Update xdg.RuntimeFile to only use the base runtime directory if it e…

…xists

If the base runtime directory exists, it is used. Otherwise the
operating system's temporary directory is used as a fallback.

base_dirs.go

@@ -1,6 +1,10 @@
 package xdg
 
-import "github.com/adrg/xdg/internal/pathutil"
+import (
+	"os"
+
+	"github.com/adrg/xdg/internal/pathutil"
+)
 
 // XDG Base Directory environment variables.
 const (
@@ -48,7 +52,13 @@ func (bd baseDirectories) cacheFile(relPath string) (string, error) {
 }
 
 func (bd baseDirectories) runtimeFile(relPath string) (string, error) {
-	return pathutil.Create(relPath, []string{bd.runtime})
+	var paths []string
+	for _, p := range pathutil.Unique([]string{bd.runtime, os.TempDir()}) {
+		if pathutil.Exists(p) {
+			paths = append(paths, p)
+		}
+	}
+	return pathutil.Create(relPath, paths)
 }
 
 func (bd baseDirectories) searchDataFile(relPath string) (string, error) {

xdg.go

@@ -161,8 +161,9 @@ func CacheFile(relPath string) (string, error) {
 // The relPath parameter must contain the name of the runtime file, and
 // optionally, a set of parent directories (e.g. appname/app.pid).
 // If the specified directories do not exist, they will be created relative
-// to the base runtime directory. On failure, an error containing the
-// attempted paths is returned.
+// to the base runtime directory. If the base runtime directory does not exist,
+// the operating system's temporary directory is used as a fallback. On failure,
+// an error containing the attempted paths is returned.
 func RuntimeFile(relPath string) (string, error) {
 	return baseDirs.runtimeFile(relPath)
 }