command/storage: automatically scan new mounts

Closes https://github.com/MusicPlayerDaemon/MPD/issues/841

src/db/plugins/simple/SimpleDatabasePlugin.cxx

@@ -428,7 +428,7 @@ IsUnsafeChar(char ch)
 	return !IsSafeChar(ch);
 }
 
-void
+bool
 SimpleDatabase::Mount(const char *local_uri, const char *storage_uri)
 {
 	if (cache_path.IsNull())
@@ -447,9 +447,11 @@ SimpleDatabase::Mount(const char *local_uri, const char *storage_uri)
 						   compress);
 	db->Open();
 
-	// TODO: update the new database instance?
+	bool exists = db->FileExists();
 
 	Mount(local_uri, std::move(db));
+
+	return exists;
 }
 
 inline DatabasePtr

src/db/plugins/simple/SimpleDatabasePlugin.hxx

@@ -103,9 +103,11 @@ public:
 
 	/**
 	 * Throws #std::runtime_error on error.
+	 *
+	 * @return false if the mounted database needs to be updated
 	 */
 	gcc_nonnull_all
-	void Mount(const char *local_uri, const char *storage_uri);
+	bool Mount(const char *local_uri, const char *storage_uri);
 
 	gcc_nonnull_all
 	bool Unmount(const char *uri) noexcept;