From 4ed7e7bd62c3f72cd7e42843e8a7b2b988256c8f Mon Sep 17 00:00:00 2001
From: "sentry-junior[bot]"
 <264270552+sentry-junior[bot]@users.noreply.github.com>
Date: Thu, 12 Nov 2026 12:19:34 +0000
Subject: [PATCH] fix(tracing): Flush pending span buckets in segment finish
 order

SpanBatcher._pending_flush was a set, so when the flusher woke after
several segments had finished, it sent their traces in hash order.
Use an insertion-ordered dict instead.

Fixes #8027
---
 sentry_sdk/_span_batcher.py        |  8 ++++---
 tests/tracing/test_span_batcher.py | 37 ++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+), 3 deletions(-)

diff --git a/sentry_sdk/_span_batcher.py b/sentry_sdk/_span_batcher.py
index ea67ff6e01..a90dbb7258 100644
--- a/sentry_sdk/_span_batcher.py
+++ b/sentry_sdk/_span_batcher.py
@@ -57,7 +57,9 @@ def __init__(
 
         self._last_full_flush: float = time.monotonic()  # drives time-based flushes
         self._flush_event = threading.Event()
-        self._pending_flush: set[str] = set()  # buckets to be flushed
+        # Buckets to be flushed, in the order they became ready. A dict, not a
+        # set, so that traces are sent in the order their segments finished.
+        self._pending_flush: dict[str, None] = {}
 
         self._flusher: "Optional[threading.Thread]" = None
         self._flusher_pid: "Optional[int]" = None
@@ -83,7 +85,7 @@ def _reset_thread_state(self) -> None:
 
         self._last_full_flush = time.monotonic()
         self._flush_event = threading.Event()
-        self._pending_flush = set()
+        self._pending_flush = {}
 
         self._flusher = None
         self._flusher_pid = None
@@ -143,7 +145,7 @@ def add(self, span: "SpanJSON") -> None:
                     or self._running_size[span["trace_id"]]
                     >= self.MAX_BYTES_BEFORE_FLUSH
                 ):
-                    self._pending_flush.add(span["trace_id"])
+                    self._pending_flush[span["trace_id"]] = None
                     notify = True
                 else:
                     notify = False
diff --git a/tests/tracing/test_span_batcher.py b/tests/tracing/test_span_batcher.py
index f249125c90..bf000e7b31 100644
--- a/tests/tracing/test_span_batcher.py
+++ b/tests/tracing/test_span_batcher.py
@@ -1,5 +1,6 @@
 import os
 import sys
+import threading
 import time
 from unittest import mock
 
@@ -479,6 +480,42 @@ def test_trace_bucket_flushes_when_segment_ends(
     assert items[1].payload["name"] == "segment span"
 
 
+def test_pending_traces_flush_in_segment_finish_order(
+    sentry_init, capture_envelopes, monkeypatch
+):
+    """Traces whose segments finish before the flusher wakes are sent in finish order."""
+    monkeypatch.setattr(SpanBatcher, "FLUSH_WAIT_TIME", 100000)
+
+    # Hold the flusher thread back until every segment below has finished, so
+    # all traces are pending when it runs.
+    flusher_may_run = threading.Event()
+    original_flush_loop = SpanBatcher._flush_loop
+
+    def delayed_flush_loop(self):
+        flusher_may_run.wait()
+        original_flush_loop(self)
+
+    monkeypatch.setattr(SpanBatcher, "_flush_loop", delayed_flush_loop)
+
+    sentry_init(traces_sample_rate=1.0, trace_lifecycle="stream")
+    envelopes = capture_envelopes()
+
+    finished_trace_ids = []
+    for i in range(10):
+        sentry_sdk.traces.new_trace()
+        with sentry_sdk.traces.start_span(name=f"segment {i}") as segment:
+            with sentry_sdk.traces.start_span(name="child"):
+                pass
+        finished_trace_ids.append(segment.trace_id)
+
+    flusher_may_run.set()
+    time.sleep(0.1)
+
+    assert [
+        envelope.headers["trace"]["trace_id"] for envelope in envelopes
+    ] == finished_trace_ids
+
+
 @pytest.mark.skipif(
     sys.platform == "win32"
     or not hasattr(os, "fork")