perf: skip lock acquisition when no orphaned requests in process_msg

Check orphaned_request_ids truthiness before acquiring the lock.  Since
orphaned requests are rare (only on timeouts), the set is almost always
empty.  Skipping the lock in the common case saves ~57 ns per response.

The unlocked truthiness check on a set is thread-safe under the GIL.
Worst case (false positive): we enter the lock block and re-check, which
is correct.  Worst case (false negative): an orphaned response is
processed normally — acceptable behavior.

Benchmark (2M iters, Python 3.14):
  Empty set (common):     80.6 -> 23.2 ns (3.47x, -57.4 ns/response)
  Non-empty set (rare):   79.7 -> 87.8 ns (+8.1 ns overhead)

Author: mykaul

benchmarks/micro/bench_orphan_lock_skip.py

@@ -0,0 +1,105 @@
+# Copyright ScyllaDB, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""
+Micro-benchmark: orphaned request lock skip in process_msg.
+
+Measures the cost of always acquiring a lock vs checking the set first.
+
+Run:
+    python benchmarks/bench_orphan_lock_skip.py
+"""
+
+import sys
+import timeit
+from threading import Lock
+
+
+def bench():
+    n = 2_000_000
+    lock = Lock()
+    orphaned_set = set()  # empty — the common case
+    stream_id = 42
+    in_flight = 100
+
+    # Old: always acquire lock
+    def old_check():
+        nonlocal in_flight
+        with lock:
+            if stream_id in orphaned_set:
+                in_flight -= 1
+                orphaned_set.remove(stream_id)
+
+    # New: check set first, skip lock if empty
+    def new_check():
+        nonlocal in_flight
+        if orphaned_set:
+            with lock:
+                if stream_id in orphaned_set:
+                    in_flight -= 1
+                    orphaned_set.remove(stream_id)
+
+    print(f"=== orphaned request lock skip ({n:,} iters) ===\n")
+
+    # Warmup
+    for _ in range(10000):
+        old_check()
+        new_check()
+
+    t_old = timeit.timeit(old_check, number=n)
+    t_new = timeit.timeit(new_check, number=n)
+    ns_old = t_old / n * 1e9
+    ns_new = t_new / n * 1e9
+    saving = ns_old - ns_new
+    speedup = ns_old / ns_new if ns_new > 0 else float('inf')
+    print(f"  Empty orphaned set (common case):")
+    print(f"    Old (always lock): {ns_old:.1f} ns")
+    print(f"    New (check first): {ns_new:.1f} ns")
+    print(f"    Saving: {saving:.1f} ns ({speedup:.2f}x)")
+
+    # Non-empty set (rare case) — should still work
+    orphaned_set_full = {99, 100, 101}
+    def old_check_full():
+        nonlocal in_flight
+        with lock:
+            if stream_id in orphaned_set_full:
+                in_flight -= 1
+                orphaned_set_full.remove(stream_id)
+
+    def new_check_full():
+        nonlocal in_flight
+        if orphaned_set_full:
+            with lock:
+                if stream_id in orphaned_set_full:
+                    in_flight -= 1
+                    orphaned_set_full.remove(stream_id)
+
+    for _ in range(10000):
+        old_check_full()
+        new_check_full()
+
+    t_old = timeit.timeit(old_check_full, number=n)
+    t_new = timeit.timeit(new_check_full, number=n)
+    ns_old = t_old / n * 1e9
+    ns_new = t_new / n * 1e9
+    diff = ns_new - ns_old
+    print(f"\n  Non-empty orphaned set (rare case):")
+    print(f"    Old (always lock): {ns_old:.1f} ns")
+    print(f"    New (check first): {ns_new:.1f} ns")
+    print(f"    Overhead: {diff:.1f} ns (extra truthiness check)")
+
+
+if __name__ == "__main__":
+    print(f"Python {sys.version}\n")
+    bench()

cassandra/connection.py

@@ -1395,11 +1395,17 @@ def process_msg(self, header, body):
                 result_metadata = None
             else:
                 need_notify_of_release = False
-                with self.lock:
-                    if stream_id in self.orphaned_request_ids:
-                        self.in_flight -= 1
-                        self.orphaned_request_ids.remove(stream_id)
-                        need_notify_of_release = True
+                # Fast path: skip lock when no orphaned requests (common case).
+                # Reading orphaned_request_ids without the lock is safe: it's a
+                # set and we only check truthiness.  A false negative just means
+                # we'll process the orphaned response normally; a false positive
+                # (rare) falls through to the locked check which is correct.
+                if self.orphaned_request_ids:
+                    with self.lock:
+                        if stream_id in self.orphaned_request_ids:
+                            self.in_flight -= 1
+                            self.orphaned_request_ids.remove(stream_id)
+                            need_notify_of_release = True
                 if need_notify_of_release and self._on_orphaned_stream_released:
                     self._on_orphaned_stream_released()