fs/StandardDirectory: add GetUserCacheDir()

Move code from CreateConfiguredDatabase() and add XDG support.  This
implements an automatic Linux fallback for the setting "db_file" if
none was specified.

NEWS

@@ -34,6 +34,7 @@ ver 0.19 (not yet released)
   - name each thread (for debugging)
 * configuration
   - allow playlist directory without music directory
+  - use XDG to auto-detect "music_directory" and "db_file"
 * new resampler option using libsoxr
 * install systemd unit for socket activation
 * Android port

src/db/Configured.cxx

@@ -23,16 +23,11 @@
 #include "config/ConfigGlobal.hxx"
 #include "config/ConfigData.hxx"
 #include "config/ConfigError.hxx"
+#include "fs/AllocatedPath.hxx"
+#include "fs/StandardDirectory.hxx"
 #include "util/Error.hxx"
 #include "Log.hxx"
 
-#ifdef ANDROID
-#include "Main.hxx"
-#include "android/Context.hxx"
-#include "fs/AllocatedPath.hxx"
-#include "plugins/simple/SimpleDatabasePlugin.hxx"
-#endif
-
 Database *
 CreateConfiguredDatabase(EventLoop &loop, DatabaseListener &listener,
 			 Error &error)
@@ -57,12 +52,9 @@ CreateConfiguredDatabase(EventLoop &loop, DatabaseListener &listener,
 	}
 
 	if (param == nullptr) {
-#ifdef ANDROID
+		/* if there is no override, use the cache directory */
-		/* if there is no override, use the Android cache
-		   directory */
 
-		const AllocatedPath cache_dir =
+		const AllocatedPath cache_dir = GetUserCacheDir();
-			context->GetCacheDir(Java::GetEnv());
 		if (cache_dir.IsNull())
 			return nullptr;
 
@@ -71,9 +63,6 @@ CreateConfiguredDatabase(EventLoop &loop, DatabaseListener &listener,
 		allocated = new config_param("database");
 		allocated->AddBlockParam("path", db_file.c_str(), -1);
 		param = allocated;
-#else
-		return nullptr;
-#endif
 	}
 
 	Database *db = DatabaseGlobalInit(loop, listener, *param,

src/fs/StandardDirectory.cxx

@@ -50,6 +50,8 @@
 #ifdef ANDROID
 #include "java/Global.hxx"
 #include "android/Environment.hxx"
+#include "android/Context.hxx"
+#include "Main.hxx"
 #endif
 
 #ifndef WIN32
@@ -252,6 +254,18 @@ AllocatedPath GetUserMusicDir()
 #endif
 }
 
+AllocatedPath
+GetUserCacheDir()
+{
+#ifdef USE_XDG
+	return GetUserDir("XDG_CACHE_DIR");
+#elif defined(ANDROID)
+	return context->GetCacheDir(Java::GetEnv());
+#else
+	return AllocatedPath::Null();
+#endif
+}
+
 #ifdef WIN32
 
 AllocatedPath GetSystemConfigDir()

src/fs/StandardDirectory.hxx

@@ -33,6 +33,13 @@ AllocatedPath GetUserConfigDir();
  */
 AllocatedPath GetUserMusicDir();
 
+/**
+ * Obtains cache directory for the current user.
+ */
+gcc_pure
+AllocatedPath
+GetUserCacheDir();
+
 #ifdef WIN32
 
 /**