Preflight checks:

put preflight check inside exchangedb start()
function, and provide a preflight method for
auditordb.

src/auditor/taler-auditor-httpd.c

@@ -372,8 +372,8 @@ handle_mhd_request (void *cls,
     { "/version", MHD_HTTP_METHOD_GET, "application/json",
       NULL, 0,
       &handle_version, MHD_HTTP_OK },
-    /* Landing page, for now tells humans to go away (FIXME: replace
-       with auditor's welcome page!) */
+    /* Landing page, for now tells humans to go away
+     * (NOTE: ideally, the reverse proxy will respond with a nicer page) */
     { "/", MHD_HTTP_METHOD_GET, "text/plain",
       "Hello, I'm the Taler auditor. This HTTP server is not for humans.\n", 0,
       &TAH_MHD_handler_static_response, MHD_HTTP_OK },

src/auditor/taler-auditor-httpd_db.c

@@ -70,7 +70,6 @@ TAH_DB_run_transaction (struct MHD_Connection *connection,
                                              "failed to establish session with database");
     return GNUNET_SYSERR;
   }
-  //  TAH_plugin->preflight (TAH_plugin->cls, session); // FIXME: needed?
   for (unsigned int retries = 0; retries < MAX_TRANSACTION_COMMIT_RETRIES;
        retries++)
   {

src/auditordb/plugin_auditordb_postgres.c

@@ -62,6 +62,8 @@ struct TALER_AUDITORDB_Session
    * Postgres connection handle.
    */
   struct GNUNET_PQ_Context *conn;
+
+  const char *transaction_name;
 };
 
 
@@ -1012,6 +1014,43 @@ postgres_get_session (void *cls)
   return session;
 }
 
+/**
+ * Do a pre-flight check that we are not in an uncommitted transaction.
+ * If we are, try to commit the previous transaction and output a warning.
+ * Does not return anything, as we will continue regardless of the outcome.
+ *
+ * @param cls the `struct PostgresClosure` with the plugin-specific state
+ * @param session the database connection
+ */
+static void
+postgres_preflight (void *cls,
+                    struct TALER_AUDITORDB_Session *session)
+{
+  struct GNUNET_PQ_ExecuteStatement es[] = {
+    GNUNET_PQ_make_execute ("ROLLBACK"),
+    GNUNET_PQ_EXECUTE_STATEMENT_END
+  };
+
+  (void) cls;
+  if (NULL == session->transaction_name)
+    return; /* all good */
+  if (GNUNET_OK ==
+      GNUNET_PQ_exec_statements (session->conn,
+                                 es))
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
+                "BUG: Preflight check committed transaction `%s'!\n",
+                session->transaction_name);
+  }
+  else
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
+                "BUG: Preflight check failed to commit transaction `%s'!\n",
+                session->transaction_name);
+  }
+  session->transaction_name = NULL;
+}
+
 
 /**
  * Start a transaction.
@@ -1029,6 +1068,8 @@ postgres_start (void *cls,
     GNUNET_PQ_EXECUTE_STATEMENT_END
   };
 
+  postgres_preflight (cls,
+                      session);
   (void) cls;
   if (GNUNET_OK !=
       GNUNET_PQ_exec_statements (session->conn,

src/exchangedb/plugin_exchangedb_postgres.c

@@ -78,7 +78,7 @@
 
 
 /**
- * Handle for a database session (per-thread, for transactions).
+ * Handler for a database session (per-thread, for transactions).
  */
 struct TALER_EXCHANGEDB_Session
 {
@@ -1711,6 +1711,17 @@ postgres_get_session (void *cls)
   return session;
 }
 
+/**
+ * Do a pre-flight check that we are not in an uncommitted transaction.
+ * If we are, try to commit the previous transaction and output a warning.
+ * Does not return anything, as we will continue regardless of the outcome.
+ *
+ * @param cls the `struct PostgresClosure` with the plugin-specific state
+ * @param session the database connection
+ */
+static void
+postgres_preflight (void *cls,
+                    struct TALER_EXCHANGEDB_Session *session);
 
 /**
  * Start a transaction.
@@ -1731,6 +1742,9 @@ postgres_start (void *cls,
     GNUNET_PQ_EXECUTE_STATEMENT_END
   };
 
+  postgres_preflight (cls,
+                      session);
+
   (void) cls;
   GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
               "Starting transaction on %p\n",
@@ -1813,7 +1827,7 @@ postgres_preflight (void *cls,
                     struct TALER_EXCHANGEDB_Session *session)
 {
   struct GNUNET_PQ_ExecuteStatement es[] = {
-    GNUNET_PQ_make_execute ("COMMIT"),
+    GNUNET_PQ_make_execute ("ROLLBACK"),
     GNUNET_PQ_EXECUTE_STATEMENT_END
   };