fix: guard resubscribeDelay against inputs that would panic

internal/nostr/subscriptions.go

@@ -101,11 +101,25 @@ func sleepCtx(ctx context.Context, d time.Duration) bool {
 // resubscribeDelay returns the randomised delay for the given attempt, where
 // attempt is 1 for the first retry. The window doubles per attempt up to
 // resubscribeMaxDelay; the returned value is uniform within it.
+//
+// Both bounds are enforced here rather than left to callers. attempt < 1 would
+// shift by a negative amount and rand.Int63n requires a positive argument, and
+// either one panics rather than degrading. An unrecovered panic in a
+// subscription goroutine takes the process down, which drops every
+// subscription at once - the synchronised reconnect this backoff exists to
+// prevent.
 func resubscribeDelay(attempt int) time.Duration {
+	if attempt < 1 {
+		attempt = 1
+	}
+
 	window := resubscribeBaseDelay << min(attempt-1, 5)
 	if window > resubscribeMaxDelay {
 		window = resubscribeMaxDelay
 	}
+	if window <= 0 {
+		return 0
+	}
 
 	return time.Duration(rand.Int63n(int64(window)))
 }

internal/nostr/subscriptions_test.go

@@ -29,6 +29,26 @@ func TestResubscribeDelayStaysWithinWindow(t *testing.T) {
 	}
 }
 
+// attempt values below 1 would shift by a negative amount, which panics at
+// runtime rather than returning a bad value. Nothing calls it that way today,
+// but the guard against it lives here rather than in the callers.
+func TestResubscribeDelayHandlesAttemptsBelowOne(t *testing.T) {
+	for _, attempt := range []int{0, -1, -1000} {
+		func() {
+			defer func() {
+				if r := recover(); r != nil {
+					t.Fatalf("resubscribeDelay(%d) panicked: %v", attempt, r)
+				}
+			}()
+
+			got := resubscribeDelay(attempt)
+			if got < 0 || got >= resubscribeBaseDelay {
+				t.Fatalf("resubscribeDelay(%d) = %v, want within [0, %v)", attempt, got, resubscribeBaseDelay)
+			}
+		}()
+	}
+}
+
 func TestResubscribeDelayGrowsWithAttempts(t *testing.T) {
 	// Compare means rather than single draws, which are random by design.
 	mean := func(attempt int) time.Duration {