Fix memory leak in 'ProxySQL_Admin::stats___mysql_global' - Closes #4976

The resource leak was associated to prepared statements (sqlite3_stmt).
Since the detection of these leaks is tricky, because the resources held
by 'SQLite3' never lose their references, a helper type
('stmt_unique_ptr') and a safer variation for 'prepare_v2' have been
introduced. This helper type should automatically handle the lifetime of
the statements, avoiding these kind of leaks in the future. The non-safe
alternative has been flagged as 'deprecated', so it can be easily
identified in later refactors or slowly removed during development.

Author: JavierJF

include/sqlite3db.h

@@ -6,7 +6,9 @@
 #undef min
 #undef max
 #include <cstdint>
+#include <memory>
 #include <vector>
+#include <utility>
 #define PROXYSQL_SQLITE3DB_PTHREAD_MUTEX
 
 #ifndef SAFE_SQLITE3_STEP2
@@ -165,6 +167,18 @@ class SQLite3_result {
 	void dump_to_stderr();
 };
 
+/**
+ * @brief Helper type for finalizing 'sqlite3_stmt' managed by smart pointers.
+ */
+struct stmt_deleter_t {
+	void operator()(sqlite3_stmt* x) const;
+};
+
+/**
+ * @brief Safe type for automatically deallocation of 'sqlite3_stmt'.
+ */
+using stmt_unique_ptr = std::unique_ptr<sqlite3_stmt, stmt_deleter_t>;
+
 class SQLite3DB {
 	private:
 	char *url;
@@ -191,7 +205,14 @@ class SQLite3DB {
 	int check_table_structure(char *table_name, char *table_def);
 	bool build_table(char *table_name, char *table_def, bool dropit);
 	bool check_and_build_table(char *table_name, char *table_def);
+	[[deprecated("Use safer alternative 'prepare_v2(const char *)'")]]
 	int prepare_v2(const char *, sqlite3_stmt **);
+	/**
+	 * @brief Prepares a query as a statement in the SQLite3DB.
+	 * @param query The query to be prepared as an 'sqlite3_stmt'.
+	 * @return A pair of with shape { err_code, stmt_unique_ptr }.
+	 */
+	std::pair<int,stmt_unique_ptr> prepare_v2(const char* query);
 	static void LoadPlugin(const char *);
 };
 

lib/ProxySQL_Admin_Stats.cpp

@@ -521,12 +521,17 @@ void ProxySQL_Admin::stats___mysql_global() {
 		"INSERT INTO stats_mysql_global VALUES " + generate_multi_rows_query(32, 2)
 	};
 
-	sqlite3_stmt* row_stmt = nullptr;
-	int rc = statsdb->prepare_v2(q_row_insert.c_str(), &row_stmt);
+	int rc = 0;
+
+	stmt_unique_ptr u_row_stmt { nullptr };
+	std::tie(rc, u_row_stmt) = statsdb->prepare_v2(q_row_insert.c_str());
 	ASSERT_SQLITE_OK(rc, statsdb);
-	sqlite3_stmt* bulk_stmt = nullptr;
-	rc = statsdb->prepare_v2(q_bulk_insert.c_str(), &bulk_stmt);
+	sqlite3_stmt* const row_stmt { u_row_stmt.get() };
+
+	stmt_unique_ptr u_bulk_stmt { nullptr };
+	std::tie(rc, u_bulk_stmt) = statsdb->prepare_v2(q_bulk_insert.c_str());
 	ASSERT_SQLITE_OK(rc, statsdb);
+	sqlite3_stmt* const bulk_stmt { u_bulk_stmt.get() };
 
 	sqlite3_bulk_step(statsdb, row_stmt, bulk_stmt, resultset, stats_mysql_global___bind_row);
 

lib/sqlite3db.cpp

@@ -260,6 +260,26 @@ int SQLite3DB::prepare_v2(const char *str, sqlite3_stmt **statement) {
 	return rc;
 }
 
+void stmt_deleter_t::operator()(sqlite3_stmt* x) const {
+	proxy_sqlite3_finalize(x);
+}
+
+std::pair<int,stmt_unique_ptr> SQLite3DB::prepare_v2(const char* query) {
+	int rc { 0 };
+	sqlite3_stmt* stmt { nullptr };
+
+	do {
+		rc = (*proxy_sqlite3_prepare_v2)(db, query, -1, &stmt, nullptr);
+
+		if (rc==SQLITE_LOCKED || rc==SQLITE_BUSY) {
+			struct timespec ts { .tv_sec = 0, .tv_nsec = USLEEP_SQLITE_LOCKED * 1000 };
+			nanosleep(&ts, nullptr);
+		}
+	} while (rc==SQLITE_LOCKED || rc==SQLITE_BUSY);
+
+	return { rc, stmt_unique_ptr(stmt) };
+}
+
 /**
  * @brief Executes a SQL statement and returns the result set.
  *