External locking for SQLite3DB()

Closes #368

include/sqlite3db.h

@@ -138,6 +138,7 @@ class SQLite3DB {
 	private:
 	char *url;
 	sqlite3 *db;
+	rwlock_t rwlock;
 	public:
 	char *get_url() const { return url; }
 	sqlite3 *get_db() const { return db; }
@@ -145,6 +146,12 @@ class SQLite3DB {
 	SQLite3DB();
 	~SQLite3DB();
 	int open(char *, int);
+
+	void rdlock();
+	void rdunlock();
+	void wrlock();
+	void wrunlock();
+
 	bool execute(const char *);
 	bool execute_statement(const char *, char **, int *, int *, SQLite3_result **);
 	int return_one_int(const char *);

lib/sqlite3db.cpp

@@ -7,6 +7,7 @@ SQLite3DB::SQLite3DB() {
 	db=NULL;
 	url=NULL;
 	assert_on_error=0;
+	spinlock_rwlock_init(&rwlock);
 }
 
 SQLite3DB::~SQLite3DB() {
@@ -161,3 +162,18 @@ bool SQLite3DB::check_and_build_table(char *table_name, char *table_def) {
 	return rcb;
 }
 
+void SQLite3DB::rdlock() {
+	spin_wrlock(&rwlock);
+}
+
+void SQLite3DB::rdunlock() {
+	spin_wrunlock(&rwlock);
+}
+
+void SQLite3DB::wrlock() {
+	spin_wrlock(&rwlock);
+}
+
+void SQLite3DB::wrunlock() {
+	spin_wrunlock(&rwlock);
+}