all: fix os resolver hangs when all server failed

For os resolver, ctrld queries against all servers concurrently, and get
the first success result back. However, if all server failed, the result
channel is not closed, causing ctrld hang.

Fixing this by closing the result channel once getting back all response
from servers.

While at it, also shorten the backoff time when waiting for network up,
ctrld should serve as fast as possible after network is available.

Updates #34

cmd/ctrld/cli.go

@@ -160,8 +160,7 @@ func initCLI() {
 				log.Fatalf("failed to unmarshal config: %v", err)
 			}
 
-			log.Println("starting ctrld ...")
-			log.Printf("version: %s\n", curVersion())
+			log.Printf("starting ctrld %s\n", curVersion())
 			oi := osinfo.New()
 			log.Printf("os: %s\n", oi.String())
 

internal/net/net.go

@@ -66,7 +66,7 @@ func supportListenIPv6Local() bool {
 }
 
 func probeStack() {
-	b := backoff.NewBackoff("probeStack", func(format string, args ...any) {}, time.Minute)
+	b := backoff.NewBackoff("probeStack", func(format string, args ...any) {}, 5*time.Second)
 	for {
 		if _, err := probeStackDialer.Dial("udp", bootstrapDNS); err == nil {
 			hasNetworkUp = true

resolver.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"net"
+	"sync"
 
 	"github.com/miekg/dns"
 )
@@ -69,8 +70,15 @@ func (o *osResolver) Resolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, error
 
 	dnsClient := &dns.Client{Net: "udp"}
 	ch := make(chan *osResolverResult, numServers)
+	var wg sync.WaitGroup
+	wg.Add(len(o.nameservers))
+	go func() {
+		wg.Wait()
+		close(ch)
+	}()
 	for _, server := range o.nameservers {
 		go func(server string) {
+			defer wg.Done()
 			answer, _, err := dnsClient.ExchangeContext(ctx, msg, server)
 			ch <- &osResolverResult{answer: answer, err: err}
 		}(server)

resolver_test.go

@@ -0,0 +1,29 @@
+package ctrld
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/miekg/dns"
+)
+
+func Test_osResolver_Resolve(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	go func() {
+		defer cancel()
+		resolver := &osResolver{nameservers: []string{"127.0.0.127:5353"}}
+		m := new(dns.Msg)
+		m.SetQuestion("controld.com.", dns.TypeA)
+		m.RecursionDesired = true
+		_, _ = resolver.Resolve(context.Background(), m)
+	}()
+
+	select {
+	case <-time.After(10 * time.Second):
+		t.Error("os resolver hangs")
+	case <-ctx.Done():
+	}
+}