lib/locations: Change default config/data location to new XDG recommendation (fixes #9178, fixes #9179) (#9180)

This makes the new default $XDG_STATE_HOME/syncthing or
~/.local/state/syncthing, while still looking in legacy locations first
for existing installs.

Note that this does not *move* existing installs, and nor should we.
Existing paths will continue to be used as-is, but the user can move the
dir into the new place if they want to use it (as they could prior to
this change as well, for that matter).

### Documentation

Needs update to the config docs about our default locations.

lib/locations/locations_test.go

@@ -0,0 +1,102 @@
+// Copyright (C) 2023 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+//go:build !windows
+
+package locations
+
+import (
+	"testing"
+
+	"golang.org/x/exp/slices"
+)
+
+func TestUnixConfigDir(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		userHome      string
+		xdgConfigHome string
+		xdgStateHome  string
+		filesExist    []string
+		expected      string
+	}{
+		// First some "new installations", no files exist previously.
+
+		// No variables set, use our current default
+		{"/home/user", "", "", nil, "/home/user/.local/state/syncthing"},
+		// Config home set, doesn't matter
+		{"/home/user", "/somewhere/else", "", nil, "/home/user/.local/state/syncthing"},
+		// State home set, use that
+		{"/home/user", "", "/var/state", nil, "/var/state/syncthing"},
+		// State home set, again config home doesn't matter
+		{"/home/user", "/somewhere/else", "/var/state", nil, "/var/state/syncthing"},
+
+		// Now some "upgrades", where we have files in the old locations.
+
+		// Config home set, a file exists in the default location
+		{"/home/user", "/somewhere/else", "", []string{"/home/user/.config/syncthing/config.xml"}, "/home/user/.config/syncthing"},
+		// State home set, a file exists in the default location
+		{"/home/user", "", "/var/state", []string{"/home/user/.config/syncthing/config.xml"}, "/home/user/.config/syncthing"},
+		// Both config home and state home set, a file exists in the default location
+		{"/home/user", "/somewhere/else", "/var/state", []string{"/home/user/.config/syncthing/config.xml"}, "/home/user/.config/syncthing"},
+
+		// Config home set, and a file exists at that place
+		{"/home/user", "/somewhere/else", "", []string{"/somewhere/else/syncthing/config.xml"}, "/somewhere/else/syncthing"},
+		// Config home and state home set, and a file exists in config home
+		{"/home/user", "/somewhere/else", "/var/state", []string{"/somewhere/else/syncthing/config.xml"}, "/somewhere/else/syncthing"},
+	}
+
+	for _, c := range cases {
+		fileExists := func(path string) bool { return slices.Contains(c.filesExist, path) }
+		actual := unixConfigDir(c.userHome, c.xdgConfigHome, c.xdgStateHome, fileExists)
+		if actual != c.expected {
+			t.Errorf("unixConfigDir(%q, %q, %q) == %q, expected %q", c.userHome, c.xdgConfigHome, c.xdgStateHome, actual, c.expected)
+		}
+	}
+}
+
+func TestUnixDataDir(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		userHome     string
+		configDir    string
+		xdgDataHome  string
+		xdgStateHome string
+		filesExist   []string
+		expected     string
+	}{
+		// First some "new installations", no files exist previously.
+
+		// No variables set, use our current default
+		{"/home/user", "", "", "", nil, "/home/user/.local/state/syncthing"},
+		// Data home set, doesn't matter
+		{"/home/user", "", "/somewhere/else", "", nil, "/home/user/.local/state/syncthing"},
+		// State home set, use that
+		{"/home/user", "", "", "/var/state", nil, "/var/state/syncthing"},
+
+		// Now some "upgrades", where we have files in the old locations.
+
+		// A database exists in the old default location, use that
+		{"/home/user", "", "", "", []string{"/home/user/.config/syncthing/index-v0.14.0.db"}, "/home/user/.config/syncthing"},
+		{"/home/user", "/config/dir", "/xdg/data/home", "/xdg/state/home", []string{"/home/user/.config/syncthing/index-v0.14.0.db"}, "/home/user/.config/syncthing"},
+
+		// A database exists in the config dir, use that
+		{"/home/user", "/config/dir", "/xdg/data/home", "/xdg/state/home", []string{"/config/dir/index-v0.14.0.db"}, "/config/dir"},
+
+		// A database exists in the old xdg data home, use that
+		{"/home/user", "/config/dir", "/xdg/data/home", "/xdg/state/home", []string{"/xdg/data/home/syncthing/index-v0.14.0.db"}, "/xdg/data/home/syncthing"},
+	}
+
+	for _, c := range cases {
+		fileExists := func(path string) bool { return slices.Contains(c.filesExist, path) }
+		actual := unixDataDir(c.userHome, c.configDir, c.xdgDataHome, c.xdgStateHome, fileExists)
+		if actual != c.expected {
+			t.Errorf("unixDataDir(%q, %q, %q, %q) == %q, expected %q", c.userHome, c.configDir, c.xdgDataHome, c.xdgStateHome, actual, c.expected)
+		}
+	}
+}