windnsapi: Fix incorrect error checking

common/windnsapi/dnsapi_test.go

@@ -0,0 +1,16 @@
+package windnsapi
+
+import (
+	"runtime"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestDNSAPI(t *testing.T) {
+	if runtime.GOOS != "windows" {
+		t.SkipNow()
+	}
+	t.Parallel()
+	require.NoError(t, FlushResolverCache())
+}

common/windnsapi/syscall_windows.go

@@ -3,4 +3,4 @@ package windnsapi
 //go:generate go run golang.org/x/sys/windows/mkwinsyscall -output zsyscall_windows.go syscall_windows.go
 
 // dnsapi.DnsFlushResolverCache is an undocumented function
-//sys FlushResolverCache() (ret error) = dnsapi.DnsFlushResolverCache
+//sys FlushResolverCache() (err error) = dnsapi.DnsFlushResolverCache

common/windnsapi/zsyscall_windows.go

@@ -43,10 +43,10 @@ var (
 	procDnsFlushResolverCache = moddnsapi.NewProc("DnsFlushResolverCache")
 )
 
-func FlushResolverCache() (ret error) {
+func FlushResolverCache() (err error) {
-	r0, _, _ := syscall.Syscall(procDnsFlushResolverCache.Addr(), 0, 0, 0, 0)
+	r1, _, e1 := syscall.Syscall(procDnsFlushResolverCache.Addr(), 0, 0, 0, 0)
-	if r0 != 0 {
+	if r1 == 0 {
-		ret = syscall.Errno(r0)
+		err = errnoErr(e1)
 	}
 	return
 }