perf: parallelize captive portal checks with asyncio.gather

Co-authored-by: Horizontal42 <75383519+Horizontal42@users.noreply.github.com>

src/netsleuth/probes/captive_portal.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import asyncio
 from urllib.parse import urlsplit
 
 import httpx
@@ -28,7 +29,10 @@ def classify_portal_response(
         if redirect_host and redirect_host != requested_host:
             return "portal", f"redirected to a different host ({redirect_host})"
     if status == 200 and body:
-        return "portal", f"200 with a non-empty body instead of the expected {expected_status}"
+        return (
+            "portal",
+            f"200 with a non-empty body instead of the expected {expected_status}",
+        )
     return "suspect", f"unexpected status {status}"
 
 
@@ -39,18 +43,23 @@ async def check_captive_portal(
     expected_status: int = 204,
     timeout: float,
 ) -> CaptivePortal:
-    checks: list[PortalCheck] = []
-    for url in urls:
+    async def _check_url(url: str) -> PortalCheck:
         try:
             response = await client.get(url, follow_redirects=False, timeout=timeout)
             verdict, evidence = classify_portal_response(
-                url, response.status_code, response.text, response.headers.get("location"), expected_status
+                url,
+                response.status_code,
+                response.text,
+                response.headers.get("location"),
+                expected_status,
+            )
+            return PortalCheck(
+                url=url, status=response.status_code, verdict=verdict, evidence=evidence
             )
         except httpx.HTTPError as exc:
-            verdict, evidence = "error", str(exc)
-            checks.append(PortalCheck(url=url, status=None, verdict=verdict, evidence=evidence))
-            continue
-        checks.append(PortalCheck(url=url, status=response.status_code, verdict=verdict, evidence=evidence))
+            return PortalCheck(url=url, status=None, verdict="error", evidence=str(exc))
+
+    checks: list[PortalCheck] = await asyncio.gather(*(_check_url(url) for url in urls))
 
     portal_checks = [c for c in checks if c.verdict == "portal"]
     if portal_checks: