Fix zero latency bug by validating ping statistics

- Added validation to detect and handle invalid zero latency measurements
- When MinRtt is 0 but packets were received, fall back to AvgRtt
- Added -debug flag to help diagnose ping statistics issues
- Added debug logging to show raw ping statistics for zero latency cases
- Prevents impossible 0s latency readings for remote servers

cmd/mullvadclosest/main.go

@@ -16,6 +16,9 @@ import (
 	"github.com/schollz/progressbar/v3"
 )
 
+// Package-level variable for debug flag
+var debugMode bool
+
 // RelayLatency holds a relay and its associated latency
 type RelayLatency struct {
 	Relay   Relay
@@ -61,6 +64,8 @@ func latencyWorker(id int, jobs <-chan RelayLatency, results chan<- RelayLatency
 				Relay:   relayLatency.Relay,
 				Latency: latency,
 			}
+		} else if debugMode {
+			fmt.Printf("Debug: Error measuring latency for %s: %v\n", relayLatency.Relay.Hostname, err)
 		}
 		_ = bar.Add(1)
 	}
@@ -220,8 +225,12 @@ func CheckMullvadExitIP() (bool, error) {
 func main() {
 	// Define force flag
 	force := flag.Bool("force", false, "Force execution even when connected to VPN")
+	debug := flag.Bool("debug", false, "Enable debug output")
 	flag.Parse()
 
+	// Set the global debug mode
+	debugMode = *debug
+
 	currentUser, err := user.Current()
 	if err != nil {
 		panic("Failed to get current user")

cmd/mullvadclosest/relays.go

@@ -52,6 +52,14 @@ func (r Relay) CheckLiveness() (bool, time.Duration, error) {
 	if stats.PacketsRecv == 0 {
 		return false, 0, nil
 	}
+
+	// Validate that we don't have a zero latency for a successful ping
+	if stats.AvgRtt == 0 {
+		// Return as live but with no initial latency measurement
+		// The actual latency will be measured in MeasureLatency
+		return true, time.Duration(1), nil
+	}
+
 	return true, stats.AvgRtt, nil
 }
 
@@ -69,6 +77,24 @@ func (r Relay) MeasureLatency() (time.Duration, error) {
 		return 0, err
 	}
 	stats := pinger.Statistics()
+
+	// Debug output
+	if debugMode && (stats.MinRtt == 0 || stats.AvgRtt == 0) {
+		fmt.Printf("Debug: %s (%s) - PacketsSent: %d, PacketsRecv: %d, MinRtt: %v, AvgRtt: %v, MaxRtt: %v\n",
+			r.Hostname, r.Ipv4AddrIn, stats.PacketsSent, stats.PacketsRecv,
+			stats.MinRtt, stats.AvgRtt, stats.MaxRtt)
+	}
+
+	// Validate the statistics - if MinRtt is 0 but we received packets, something is wrong
+	if stats.PacketsRecv > 0 && stats.MinRtt == 0 {
+		// Use average RTT if available and non-zero
+		if stats.AvgRtt > 0 {
+			return stats.AvgRtt, nil
+		}
+		// If all RTT values are 0, this is likely a bug - return an error
+		return 0, fmt.Errorf("invalid zero latency for relay %s", r.Hostname)
+	}
+
 	return stats.MinRtt, nil
 }