monitor/polling: reduce Cloud Run polling interval.

Cloud Run's routes never change at runtime. Don't poll it for
route changes very often.

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

wgengine/monitor/polling.go

@@ -7,7 +7,9 @@
 package monitor
 
 import (
+	"bytes"
 	"errors"
+	"os"
 	"runtime"
 	"sync"
 	"time"
@@ -42,6 +44,20 @@ func (pm *pollingMon) Close() error {
 	return nil
 }
 
+func (pm *pollingMon) isCloudRun() bool {
+	// https: //cloud.google.com/run/docs/reference/container-contract#env-vars
+	if os.Getenv("K_REVISION") == "" || os.Getenv("K_CONFIGURATION") == "" ||
+		os.Getenv("K_SERVICE") == "" || os.Getenv("PORT") == "" {
+		return false
+	}
+	vers, err := os.ReadFile("/proc/version")
+	if err != nil {
+		pm.logf("Failed to read /proc/version: %v", err)
+		return false
+	}
+	return string(bytes.TrimSpace(vers)) == "Linux version 4.4.0 #1 SMP Sun Jan 10 15:06:54 PST 2016"
+}
+
 func (pm *pollingMon) Receive() (message, error) {
 	d := 10 * time.Second
 	if runtime.GOOS == "android" {
@@ -50,8 +66,12 @@ func (pm *pollingMon) Receive() (message, error) {
 		// https://github.com/tailscale/tailscale/issues/1427
 		d = 10 * time.Minute
 	}
-	// TODO: detect if we're running in Cloud Run, and reduce frequency of
-	// polling as its routes never change.
+	if pm.isCloudRun() {
+		// Cloud Run routes never change at runtime. the containers are killed within
+		// 15 minutes by default, set the interval long enough to be effectively infinite.
+		pm.logf("monitor polling: Cloud Run detected, reduce polling interval to 24h")
+		d = 24 * time.Hour
+	}
 	ticker := time.NewTicker(d)
 	defer ticker.Stop()
 	base := pm.m.InterfaceState()