perf: batch-filter drop_tablets_by_host_id instead of triple pop

Replace the per-tablet reversed pop() loop (O(k*n) for each of three
parallel lists) with a single-pass index filter that rebuilds the
lists once.  This avoids repeated list element shifting and scales
better when many tablets are dropped at once.

Benchmark (3 replicas/tablet, ~33% dropped):
  Tablets   Old (triple-pop)   New (batch-filter)   Speedup
     100          123 us             128 us           ~1.0x
   1,000        1,375 us           1,113 us           1.24x
  10,000       25,429 us          13,079 us           1.94x

Add 3 unit tests for drop_tablets_by_host_id covering matching,
None host_id, and nonexistent host_id.

Author: mykaul

cassandra/tablets.py

@@ -96,15 +96,16 @@ def drop_tablets_by_host_id(self, host_id: Optional[UUID]):
             return
         with self._lock:
             for key, tablets in self._tablets.items():
-                to_be_deleted = []
-                for tablet_id, tablet in enumerate(tablets):
-                    if tablet.replica_contains_host_id(host_id):
-                        to_be_deleted.append(tablet_id)
-
-                for tablet_id in reversed(to_be_deleted):
-                    tablets.pop(tablet_id)
-                    self._first_tokens[key].pop(tablet_id)
-                    self._last_tokens[key].pop(tablet_id)
+                # Filter in one pass instead of popping one-by-one (O(n) vs O(k*n))
+                keep = [i for i, t in enumerate(tablets)
+                        if not t.replica_contains_host_id(host_id)]
+                if len(keep) == len(tablets):
+                    continue  # nothing to drop
+                self._tablets[key] = [tablets[i] for i in keep]
+                first = self._first_tokens[key]
+                last = self._last_tokens[key]
+                self._first_tokens[key] = [first[i] for i in keep]
+                self._last_tokens[key] = [last[i] for i in keep]
 
     def add_tablet(self, keyspace, table, tablet):
         with self._lock:

tests/unit/test_tablets.py

@@ -184,3 +184,38 @@ def gen():
         self.assertEqual(t._replica_dict, {u1: 3, u2: 7})
         self.assertTrue(t.replica_contains_host_id(u1))
         self.assertTrue(t.replica_contains_host_id(u2))
+
+
+class DropTabletsByHostIdTest(unittest.TestCase):
+    """Tests for Tablets.drop_tablets_by_host_id batch-filter path."""
+
+    def test_drop_removes_matching_tablets(self):
+        u1 = UUID('12345678-1234-5678-1234-567812345678')
+        u2 = UUID('87654321-4321-8765-4321-876543218765')
+        t1 = Tablet(0, 100, [(u1, 0)])
+        t2 = Tablet(100, 200, [(u2, 0)])
+        t3 = Tablet(200, 300, [(u1, 1), (u2, 1)])
+        tablets = Tablets({("ks", "tb"): [t1, t2, t3]})
+
+        tablets.drop_tablets_by_host_id(u1)
+
+        remaining = tablets._tablets[("ks", "tb")]
+        self.assertEqual(len(remaining), 1)
+        self.assertIs(remaining[0], t2)
+        # Verify token index lists are in sync
+        self.assertEqual(tablets._first_tokens[("ks", "tb")], [100])
+        self.assertEqual(tablets._last_tokens[("ks", "tb")], [200])
+
+    def test_drop_none_host_id_is_noop(self):
+        t1 = Tablet(0, 100, [("host1", 0)])
+        tablets = Tablets({("ks", "tb"): [t1]})
+        tablets.drop_tablets_by_host_id(None)
+        self.assertEqual(len(tablets._tablets[("ks", "tb")]), 1)
+
+    def test_drop_nonexistent_host_id_is_noop(self):
+        u1 = UUID('12345678-1234-5678-1234-567812345678')
+        u_missing = UUID('aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee')
+        t1 = Tablet(0, 100, [(u1, 0)])
+        tablets = Tablets({("ks", "tb"): [t1]})
+        tablets.drop_tablets_by_host_id(u_missing)
+        self.assertEqual(len(tablets._tablets[("ks", "tb")]), 1)