Obtain database backend type from the world's world.mt file by default

README.rst

@@ -122,7 +122,10 @@ max-y:
     Don't draw nodes above this y value, `--max-y 75`
 
 backend:
-    Use specific map backend, supported: sqlite3, leveldb, redis, `--backend leveldb`
+    Use specific map backend, supported: auto, sqlite3, leveldb, redis, `--backend leveldb`
+
+    By default, the backend is 'auto', i.e. it is determined from the backend
+    setting in the world's world.mt file (if found).
 
 geometry <geometry>:
     (see below, under 'centergeometry')

TileGenerator.cpp

@@ -378,27 +378,79 @@ void TileGenerator::parseColorsStream(std::istream &in, const std::string &filen
 	}
 }
 
+std::string TileGenerator::getWorldDatabaseBackend(const std::string &input)
+{
+	string backend;
+
+	std::string worldFile = input + PATH_SEPARATOR + "world.mt";
+	ifstream in;
+	in.open(worldFile.c_str(), ifstream::in);
+	if (!in.is_open())
+		throw std::runtime_error(std::string("Failed to open world.mt file '") + worldFile + "'");
+
+	std::string line;
+	int linenr = 0;
+	for (std::getline(in,line); in.good(); std::getline(in,line)) {
+		linenr++;
+		istringstream iline;
+		iline.str(line);
+		iline >> std::skipws;
+		string variable;
+		string eq;
+		iline >> variable;
+		if (variable != "backend")
+			continue;
+		iline >> eq;
+		iline >> backend;
+		if (in.fail() || eq != "=") {
+			ostringstream oss;
+			oss << "Error parsing 'backend' in world.mt file at line " << linenr;
+			throw std::runtime_error(oss.str());
+		}
+	}
+	in.close();
+	if (backend == "")
+		backend = "sqlite3";
+	return backend;
+}
+
 void TileGenerator::openDb(const std::string &input)
 {
-	if (false) {
-	}
+	string backend = m_backend;
+	bool unsupported = false;
+	if (m_backend == "auto")
+		backend = getWorldDatabaseBackend(input);
+
+	if(backend == "sqlite3") {
 #if USE_SQLITE3
-	else if(m_backend == "sqlite3") {
 		DBSQLite3 *db;
 		m_db = db = new DBSQLite3(input);
 		db->cacheWorldRow = m_sqliteCacheWorldRow;
+#else
+		unsupported = true;
+#endif
 	}
-#endif
+	else if (backend == "leveldb") {
 #if USE_LEVELDB
-	else if(m_backend == "leveldb")
 		m_db = new DBLevelDB(input);
+#else
+		unsupported = true;
 #endif
+	}
+	else if (backend == "redis") {
 #if USE_REDIS
-	else if(m_backend == "redis")
 		m_db = new DBRedis(input);
+#else
+		unsupported = true;
 #endif
+	}
+	else if (m_backend == "auto")
+		throw std::runtime_error(((std::string) "World uses unrecognised database backend: ") + backend);
 	else
-		throw std::runtime_error(((std::string) "Unknown map backend: ") + m_backend);
+		throw std::runtime_error(((std::string) "Internal error: unknown database backend: ") + m_backend);
+
+	if (unsupported)
+		throw std::runtime_error(((std::string) "World uses backend '") + backend + ", which was not enabled at compile-time.");
 }
 
 void TileGenerator::loadBlocks()

TileGenerator.h

@@ -73,6 +73,7 @@ public:
 
 private:
 	void parseColorsStream(std::istream &in, const std::string &filename);
+	std::string getWorldDatabaseBackend(const std::string &input);
 	void openDb(const std::string &input);
 	void loadBlocks();
 	BlockPos decodeBlockPos(int64_t blockId) const;

config.h

@@ -21,45 +21,44 @@
 #ifdef USE_CMAKE_CONFIG_H
 #include "cmake_config.h"
 #else
+#define USE_SQLITE3 1
 #define USE_LEVELDB 0
+#define USE_REDIS 0
 #endif
 
 // List of possible database names (for usage message)
 #if USE_SQLITE3
-#define USAGE_NAME_SQLITE "sqlite3"
+#define USAGE_NAME_SQLITE "/sqlite3"
 #else
 #define USAGE_NAME_SQLITE
 #endif
-#if USE_SQLITE3 && USE_LEVELDB
-#define USAGE_SEP_SQLITE_LEVELDB "/"
-#else
-#define USAGE_SEP_SQLITE_LEVELDB
-#endif
+
 #if USE_LEVELDB
-#define USAGE_NAME_LEVELDB "leveldb"
+#define USAGE_NAME_LEVELDB "/leveldb"
 #else
 #define USAGE_NAME_LEVELDB
 #endif
-#if USE_LEVELDB && USE_REDIS
-#define USAGE_SEP_LEVELDB_REDIS "/"
-#else
-#define USAGE_SEP_LEVELDB_REDIS
-#endif
+
 #if USE_REDIS
-#define USAGE_NAME_REDIS "redis"
+#define USAGE_NAME_REDIS "/redis"
 #else
 #define USAGE_NAME_REDIS
 #endif
-#define USAGE_DATABASES USAGE_NAME_SQLITE USAGE_SEP_SQLITE_LEVELDB USAGE_NAME_LEVELDB USAGE_SEP_LEVELDB_REDIS USAGE_NAME_REDIS
 
-// default database to use
-#if USE_SQLITE3
-#define DEFAULT_BACKEND "sqlite3"
-#elif USE_LEVELDB
-#define DEFAULT_BACKEND "leveldb"
-#elif USE_REDIS
-#define DEFAULT_BACKEND "redis"
-#else
+#define USAGE_DATABASES "auto" USAGE_NAME_SQLITE USAGE_NAME_LEVELDB USAGE_NAME_REDIS
+
+#if !USE_SQLITE3 && !USE_LEVELDB && !USE_REDIS
 #error No database backends configured !
 #endif
 
+// default database to use
+#if USE_SQLITE3 && !USE_LEVELDB && !USE_REDIS
+#define DEFAULT_BACKEND "sqlite3"
+#elif !USE_SQLITE3 && USE_LEVELDB && !USE_REDIS
+#define DEFAULT_BACKEND "leveldb"
+#elif !USE_SQLITE3 && !USE_LEVELDB && USE_REDIS
+#define DEFAULT_BACKEND "redis"
+#else
+#define DEFAULT_BACKEND "auto"
+#endif
+