net/dnsfallback: allow setting log function (#5550)

This broke a test in corp that enforces we don't use the log package.

Signed-off-by: Andrew Dunham <andrew@tailscale.com>

net/dnsfallback/dnsfallback.go

@@ -29,7 +29,9 @@ import (
 	"tailscale.com/net/netns"
 	"tailscale.com/net/tlsdial"
 	"tailscale.com/net/tshttpproxy"
+	"tailscale.com/syncs"
 	"tailscale.com/tailcfg"
+	"tailscale.com/types/logger"
 )
 
 func Lookup(ctx context.Context, host string) ([]netip.Addr, error) {
@@ -77,16 +79,16 @@ func Lookup(ctx context.Context, host string) ([]netip.Addr, error) {
 		if err := ctx.Err(); err != nil {
 			return nil, err
 		}
-		log.Printf("trying bootstrapDNS(%q, %q) for %q ...", cand.dnsName, cand.ip, host)
+		logf("trying bootstrapDNS(%q, %q) for %q ...", cand.dnsName, cand.ip, host)
 		ctx, cancel := context.WithTimeout(ctx, 3*time.Second)
 		defer cancel()
 		dm, err := bootstrapDNSMap(ctx, cand.dnsName, cand.ip, host)
 		if err != nil {
-			log.Printf("bootstrapDNS(%q, %q) for %q error: %v", cand.dnsName, cand.ip, host, err)
+			logf("bootstrapDNS(%q, %q) for %q error: %v", cand.dnsName, cand.ip, host, err)
 			continue
 		}
 		if ips := dm[host]; len(ips) > 0 {
-			log.Printf("bootstrapDNS(%q, %q) for %q = %v", cand.dnsName, cand.ip, host, ips)
+			logf("bootstrapDNS(%q, %q) for %q = %v", cand.dnsName, cand.ip, host, ips)
 			return ips, nil
 		}
 	}
@@ -99,7 +101,7 @@ func Lookup(ctx context.Context, host string) ([]netip.Addr, error) {
 // serverName and serverIP of are, say, "derpN.tailscale.com".
 // queryName is the name being sought (e.g. "controlplane.tailscale.com"), passed as hint.
 func bootstrapDNSMap(ctx context.Context, serverName string, serverIP netip.Addr, queryName string) (dnsMap, error) {
-	dialer := netns.NewDialer(log.Printf)
+	dialer := netns.NewDialer(logf)
 	tr := http.DefaultTransport.(*http.Transport).Clone()
 	tr.Proxy = tshttpproxy.ProxyFromEnvironment
 	tr.DialContext = func(ctx context.Context, netw, addr string) (net.Conn, error) {
@@ -201,7 +203,7 @@ func UpdateCache(c *tailcfg.DERPMap) {
 
 	d, err := json.Marshal(c)
 	if err != nil {
-		log.Printf("[v1] dnsfallback: UpdateCache error marshaling: %v", err)
+		logf("[v1] dnsfallback: UpdateCache error marshaling: %v", err)
 		return
 	}
 
@@ -213,11 +215,10 @@ func UpdateCache(c *tailcfg.DERPMap) {
 	if cachePath != "" {
 		err = atomicfile.WriteFile(cachePath, d, 0600)
 		if err != nil {
-			log.Printf("[v1] dnsfallback: UpdateCache error writing: %v", err)
+			logf("[v1] dnsfallback: UpdateCache error writing: %v", err)
 			return
 		}
 	}
-	log.Printf("[v2] dnsfallback: UpdateCache succeeded")
 }
 
 // SetCachePath sets the path to the on-disk DERP map cache that we store and
@@ -231,17 +232,34 @@ func SetCachePath(path string) {
 
 	f, err := os.Open(path)
 	if err != nil {
-		log.Printf("[v1] dnsfallback: SetCachePath error reading %q: %v", path, err)
+		logf("[v1] dnsfallback: SetCachePath error reading %q: %v", path, err)
 		return
 	}
 	defer f.Close()
 
 	dm := new(tailcfg.DERPMap)
 	if err := json.NewDecoder(f).Decode(dm); err != nil {
-		log.Printf("[v1] dnsfallback: SetCachePath error decoding %q: %v", path, err)
+		logf("[v1] dnsfallback: SetCachePath error decoding %q: %v", path, err)
 		return
 	}
 
 	cachedDERPMap.Store(dm)
-	log.Printf("[v2] dnsfallback: SetCachePath loaded cached DERP map")
+	logf("[v2] dnsfallback: SetCachePath loaded cached DERP map")
+}
+
+// logfunc stores the logging function to use for this package.
+var logfunc syncs.AtomicValue[logger.Logf]
+
+// SetLogger sets the logging function that this package will use. The default
+// logger if this function is not called is 'log.Printf'.
+func SetLogger(log logger.Logf) {
+	logfunc.Store(log)
+}
+
+func logf(format string, args ...any) {
+	if lf := logfunc.Load(); lf != nil {
+		lf(format, args...)
+	} else {
+		log.Printf(format, args...)
+	}
 }

net/dnsfallback/dnsfallback_test.go

@@ -25,6 +25,11 @@ func TestGetDERPMap(t *testing.T) {
 }
 
 func TestCache(t *testing.T) {
+	oldlog := logfunc.Load()
+	SetLogger(t.Logf)
+	t.Cleanup(func() {
+		SetLogger(oldlog)
+	})
 	cacheFile := filepath.Join(t.TempDir(), "cache.json")
 
 	// Write initial cache value
@@ -101,6 +106,11 @@ func TestCache(t *testing.T) {
 }
 
 func TestCacheUnchanged(t *testing.T) {
+	oldlog := logfunc.Load()
+	SetLogger(t.Logf)
+	t.Cleanup(func() {
+		SetLogger(oldlog)
+	})
 	cacheFile := filepath.Join(t.TempDir(), "cache.json")
 
 	// Write initial cache value