lib/utils: add the "Utils.with_nbd_connect_unix" helper function

Connecting to an NBD server temporarily, for a "one-shot" operation, is
quite similar to "Std_utils.with_open_in" and "Std_utils.with_open_out",
as there are cleanup operations regardless of whether the "one-shot"
operation completes successfully or throws an exception.

Introduce the "Utils.with_nbd_connect_unix" function, which takes a Unix
domain socket pathname, a list of metadata contexts to request from the
NBD server, and calls a function with the live NBD server connection.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2027598
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20211210113537.10907-2-lersek@redhat.com>
Acked-by: Richard W.M. Jones <rjones@redhat.com>

lib/Makefile.am

@@ -80,7 +80,7 @@ BOBJECTS = config.cmo $(SOURCES_ML:.ml=.cmo)
 XOBJECTS = $(BOBJECTS:.cmo=.cmx)
 
 OCAMLPACKAGES = \
-	-package str,unix \
+	-package str,unix,nbd \
 	-I $(builddir) \
 	-I $(top_builddir)/common/mlgettext \
 	-I $(top_builddir)/common/mlpcre \

lib/utils.ml

@@ -164,3 +164,15 @@ let rec wait_for_file filename timeout =
   )
 
 let metaversion = Digest.to_hex (Digest.string Config.package_version_full)
+
+let with_nbd_connect_unix ~socket ~meta_contexts ~f =
+  let nbd = NBD.create () in
+  protect
+    ~f:(fun () ->
+          List.iter (NBD.add_meta_context nbd) meta_contexts;
+          NBD.connect_unix nbd socket;
+          protect
+            ~f:(fun () -> f nbd)
+            ~finally:(fun () -> NBD.shutdown nbd)
+       )
+    ~finally:(fun () -> NBD.close nbd)

lib/utils.mli

@@ -77,3 +77,13 @@ val metaversion : string
 
     Eventually we may switch to using an "open metadata" format instead
     (eg. XML). *)
+
+val with_nbd_connect_unix : socket:string ->
+                            meta_contexts:string list ->
+                            f:(NBD.t -> 'a) ->
+                            'a
+(** [with_nbd_connect_unix socket meta_contexts f] calls function [f] with the
+    NBD server at Unix domain socket [socket] connected, and the metadata
+    contexts in [meta_contexts] requested (each of which is not necessarily
+    supported by the server though). The connection is torn down either on
+    normal return or if the function [f] throws an exception. *)