[Libguestfs] [virt-v2v PATCH v2 1/3] lib/utils: add the "Utils.with_nbd_connect_unix" helper function

[Laszlo Ersek]

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 at redhat.com>
---

Notes:
    v2:
    
    - move the function to the Utils module, from Nbdkit [Rich]
    
    - rename the function to "with_nbd_connect_unix" (from
      "with_connect_unix")
    
    - update the commit message

 lib/Makefile.am |  2 +-
 lib/utils.mli   | 10 ++++++++++
 lib/utils.ml    | 12 ++++++++++++
 3 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/lib/Makefile.am b/lib/Makefile.am
index c274b9ecf6c7..1fab25b326f5 100644
--- a/lib/Makefile.am
+++ b/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 \
diff --git a/lib/utils.mli b/lib/utils.mli
index c9b4bd631d65..f41e825e747d 100644
--- a/lib/utils.mli
+++ b/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. *)
diff --git a/lib/utils.ml b/lib/utils.ml
index c18a467cb57c..86967f342e6e 100644
--- a/lib/utils.ml
+++ b/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)
-- 
2.19.1.3.g30247aa5d201