feat: dynamic health test concurrency

Auto-scale test concurrency to ~10% of proxy count, capped by
test_concurrency config ceiling (default raised from 5 to 25).
Prevents saturating upstream Tor when pool size varies.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_pool.py

@@ -155,6 +155,102 @@ class TestProxyPoolWeight:
         pool.report_failure(hop)  # should not raise
 
 
+class TestDynamicConcurrency:
+    """Test dynamic health test concurrency scaling."""
+
+    def test_scales_to_ten_percent(self):
+        import asyncio
+        from unittest.mock import AsyncMock, patch
+
+        cfg = ProxyPoolConfig(sources=[], test_concurrency=25)
+        pool = ProxyPool(cfg, [], timeout=10.0)
+
+        now = time.time()
+        # add 100 proxies -> effective concurrency = max(3, min(100//10, 25)) = 10
+        for i in range(100):
+            hop = ChainHop(proto="socks5", host=f"10.0.{i // 256}.{i % 256}", port=1080)
+            key = f"socks5://10.0.{i // 256}.{i % 256}:1080"
+            pool._proxies[key] = ProxyEntry(hop=hop, alive=False, last_seen=now)
+
+        captured = {}
+
+        original_semaphore = asyncio.Semaphore
+
+        def capture_semaphore(value):
+            captured["concurrency"] = value
+            return original_semaphore(value)
+
+        with (
+            patch.object(pool, "_test_proxy", new_callable=AsyncMock, return_value=True),
+            patch("s5p.pool.asyncio.Semaphore", side_effect=capture_semaphore),
+        ):
+            asyncio.run(pool._run_health_tests())
+
+        assert captured["concurrency"] == 10
+
+    def test_minimum_of_three(self):
+        import asyncio
+        from unittest.mock import AsyncMock, patch
+
+        cfg = ProxyPoolConfig(sources=[], test_concurrency=25)
+        pool = ProxyPool(cfg, [], timeout=10.0)
+
+        now = time.time()
+        # 5 proxies -> 5//10=0, but min is 3
+        for i in range(5):
+            hop = ChainHop(proto="socks5", host=f"10.0.0.{i}", port=1080)
+            pool._proxies[f"socks5://10.0.0.{i}:1080"] = ProxyEntry(
+                hop=hop, alive=False, last_seen=now,
+            )
+
+        captured = {}
+
+        original_semaphore = asyncio.Semaphore
+
+        def capture_semaphore(value):
+            captured["concurrency"] = value
+            return original_semaphore(value)
+
+        with (
+            patch.object(pool, "_test_proxy", new_callable=AsyncMock, return_value=True),
+            patch("s5p.pool.asyncio.Semaphore", side_effect=capture_semaphore),
+        ):
+            asyncio.run(pool._run_health_tests())
+
+        assert captured["concurrency"] == 3
+
+    def test_capped_by_config(self):
+        import asyncio
+        from unittest.mock import AsyncMock, patch
+
+        cfg = ProxyPoolConfig(sources=[], test_concurrency=5)
+        pool = ProxyPool(cfg, [], timeout=10.0)
+
+        now = time.time()
+        # 1000 proxies -> 1000//10=100, capped at 5
+        for i in range(1000):
+            h = f"10.{i // 65536}.{(i // 256) % 256}.{i % 256}"
+            hop = ChainHop(proto="socks5", host=h, port=1080)
+            key = str(hop)
+            pool._proxies[key] = ProxyEntry(hop=hop, alive=False, last_seen=now)
+
+        captured = {}
+
+        original_semaphore = asyncio.Semaphore
+
+        def capture_semaphore(value):
+            captured["concurrency"] = value
+            return original_semaphore(value)
+
+        with (
+            patch.object(pool, "_test_proxy", new_callable=AsyncMock, return_value=True),
+            patch("s5p.pool.asyncio.Semaphore", side_effect=capture_semaphore),
+        ):
+            asyncio.run(pool._run_health_tests())
+
+        assert captured["concurrency"] == 5
+
+
 class TestProxyPoolHealthTests:
     """Test selective health testing."""