oxenstored: do not commit read-only transactions

The packet telling us to end the transaction has always carried an
argument telling us whether to commit.

If the transaction made no modifications to the tree, now we ignore
that argument and do not commit: it is just a waste of effort.

This makes read-only transactions immune to conflicts, and means that
we do not need to store any of their details in the history that is
used for assigning blame for conflicts.

We count a transaction as a read-only transaction only if it contains
no operations that modified the tree.

This means that (for example) a transaction that creates a new node
then deletes it would NOT count as read-only, even though it makes no
change overall. A more sophisticated algorithm could judge the
transaction based on comparison of its initial and final states, but
this would add complexity and computational cost.

Signed-off-by: Thomas Sanders <thomas.sanders@citrix.com>
Reviewed-by: Jonathan Davies <jonathan.davies@citrix.com>

tools/ocaml/xenstored/process.ml

@@ -378,6 +378,7 @@ let do_transaction_end con t domains cons data =
 		| x :: _   -> raise (Invalid_argument x)
 		| _        -> raise Invalid_Cmd_Args
 		in
+	let commit = commit && not (Transaction.is_read_only t) in
 	let success =
 		let commit = if commit then Some (fun con trans -> transaction_replay con trans domains cons) else None in
 		History.end_transaction t con (Transaction.get_id t) commit in

tools/ocaml/xenstored/transaction.ml

@@ -128,6 +128,7 @@ let make ?(internal=false) id store =
 let get_store t = t.store
 let get_paths t = t.paths
 
+let is_read_only t = t.paths = []
 let add_wop t ty path = t.paths <- (ty, path) :: t.paths
 let add_operation ~perm t request response =
 	if !Define.maxrequests >= 0