Add a store handle that shares one mapping

Opening a draft by path opened the directory and the file, mapped the
whole file, read the head, and undid all of it on close: nine system
calls for every GET. pure_store_open keeps the store open for one owner
thread; drafts opened through it share a reference-counted mapping of the
newest version, and opening one only checks whether the file changed (its
identity and its head: two system calls). A commit of its drafts maps the
new version once and moves them all onto it. A compaction by another
writer is noticed and followed, while drafts of the old file keep reading
it and relocate their extents when they commit.

pure_store_data gives modules the same through a handle.

1 files changed, 3 insertions, 0 deletions

diff --git a/prototypes/connections/module-store.h b/prototypes/connections/module-store.h
index 0a4b494..d14ab3d 100644
--- a/prototypes/connections/module-store.h
+++ b/prototypes/connections/module-store.h
@@ -5,4 +5,7 @@
  * 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);
+/* The same through a store handle (state.h): opening a document or state
+ * checks only whether the store changed. The handle outlives the app. */
+PureData pure_store_data(PureStore *);
 #endif