run with scissors

Switch we switched to processes for concurrency, we can tell SQLite to
disable its intra-process locks.

src/tilemaker.cpp

@@ -107,28 +107,22 @@ int main(const int argc, const char* argv[]) {
 		return 1;
 	}
 
-	if (false) {
-		// See https://github.com/xerial/sqlite-jdbc/issues/59#issuecomment-162115704
-		int rv;
-		rv = sqlite3_config(SQLITE_CONFIG_MEMSTATUS, 0);
-		if (rv) {
-			std::cerr << "fatal: sqlite3_config(SQLITE_CONFIG_MEMSTATUS)=" << std::to_string(rv) << std::endl;
-			return 1;
-		}
+	// See https://github.com/xerial/sqlite-jdbc/issues/59#issuecomment-162115704
+	int rv;
+	rv = sqlite3_config(SQLITE_CONFIG_MEMSTATUS, 0);
+	if (rv) {
+		std::cerr << "fatal: sqlite3_config(SQLITE_CONFIG_MEMSTATUS)=" << std::to_string(rv) << std::endl;
+		return 1;
+	}
 
-		// Per https://www.sqlite.org/c3ref/c_config_covering_index_scan.html#sqliteconfigsinglethread,
-		// this should break everything.
-		//
-		// But things seem to still work, and this avoids the locking we were seeing.
-		//
-		// I would have thought we needed SQLITE_CONFIG_MULTITHREAD ? I feel like
-		// I'm missing something pretty big here, but let's go until we hit a wall.
-		rv = sqlite3_config(SQLITE_CONFIG_SINGLETHREAD);
-		if (rv) {
-			std::cerr << "fatal: sqlite3_config(SQLITE_CONFIG_SINGLETHREAD)=" << std::to_string(rv) << std::endl;
-			return 1;
-		}
+	// We only use a single thread, so tell SQLite to skip its internal locking.
+	// See https://www.sqlite.org/c3ref/c_config_covering_index_scan.html#sqliteconfigsinglethread
+	rv = sqlite3_config(SQLITE_CONFIG_SINGLETHREAD);
+	if (rv) {
+		std::cerr << "fatal: sqlite3_config(SQLITE_CONFIG_SINGLETHREAD)=" << std::to_string(rv) << std::endl;
+		return 1;
 	}
+
 	std::vector<std::shared_ptr<Input>> inputs;
 	for (auto filename : filenames) {
 		std::shared_ptr<Input> input = std::make_shared<Input>();