Fix differential CI with ClickHouse 26.9

tests/differential/conftest.py

@@ -83,7 +83,13 @@ def run(
         :param path: Optional persistent ``--path`` directory. By default
             ``clickhouse local`` uses an ephemeral temp dir per process.
         """
-        args = [self.binary, "local", "--query", sql, "--format", fmt]
+        # Select the output format in SQL rather than with the CLI's generic
+        # ``--format`` option.  Since ClickHouse 26.9, that option also selects
+        # the INSERT input format, causing inline ``VALUES`` tuples in the seed
+        # script to be parsed as TSV.  The SQL FORMAT clause works on both the
+        # old 26.5 baseline and current releases.
+        formatted_sql = f"{sql.rstrip().rstrip(';')} FORMAT {fmt}"
+        args = [self.binary, "local", "--query", formatted_sql]
         if path is not None:
             args += ["--path", path]
         proc = subprocess.run(

tests/test_differential_runner.py

@@ -0,0 +1,42 @@
+"""Unit tests for the ClickHouse differential-test subprocess wrapper."""
+
+from __future__ import annotations
+
+import subprocess
+
+from tests.differential import conftest as differential_conftest
+
+
+def test_clickhouse_local_sets_only_the_output_format(monkeypatch):
+    """The output format must not override how inline VALUES are parsed."""
+    calls = []
+
+    def fake_run(args, **kwargs):
+        calls.append((args, kwargs))
+        return subprocess.CompletedProcess(args, 0, stdout="3\n", stderr="")
+
+    monkeypatch.setattr(differential_conftest.subprocess, "run", fake_run)
+
+    sql = (
+        "CREATE TABLE t (x UInt8) ENGINE=Memory; "
+        "INSERT INTO t VALUES (1), (2); "
+        "SELECT sum(x) FROM t"
+    )
+    runner = differential_conftest.ClickHouseLocal("/tmp/clickhouse")
+
+    assert runner.run(sql) == "3\n"
+    assert calls == [
+        (
+            [
+                "/tmp/clickhouse",
+                "local",
+                "--query",
+                f"{sql} FORMAT TabSeparated",
+            ],
+            {
+                "capture_output": True,
+                "text": True,
+                "timeout": 60,
+            },
+        )
+    ]