Make gnc_dbi_safe_sync_all safer.

With SQLite3 and PGSql perform all of the safe_sync actions in a SQL transaction.
Unfortunately MySQL commits the transaction on the first schema-altering
query (CREATE_TABLE in this case) without decrementing sql_savepoint, so
raising an error when we try to release the (non-existent) save point at
the end of writing the tables, so we have to fall back on detecting a failed
safe_sync at the next connection attempt.

Add a GncDbiSqlConnection::check_and_rollback_failed_save() to restore
the database after a failed safe_save; this is performed at the next connection.

src/backend/dbi/gnc-backend-dbi.cpp

@@ -866,6 +866,49 @@ GncDbiBackend<Type>::safe_sync (QofBook* book)
     g_return_if_fail (conn != nullptr);
     g_return_if_fail (book != nullptr);
 
+    ENTER ("book=%p, primary=%p", book, m_book);
+    if (!conn->begin_transaction())
+    {
+        LEAVE("Failed to obtain a transaction.");
+        return;
+    }
+    if (!conn->table_operation (TableOpType::backup))
+    {
+        conn->rollback_transaction();
+        LEAVE ("Failed to rename tables");
+        return;
+    }
+    if (!conn->drop_indexes())
+    {
+        conn->rollback_transaction();
+        LEAVE ("Failed to drop indexes");
+        return;
+    }
+
+    sync(m_book);
+    if (check_error())
+    {
+        conn->rollback_transaction();
+        LEAVE ("Failed to create new database tables");
+        return;
+    }
+    conn->table_operation (TableOpType::drop_backup);
+    conn->commit_transaction();
+    LEAVE ("book=%p", m_book);
+}
+/* MySQL commits the transaction and all savepoints after the first CREATE
+ * TABLE, crashing when we try to RELEASE SAVEPOINT because the savepoint
+ * doesn't exist after the commit. We must run without a wrapping transaction in
+ * that case.
+ */
+template <> void
+GncDbiBackend<DbType::DBI_MYSQL>::safe_sync (QofBook* book)
+{
+    auto conn = dynamic_cast<GncDbiSqlConnection*>(m_conn);
+
+    g_return_if_fail (conn != nullptr);
+    g_return_if_fail (book != nullptr);
+
     ENTER ("book=%p, primary=%p", book, m_book);
     if (!conn->table_operation (TableOpType::backup))
     {

src/backend/dbi/gnc-dbisqlconnection.cpp

@@ -91,6 +91,11 @@ GncDbiSqlConnection::GncDbiSqlConnection (DbType type, QofBackend* qbe,
 {
     if (!lock_database(ignore_lock))
         throw std::runtime_error("Failed to lock database!");
+    if (!check_and_rollback_failed_save())
+    {
+        unlock_database();
+        throw std::runtime_error("A failed safe-save was detected and rolling it back failed.");
+    }
 }
 
 bool
@@ -219,6 +224,15 @@ GncDbiSqlConnection::unlock_database ()
     m_qbe->set_error (ERR_BACKEND_SERVER_ERR);
 }
 
+bool
+GncDbiSqlConnection::check_and_rollback_failed_save()
+{
+    auto backup_tables = m_provider->get_table_list(m_conn, "%back");
+    if (backup_tables.empty())
+        return true;
+    return table_operation(rollback);
+}
+
 GncDbiSqlConnection::~GncDbiSqlConnection()
 {
     if (m_conn)
@@ -314,8 +328,8 @@ GncDbiSqlConnection::begin_transaction () noexcept
             result = dbi_conn_queryf (m_conn, "BEGIN");
         else
         {
-            std::ostringstream savepoint("savepoint_");
-            savepoint << m_sql_savepoint;
+            std::ostringstream savepoint;
+            savepoint << "savepoint_" << m_sql_savepoint;
             result = dbi_conn_queryf(m_conn, "SAVEPOINT %s",
                                      savepoint.str().c_str());
         }
@@ -348,8 +362,8 @@ GncDbiSqlConnection::rollback_transaction () noexcept
         result = dbi_conn_query (m_conn, "ROLLBACK");
     else
     {
-        std::ostringstream savepoint("savepoint_");
-        savepoint << m_sql_savepoint;
+        std::ostringstream savepoint;
+        savepoint << "savepoint_" << m_sql_savepoint - 1;
         result = dbi_conn_queryf(m_conn, "ROLLBACK TO SAVEPOINT %s",
                                  savepoint.str().c_str());
     }
@@ -381,8 +395,8 @@ GncDbiSqlConnection::commit_transaction () noexcept
         result = dbi_conn_queryf (m_conn, "COMMIT");
     else
     {
-        std::ostringstream savepoint("savepoint_");
-        savepoint << m_sql_savepoint;
+        std::ostringstream savepoint;
+        savepoint << "savepoint_" << m_sql_savepoint - 1;
         result = dbi_conn_queryf(m_conn, "RELEASE SAVEPOINT %s",
                                  savepoint.str().c_str());
     }

src/backend/dbi/gnc-dbisqlconnection.hpp

@@ -110,6 +110,7 @@ private:
     unsigned int m_sql_savepoint;
     bool lock_database(bool ignore_lock);
     void unlock_database();
+    bool check_and_rollback_failed_save();
 
 };
 

src/backend/sql/gnc-sql-backend.cpp

@@ -673,9 +673,7 @@ GncSqlBackend::init_version_info() noexcept
 bool
 GncSqlBackend::reset_version_info() noexcept
 {
-    bool ok = true;
-    if (!m_conn->does_table_exist (VERSION_TABLE_NAME))
-        ok = create_table (VERSION_TABLE_NAME, version_table);
+    bool ok = create_table (VERSION_TABLE_NAME, version_table);
     m_versions.clear();
     set_table_version ("Gnucash", gnc_prefs_get_long_version ());
     set_table_version ("Gnucash-Resave", GNUCASH_RESAVE_VERSION);