenizer-t: test_tokenizer-t.cpp $(TAP_LDIR)/libtap$(SHLIB_EXT)
 test_mysql_query_digests_stages-t: test_mysql_query_digests_stages-t.cpp $(TAP_LDIR)/libtap$(SHLIB_EXT)
 	$(CXX) $< $(IDIRS) $(LDIRS) $(OPT) $(MYLIBS) -o $@
 
-.PHONY: test_mysqlx_plugin_load-t
-test_mysqlx_plugin_load-t:
-	$(MAKE) -C unit test_mysqlx_plugin_load-t
-	ln -fs unit/test_mysqlx_plugin_load-t $@
-
-.PHONY: test_mysqlx_admin_tables-t
-test_mysqlx_admin_tables-t:
-	$(MAKE) -C unit test_mysqlx_admin_tables-t
-	ln -fs unit/test_mysqlx_admin_tables-t $@
+MYSQLX_BRIDGE_TESTS := test_mysqlx_plugin_load-t test_mysqlx_admin_tables-t
+
+# Build both bridge tests in one unit submake. Separate recursive makes do not
+# share a dependency graph and can otherwise rebuild common objects such as
+# unit/obj/test_init.o concurrently.
+.PHONY: mysqlx-bridge-tests $(MYSQLX_BRIDGE_TESTS)
+mysqlx-bridge-tests:
+	$(MAKE) -C unit $(MYSQLX_BRIDGE_TESTS)
+
+$(MYSQLX_BRIDGE_TESTS): mysqlx-bridge-tests
+	ln -fs unit/$@ $@
 
 # test_mysqlx_listener_smoke-t was retired with the dormant MysqlxWorker
 # path in 98aee7db2. Listener-lifecycle coverage now lives in