metrics: don't clock-rebase unset time sentinels in ReqTimeStats deserialization (#34335)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

python/sglang/srt/observability/req_time_stats.py

@@ -363,7 +363,7 @@ class ReqTimeStatsBase:
                 state["trace_ctx"] = TraceNullContext()
 
         for key in state.keys():
-            if key.endswith("time"):
+            if key.endswith("time") and state[key]:
                 state[key] = convert_time_cross_thread(
                     state[key],
                     state["diff_realtime_monotonic"],

test/registered/unit/observability/test_req_time_stats.py

@@ -0,0 +1,44 @@
+"""Unit tests for ReqTimeStats IPC serialization.
+
+ReqTimeStatsBase.__setstate__ rebases perf_counter fields onto the receiving
+process's clock anchor. Rebasing a field that was never stamped (0.0) turns
+the sentinel into a tiny epsilon (sender_diff - receiver_diff), which defeats
+== 0.0 / > 0.0 "was this stamped?" checks downstream. Concretely, a PD decode
+server never stamps prefill_finished_time locally; if the sentinel arrives at
+the tokenizer as an epsilon, first-token bookkeeping mistakes it for a real
+stamp and the TTFT / inter-token-latency histograms record ~node-uptime-sized
+garbage samples.
+"""
+
+import pickle
+import unittest
+from unittest import mock
+
+import sglang.srt.observability.req_time_stats as rts
+from sglang.test.ci.ci_register import register_cpu_ci
+from sglang.test.test_utils import CustomTestCase
+
+register_cpu_ci(est_time=5, suite="base-a-test-cpu")
+
+
+class TestSetstatePreservesUnsetTimeSentinels(CustomTestCase):
+    def test_two_hop_round_trip(self):
+        src = rts.SchedulerReqTimeStats()
+        src.enable_metrics = True
+        src.wait_queue_entry_time = 123.456
+        src.prefill_finished_time = 0.0
+
+        with mock.patch.object(rts, "global_diff_realtime_monotonic", 1_000_000.0):
+            blob = pickle.dumps(src)
+        with mock.patch.object(rts, "global_diff_realtime_monotonic", 1_000_005.0):
+            hop1 = pickle.loads(blob)
+            blob2 = pickle.dumps(hop1)
+        with mock.patch.object(rts, "global_diff_realtime_monotonic", 1_000_009.0):
+            hop2 = pickle.loads(blob2)
+
+        self.assertEqual(hop2.prefill_finished_time, 0.0)
+        self.assertAlmostEqual(hop2.wait_queue_entry_time, 123.456 - 9.0)
+
+
+if __name__ == "__main__":
+    unittest.main()