gh-152721: Fix quadratic RLE replay time in the profiling binary reader

Author: tonghuaroot

Lib/test/test_profiling/test_sampling_profiler/test_binary_format.py

@@ -1175,6 +1175,49 @@ def test_alternating_threads_status_changes(self):
         self.assertEqual(count, 100)
         self.assert_samples_equal(samples, collector)
 
+    def test_rle_alternating_status_batches_correctly(self):
+        """A repeat record whose status alternates every sample replays as N
+        single-status batches with the right cumulative timestamps."""
+        class BatchCollector:
+            def __init__(self):
+                self.batches = []
+
+            def collect(self, stack_frames, timestamps_us):
+                for interp in stack_frames:
+                    for thread in interp.threads:
+                        self.batches.append(
+                            (thread.status, list(timestamps_us))
+                        )
+
+            def export(self, filename):
+                pass
+
+        num_samples = 2000
+        frame = make_frame("rle.py", 42, "rle_func")
+        with tempfile.NamedTemporaryFile(suffix=".bin", delete=False) as f:
+            filename = f.name
+        self.temp_files.append(filename)
+
+        writer = BinaryCollector(filename, 1000, compression="none")
+        expected = []
+        for i in range(num_samples):
+            status = THREAD_STATUS_HAS_GIL if i % 2 else 0
+            ts = 1000 + i
+            expected.append((status, [ts]))
+            sample = [
+                make_interpreter(0, [make_thread(1, [frame], status)])
+            ]
+            writer.collect(sample, timestamp_us=ts)
+        writer.export(None)
+
+        collector = BatchCollector()
+        with BinaryReader(filename) as reader:
+            count = reader.replay_samples(collector)
+
+        self.assertEqual(count, num_samples)
+        self.assertEqual(len(collector.batches), num_samples)
+        self.assertEqual(collector.batches, expected)
+
 
 class TestBinaryStress(BinaryFormatTestBase):
     """Randomized stress tests for binary format."""

Misc/NEWS.d/next/Library/2026-07-01-18-00-00.gh-issue-152721.rlequad.rst

@@ -0,0 +1,2 @@
+Fix quadratic replay time in the :mod:`profiling.sampling` binary reader when a
+profile's run-length-encoded samples alternate thread status.

Modules/_remote_debugging/binary_io_reader.c

@@ -1105,7 +1105,8 @@ binary_reader_replay(BinaryReader *reader, PyObject *collector, PyObject *progre
                             return -1;
                         }
                     }
-                    timestamps_list = PyList_New(count - i);
+                    /* Exact-size the list; alloc+trim is O(count^2). */
+                    timestamps_list = PyList_New(0);
                     if (!timestamps_list) {
                         return -1;
                     }
@@ -1118,7 +1119,13 @@ binary_reader_replay(BinaryReader *reader, PyObject *collector, PyObject *progre
                     Py_DECREF(timestamps_list);
                     return -1;
                 }
-                PyList_SET_ITEM(timestamps_list, batch_idx++, ts_obj);
+                int append_rc = PyList_Append(timestamps_list, ts_obj);
+                Py_DECREF(ts_obj);
+                if (append_rc < 0) {
+                    Py_DECREF(timestamps_list);
+                    return -1;
+                }
+                batch_idx++;
             }
 
             /* Emit final batch */