os: check relative paths in UserConfigDir and UserCacheDir

src/os/file.go

@@ -472,8 +472,8 @@ func TempDir() string {
 // On Windows, it returns %LocalAppData%.
 // On Plan 9, it returns $home/lib/cache.
 //
-// If the location cannot be determined (for example, $HOME is not defined),
-// then it will return an error.
+// If the location cannot be determined (for example, $HOME is not defined) or
+// the path in $XDG_CACHE_HOME is relative, then it will return an error.
 func UserCacheDir() (string, error) {
 	var dir string
 
@@ -506,6 +506,8 @@ func UserCacheDir() (string, error) {
 				return "", errors.New("neither $XDG_CACHE_HOME nor $HOME are defined")
 			}
 			dir += "/.cache"
+		} else if !filepathlite.IsAbs(dir) {
+			return "", errors.New("path in $XDG_CACHE_HOME is relative")
 		}
 	}
 
@@ -523,8 +525,8 @@ func UserCacheDir() (string, error) {
 // On Windows, it returns %AppData%.
 // On Plan 9, it returns $home/lib.
 //
-// If the location cannot be determined (for example, $HOME is not defined),
-// then it will return an error.
+// If the location cannot be determined (for example, $HOME is not defined) or
+// the path in $XDG_CONFIG_HOME is relative, then it will return an error.
 func UserConfigDir() (string, error) {
 	var dir string
 
@@ -557,6 +559,8 @@ func UserConfigDir() (string, error) {
 				return "", errors.New("neither $XDG_CONFIG_HOME nor $HOME are defined")
 			}
 			dir += "/.config"
+		} else if !filepathlite.IsAbs(dir) {
+			return "", errors.New("path in $XDG_CONFIG_HOME is relative")
 		}
 	}
 

src/os/os_test.go

@@ -2817,6 +2817,33 @@ func TestUserCacheDir(t *testing.T) {
 	}
 }
 
+func TestUserCacheDirXDGConfigDirEnvVar(t *testing.T) {
+	switch runtime.GOOS {
+	case "windows", "darwin", "plan9":
+		t.Skip("$XDG_CACHE_HOME is effective only on Unix systems")
+	}
+
+	wd, err := Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("XDG_CACHE_HOME", wd)
+
+	dir, err := UserCacheDir()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if dir != wd {
+		t.Fatalf("UserCacheDir returned %q; want the value of $XDG_CACHE_HOME %q", dir, wd)
+	}
+
+	t.Setenv("XDG_CACHE_HOME", "some-dir")
+	_, err = UserCacheDir()
+	if err == nil {
+		t.Fatal("UserCacheDir succeeded though $XDG_CACHE_HOME contains a relative path")
+	}
+}
+
 func TestUserConfigDir(t *testing.T) {
 	t.Parallel()
 
@@ -2841,6 +2868,33 @@ func TestUserConfigDir(t *testing.T) {
 	}
 }
 
+func TestUserConfigDirXDGConfigDirEnvVar(t *testing.T) {
+	switch runtime.GOOS {
+	case "windows", "darwin", "plan9":
+		t.Skip("$XDG_CONFIG_HOME is effective only on Unix systems")
+	}
+
+	wd, err := Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("XDG_CONFIG_HOME", wd)
+
+	dir, err := UserConfigDir()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if dir != wd {
+		t.Fatalf("UserConfigDir returned %q; want the value of $XDG_CONFIG_HOME %q", dir, wd)
+	}
+
+	t.Setenv("XDG_CONFIG_HOME", "some-dir")
+	_, err = UserConfigDir()
+	if err == nil {
+		t.Fatal("UserConfigDir succeeded though $XDG_CONFIG_HOME contains a relative path")
+	}
+}
+
 func TestUserHomeDir(t *testing.T) {
 	t.Parallel()