mllib: Add 'protect' function (like try/finally).

Use the function in the virt-v2v [Windows] module.

This is just refactoring.  There is no visible change in
functionality.

mllib/common_utils.ml

@@ -273,6 +273,13 @@ let may f = function
 
 type ('a, 'b) maybe = Either of 'a | Or of 'b
 
+let protect ~f ~finally =
+  let r =
+    try Either (f ())
+    with exn -> Or exn in
+  finally ();
+  match r with Either ret -> ret | Or exn -> raise exn
+
 let istty chan =
   Unix.isatty (Unix.descr_of_out_channel chan)
 

mllib/common_utils.mli

@@ -130,6 +130,18 @@ val may : ('a -> unit) -> 'a option -> unit
 type ('a, 'b) maybe = Either of 'a | Or of 'b
 (** Like the Haskell [Either] type. *)
 
+val protect : f:(unit -> 'a) -> finally:(unit -> unit) -> 'a
+(** Execute [~f] and afterwards execute [~finally].
+
+    If [~f] throws an exception then [~finally] is run and the
+    original exception from [~f] is re-raised.
+
+    If [~finally] throws an exception, then the original exception
+    is lost. (NB: Janestreet core {!Exn.protectx}, on which this
+    function is modelled, doesn't throw away the exception in this
+    case, but requires a lot more work by the caller.  Perhaps we
+    will change this in future.) *)
+
 val prog : string
 (** The program name (derived from {!Sys.executable_name}). *)
 

v2v/windows.ml

@@ -55,16 +55,16 @@ and (=~) str rex =
 let with_hive (g : Guestfs.guestfs) hive_filename ~write f =
   let verbose = verbose () in
   g#hivex_open ~write ~verbose (* ~debug:verbose *) hive_filename;
-  let r =
-    try
+  protect ~f:(
+    fun () ->
       let root = g#hivex_root () in
       let ret = f root in
       if write then g#hivex_commit None;
-      Either ret
-    with exn ->
-      Or exn in
-  g#hivex_close ();
-  match r with Either ret -> ret | Or exn -> raise exn
+      ret
+  ) ~finally:(
+    fun () ->
+      g#hivex_close ()
+  )
 
 (* Find the given node in the current hive, relative to the starting
  * point.  Returns [None] if the node is not found.