Reinstate and deprecate original transaction function

To reduce disruption to existing users of the transaction function,
this commit replaces the original function (alongside the new
alternatives) and adds comments to say it is deprecated and to
warn explicitly about its potential to retry for ever.

Signed-off-by: Thomas Sanders <thomas.sanders@citrix.com>

client_unix/xs_client_unix.ml

@@ -406,5 +406,11 @@ module Client = functor(IO: IO with type 'a t = 'a) -> struct
     with Eagain when (attempts > 1) ->
       transaction_attempts (attempts-1) client f
 
+  (** Deprecated: retries for ever on repeated Eagain *)
+  let rec transaction client f =
+    let (h, result) = _transaction_leave_open client f in
+    try _commit h result
+    with Eagain -> transaction client f
+
 end
 

client_unix/xs_client_unix.mli

@@ -77,6 +77,12 @@ module Client : functor(IO: IO) -> sig
       attempts. If the last of those fails with a conflict, the Eagain
       exception will be raised to the caller. *)
 
+  val transaction : client -> (handle -> 'a IO.t) -> 'a IO.t
+  (** DEPRECATED!
+      Access xenstore with a single transaction.  On conflict the
+      operation will be repeated INDEFINITELY, with no guarantee
+      of eventual success or termination. *)
+
   val wait : client -> (handle -> 'a IO.t) -> 'a Task.u
   (** Wait for some condition to become true and return a value.  The
       function argument should throw Eagain if the condition is not