Implement exit node check-in tracking and adjust logging for connection errors

server/lib/exitNodes/exitNodeCheckIn.ts

@@ -0,0 +1,14 @@
+// Tracks, per process lifetime, whether a given exit node has ever checked in
+// (called /gerbil/get-config) since this Pangolin instance started. This lets
+// callers distinguish "gerbil hasn't come up yet" (expected briefly after a
+// restart, since gerbil depends on pangolin's container starting first) from
+// "gerbil was reachable and now isn't" (a real problem worth an error log).
+const checkedInExitNodeIds = new Set<number>();
+
+export function markExitNodeCheckedIn(exitNodeId: number): void {
+    checkedInExitNodeIds.add(exitNodeId);
+}
+
+export function hasExitNodeCheckedIn(exitNodeId: number): boolean {
+    return checkedInExitNodeIds.has(exitNodeId);
+}