vpn: dump wg transfer counters + routes on handshake timeout for diagnosis

yt_downloader/vpn.py

@@ -220,7 +220,7 @@ def down(cfg) -> None:
     _sudo(["rm", "-rf", f"/etc/netns/{ns}"])
 
 
-def wait_for_handshake(cfg, timeout: float = 15.0) -> bool:
+def wait_for_handshake(cfg, timeout: float = 20.0) -> bool:
     import time
     deadline = time.monotonic() + timeout
     while time.monotonic() < deadline:
@@ -230,6 +230,20 @@ def wait_for_handshake(cfg, timeout: float = 15.0) -> bool:
     return has_handshake(cfg)
 
 
+def _diagnose(cfg) -> str:
+    """Collect tunnel state to explain why a handshake isn't happening."""
+    ns, iface = netns_name(cfg), iface_name(cfg)
+    parts = []
+    for title, args in (
+        ("wg", ["ip", "netns", "exec", ns, "wg", "show", iface]),
+        ("routes", ["ip", "netns", "exec", ns, "ip", "route", "show", "default"]),
+        ("rules", ["ip", "netns", "exec", ns, "ip", "rule", "show"]),
+    ):
+        rc, out = _sudo_out(args)
+        parts.append(f"--- {title} ---\n{out or '(none)'}")
+    return "\n".join(parts)
+
+
 def ensure_up(cfg) -> None:
     if has_handshake(cfg):
         return
@@ -237,11 +251,15 @@ def ensure_up(cfg) -> None:
     down(cfg)  # converge from any stale half-configured state
     up(cfg)
     if not wait_for_handshake(cfg):
+        log.error("no WireGuard handshake within timeout — tunnel state:\n%s",
+                  _diagnose(cfg))
         down(cfg)
         raise RuntimeError(
             "WireGuard tunnel came up but no handshake was established — "
-            "check the endpoint/reachability and that the tunnel is actually "
-            "active. Refusing to download over a dead tunnel."
+            "see tunnel state above. If 'transfer' shows sent bytes but 0 "
+            "received, the endpoint is unreachable (try a different Mullvad "
+            "server); if 0 sent, the namespace has no route out. "
+            "Refusing to download over a dead tunnel."
         )