the previous patch didn't cover test_version_mismatch; this one is far more universal

test_runner/fixtures/neon_fixtures.py

@@ -460,6 +460,7 @@ class NeonEnvBuilder:
         self.test_name = test_name
         self.compatibility_neon_binpath = compatibility_neon_binpath
         self.compatibility_pg_distrib_dir = compatibility_pg_distrib_dir
+        self.test_may_use_compatibility_snapshot_binaries = False
         self.version_combination = combination
         self.mixdir = self.test_output_dir / "mixdir_neon"
         if self.version_combination is not None:
@@ -471,6 +472,7 @@ class NeonEnvBuilder:
             ), "the environment variable COMPATIBILITY_POSTGRES_DISTRIB_DIR is required when using mixed versions"
             self.mixdir.mkdir(mode=0o755, exist_ok=True)
             self._mix_versions()
+            self.test_may_use_compatibility_snapshot_binaries = True
 
     def init_configs(self, default_remote_storage_if_missing: bool = True) -> NeonEnv:
         # Cannot create more than one environment from one builder
@@ -1133,7 +1135,13 @@ class NeonEnv:
             # Concurrent IO (https://github.com/neondatabase/neon/issues/9378):
             # enable concurrent IO by default in tests and benchmarks.
             # Compat tests are exempt because old versions fail to parse the new config.
-            if self.pageserver_get_vectored_concurrent_io is not None:
+            get_vectored_concurrent_io = self.pageserver_get_vectored_concurrent_io
+            if config.test_may_use_compatibility_snapshot_binaries:
+                log.info(
+                    "Forcing use of binary-built-in default to avoid forward-compatibility related test failures"
+                )
+                get_vectored_concurrent_io = None
+            if get_vectored_concurrent_io is not None:
                 ps_cfg["get_vectored_concurrent_io"] = {
                     "mode": self.pageserver_get_vectored_concurrent_io,
                 }

test_runner/regress/test_compatibility.py

@@ -251,14 +251,7 @@ def test_forward_compatibility(
         os.environ.get("ALLOW_FORWARD_COMPATIBILITY_BREAKAGE", "false").lower() == "true"
     )
 
-    # Concurrent I/O (https://github.com/neondatabase/neon/issues/9378):
-    # As of 2025-10-22, the compatibility snapshot (=last release) does not
-    # yet support the pageserver.toml config option that is set in the toml
-    # if pageserver_get_vectored_concurrent_io is not None. So, don't set it
-    # to make this test pass. We will not start using the new config option
-    # until we are certain that we will never roll back to a release that
-    # doesn't understand the config option.
-    neon_env_builder.pageserver_get_vectored_concurrent_io = None
+    neon_env_builder.test_may_use_compatibility_snapshot_binaries = True
 
     try:
         neon_env_builder.num_safekeepers = 3