make cross-platform EADDRINUSE check

cmd/humanlog/localhost.go

@@ -29,9 +29,9 @@ import (
 	"github.com/rs/cors"
 	"golang.org/x/net/http2"
 	"golang.org/x/net/http2/h2c"
-	"golang.org/x/sys/unix"
 
 	// imported for side-effect
+	internal_errors "github.com/humanlogio/humanlog/internal/errors"
 	_ "github.com/humanlogio/humanlog/internal/memstorage"
 )
 
@@ -51,7 +51,7 @@ func isEADDRINUSE(err error) bool {
 	if !ok {
 		return false
 	}
-	return nserrno == unix.EADDRINUSE
+	return internal_errors.IsSocketInUse(nserrno)
 }
 
 func startLocalhostServer(

internal/errors/errors_unix.go

@@ -0,0 +1,13 @@
+//go:build !windows
+
+package errors
+
+import (
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+func IsSocketInUse(errno syscall.Errno) bool {
+	return errno == unix.EADDRINUSE
+}

internal/errors/errors_windows.go

@@ -0,0 +1,13 @@
+//go:build windows
+
+package errors
+
+import (
+	"syscall"
+
+	"golang.org/x/sys/windows"
+)
+
+func IsSocketInUse(errno syscall.Errno) bool {
+	return errno == windows.WSAEADDRINUSE
+}