net: create resolver semaphore channels eagerly

src/net/dnsclient_unix.go

@@ -363,7 +363,12 @@ type resolverConfig struct {
 	dnsConfig atomic.Pointer[dnsConfig] // parsed resolv.conf structure used in lookups
 }
 
-var resolvConf resolverConfig
+// ch is created here rather than in init: a channel created inside a synctest
+// bubble stays associated with it, and operating on such a channel from
+// outside the bubble is a fatal error.
+var resolvConf = resolverConfig{
+	ch: make(chan struct{}, 1),
+}
 
 func getSystemDNSConfig() *dnsConfig {
 	return getSystemDNSConfigNamed("/etc/resolv.conf")
@@ -380,10 +385,6 @@ func (conf *resolverConfig) init() {
 	// resolv.conf twice the first time.
 	conf.dnsConfig.Store(dnsReadConfig("/etc/resolv.conf"))
 	conf.lastChecked = time.Now()
-
-	// Prepare ch so that only one update of resolverConfig may
-	// run at once.
-	conf.ch = make(chan struct{}, 1)
 }
 
 // distantFuture is a sentinel time used for tests to signal that

src/net/dnsclient_unix_test.go

@@ -10,6 +10,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"internal/testenv"
 	"maps"
 	"os"
 	"path"
@@ -21,6 +22,7 @@ import (
 	"sync"
 	"sync/atomic"
 	"testing"
+	"testing/synctest"
 	"time"
 
 	"golang.org/x/net/dns/dnsmessage"
@@ -2881,3 +2883,28 @@ func TestEmptyResolvConfReplacedWithConfHaingNameservers(t *testing.T) {
 		t.Fatal("resolv.conf was not re-loaded")
 	}
 }
+
+// Run in a subprocess: a bubble violation is fatal, not a recoverable panic.
+func TestResolverConfigSemaphoresNotBubbled(t *testing.T) {
+	if os.Getenv("GO_NET_TEST_BUBBLE_HELPER") == "1" {
+		synctest.Test(t, func(t *testing.T) {
+			getSystemDNSConfig()
+			getSystemNSS()
+		})
+		// getSystemDNSConfig would skip the send on "options no-reload".
+		if resolvConf.tryAcquireSema() {
+			resolvConf.releaseSema()
+		}
+		if nssConfig.tryAcquireSema() {
+			nssConfig.releaseSema()
+		}
+		return
+	}
+	testenv.MustHaveExec(t)
+	cmd := testenv.Command(t, testenv.Executable(t), "-test.run=^"+t.Name()+"$", "-test.count=1")
+	cmd = testenv.CleanCmdEnv(cmd)
+	cmd.Env = append(cmd.Env, "GO_NET_TEST_BUBBLE_HELPER=1")
+	if out, err := cmd.CombinedOutput(); err != nil {
+		t.Errorf("helper failed: %v\n%s", err, out)
+	}
+}

src/net/nss.go

@@ -16,7 +16,10 @@ const (
 	nssConfigPath = "/etc/nsswitch.conf"
 )
 
-var nssConfig nsswitchConfig
+// ch is created here rather than in init; see resolvConf.
+var nssConfig = nsswitchConfig{
+	ch: make(chan struct{}, 1),
+}
 
 type nsswitchConfig struct {
 	initOnce sync.Once // guards init of nsswitchConfig
@@ -42,7 +45,6 @@ func getSystemNSS() *nssConf {
 func (conf *nsswitchConfig) init() {
 	conf.nssConf = parseNSSConfFile("/etc/nsswitch.conf")
 	conf.lastChecked = time.Now()
-	conf.ch = make(chan struct{}, 1)
 }
 
 // tryUpdate tries to update conf.