chore(pgproto): add a test for large messages

test/pgproto/large_messages_test.py

+from conftest import Postgres
+import psycopg
+
+
+def test_large_messages(postgres: Postgres):
+    user = "postgres"
+    password = "P@ssw0rd"
+    host = postgres.host
+    port = postgres.port
+
+    postgres.instance.sql(f"CREATE USER \"{user}\" WITH PASSWORD '{password}' USING md5")
+    postgres.instance.sql(f'GRANT CREATE TABLE TO "{user}"', sudo=True)
+    conn = psycopg.connect(f"user={user} password={password} host={host} port={port} sslmode=disable")
+    conn.autocommit = True
+
+    conn.execute(
+        """
+        create table "t" (
+            "id" int not null primary key,
+            "value" text
+        )
+        option (timeout = 3);
+        """
+    )
+
+    count = 100
+    payload = "A" * 2**17  # 128 KiB
+
+    # check if pgproto can read messages that are larger than the initial buffer size
+    dml = """ INSERT INTO "t" VALUES """
+    # TODO: dml += ",".join(f""" ({i}, %(payload)s::text """ for i in range(count))
+    dml += ",".join(f""" ({i}, cast (%(payload)s as text)) """ for i in range(count))
+    conn.execute(dml, {"payload": payload})
+
+    # check if pgproto can send messages that are larger than the initial buffer size
+    cur = conn.execute(""" SELECT * FROM "t" """)
+    assert sorted(cur.fetchall()) == [(i, payload) for i in range(count)]