fix(load-snapshot): avoid duplicate zmq bind in multi-tokenizer mode (#27145)

Co-authored-by: Lianmin Zheng <lianminzheng@gmail.com>

test/registered/unit/managers/test_load_snapshot_backends.py

@@ -16,6 +16,7 @@ from sglang.srt.managers.load_snapshot import (
     create_load_snapshot_reader,
     create_load_snapshot_writer,
     should_use_zmq,
+    zmq_reader_owner,
 )
 from sglang.test.ci.ci_register import register_cpu_ci
 from sglang.test.test_utils import CustomTestCase, maybe_stub_sgl_kernel
@@ -217,13 +218,16 @@ class TestFactoryFunctions(CustomTestCase):
             dp_size=1,
             load_balance_method="round_robin",
             node_rank=0,
+            tokenizer_worker_num=1,
         )
         port_args = SimpleNamespace(instance_id="test_shm_factory")
         writer = create_load_snapshot_writer(
             server_args, port_args, dp_size=1, dp_rank=0
         )
         self.assertIsInstance(writer, ShmLoadSnapshotWriter)
-        reader = create_load_snapshot_reader(server_args, port_args, caller="tokenizer")
+        reader = create_load_snapshot_reader(
+            server_args, port_args, caller="TokenizerManager"
+        )
         self.assertIsInstance(reader, ShmLoadSnapshotReader)
         reader.close()
         writer.close()
@@ -240,6 +244,7 @@ class TestFactoryFunctions(CustomTestCase):
             dp_size=1,
             load_balance_method="round_robin",
             node_rank=0,
+            tokenizer_worker_num=1,
         )
         port_args = SimpleNamespace(instance_id="test_zmq_factory")
         os.environ["SGLANG_LOAD_SNAPSHOT_USE_ZMQ"] = "1"
@@ -249,7 +254,7 @@ class TestFactoryFunctions(CustomTestCase):
             )
             self.assertIsInstance(writer, ZmqLoadSnapshotWriter)
             reader = create_load_snapshot_reader(
-                server_args, port_args, caller="tokenizer"
+                server_args, port_args, caller="TokenizerManager"
             )
             self.assertIsInstance(reader, ZmqShmLoadSnapshotReader)
             reader.close()
@@ -270,6 +275,71 @@ class TestFactoryFunctions(CustomTestCase):
         self.assertFalse(should_use_zmq(args))
 
 
+class TestZmqReaderOwner(CustomTestCase):
+    """At most one process binds the zmq PULL socket across all callers."""
+
+    CALLERS = ("TokenizerManager", "MultiTokenizerRouter", "DataParallelController")
+
+    @staticmethod
+    def _args(**overrides):
+        base = dict(
+            enable_dp_attention=True,
+            nnodes=2,
+            node_rank=0,
+            dp_size=1,
+            load_balance_method="round_robin",
+            tokenizer_worker_num=1,
+        )
+        base.update(overrides)
+        return SimpleNamespace(**base)
+
+    def _owners(self, args):
+        return {c for c in self.CALLERS if zmq_reader_owner(args, c)}
+
+    def test_zmq_disabled_no_owner(self):
+        args = self._args(enable_dp_attention=False, nnodes=1)
+        self.assertEqual(self._owners(args), set())
+
+    def test_non_zero_node_rank_no_owner(self):
+        args = self._args(node_rank=1, dp_size=4, tokenizer_worker_num=8)
+        self.assertEqual(self._owners(args), set())
+
+    def test_tokenizer_manager_owns_when_dp1(self):
+        self.assertEqual(self._owners(self._args(dp_size=1)), {"TokenizerManager"})
+
+    def test_multi_tokenizer_router_owns_in_multi_tokenizer_dp1(self):
+        args = self._args(dp_size=1, tokenizer_worker_num=8)
+        self.assertEqual(self._owners(args), {"MultiTokenizerRouter"})
+
+    def test_multi_tokenizer_router_owns_in_multi_tokenizer_round_robin(self):
+        args = self._args(dp_size=4, tokenizer_worker_num=8)
+        self.assertEqual(self._owners(args), {"MultiTokenizerRouter"})
+
+    def test_data_parallel_controller_owns_load_aware(self):
+        for method in ("total_tokens", "total_requests"):
+            args = self._args(
+                dp_size=4, tokenizer_worker_num=8, load_balance_method=method
+            )
+            self.assertEqual(self._owners(args), {"DataParallelController"})
+
+    def test_tokenizer_manager_owns_dp4_round_robin(self):
+        args = self._args(dp_size=4, tokenizer_worker_num=1)
+        self.assertEqual(self._owners(args), {"TokenizerManager"})
+
+    def test_at_most_one_owner_across_configs(self):
+        for dp_size in (1, 4):
+            for tw in (1, 8):
+                for method in ("round_robin", "total_tokens", "total_requests"):
+                    for node_rank in (0, 1):
+                        args = self._args(
+                            dp_size=dp_size,
+                            tokenizer_worker_num=tw,
+                            load_balance_method=method,
+                            node_rank=node_rank,
+                        )
+                        self.assertLessEqual(len(self._owners(args)), 1, args)
+
+
 class TestZmqAddr(CustomTestCase):
     def test_ipc_for_single_node(self):
         port_args = SimpleNamespace(instance_id="myinstance")