[Libguestfs] [virt-v2v PATCH 1/3] lib/nbdkit: add the "Nbdkit.with_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 "Nbdkit.with_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>
---
 lib/Makefile.am |  2 +-
 lib/nbdkit.mli  | 10 ++++++++++
 lib/nbdkit.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/nbdkit.mli b/lib/nbdkit.mli
index ae19295186ed..825755e61dbe 100644
--- a/lib/nbdkit.mli
+++ b/lib/nbdkit.mli
@@ -107,3 +107,13 @@ val run_unix : ?socket:string -> cmd -> string * int
     The --exit-with-parent, --foreground, --pidfile, --newstyle and
     --unix flags are added automatically.  Other flags are set as
     in the {!cmd} struct. *)
+
+val with_connect_unix : socket:string ->
+                        meta_contexts:string list ->
+                        f:(NBD.t -> 'a) ->
+                        'a
+(** [with_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/nbdkit.ml b/lib/nbdkit.ml
index 8d0bfdf9cff1..43e0fc5c728a 100644
--- a/lib/nbdkit.ml
+++ b/lib/nbdkit.ml
@@ -217,3 +217,15 @@ If the messages above are not sufficient to diagnose the problem then add the 
   chmod socket 0o777;
 
   socket, pid
+
+let with_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