Keep app states in the tree store

When the data host keeps states (PureData.state and .save, implemented by
pure_tree_data), every state of an app lives in the store: a text state's
draft is its stored value, a list mirrors each change into a stored draft
(record header, then items), and numbers, cursors and indices are stored
together by name. An app starts from what is stored, moves list fields by
name when a record changed, resets cursors and indices that no longer fit,
and refuses damaged stored values.

Each key, text and message event publishes the states it changed together
with the message's document in one store commit, or fails and leaves
memory and store as they were. Frame changes go with the next event or
pure_app_save. pure-run takes --store PATH.

Tests run the same random events on an app in memory and on a stored app
that restarts every few events and require equal states after every step;
kill a process at every commit stage of an event that changes a number, a
text and a list; and check frames, message atomicity and migration by
restart and by replacement. A stored event costs about 10.6 ms more on the
Pi's SD card, mostly two fsyncs.

1 files changed, 3 insertions, 2 deletions

diff --git a/prototypes/connections/module-store.h b/prototypes/connections/module-store.h
index 5f1c1a8..0a4b494 100644
--- a/prototypes/connections/module-store.h
+++ b/prototypes/connections/module-store.h
@@ -1,7 +1,8 @@
 #ifndef PURE_MODULE_STORE_H
 #define PURE_MODULE_STORE_H
 #include "module.h"
-/* Keyed module collections on the selected tree store: one event opens the
- * message key as a held draft and commits it. The path outlives the app. */
+/* Keyed module collections and the app's states on the selected tree store:
+ * one event opens the message key as a held draft and commits it together
+ * with the states the event changed. The path outlives the app. */
 PureData pure_tree_data(const char *path);
 #endif