[Libguestfs] [libnbd PATCH] RFC: api: Add nbd_supports_vsock()

[Eric Blake]

Similar to nbd_supports_tls(), it is nice to know from a feature probe
whether we are likely to have VSOCK support before even trying more
expensive APIs like nbd_connect_uri("nbd+vsock://...").

---

https://bugzilla.redhat.com/show_bug.cgi?id=2069558 documents a case
where AF_VSOCK is compiled, but vsock still doesn't work because
vsock_loopback is not loaded.  Should we make nbd_supports_vsock()
return true only when ALL aspects of vsock are known to be working?
---
 generator/API.ml | 25 ++++++++++++++++++++++---
 lib/handle.c     | 17 +++++++++++++++++
 2 files changed, 39 insertions(+), 3 deletions(-)

diff --git a/generator/API.ml b/generator/API.ml
index b377b9f..7ec85ca 100644
--- a/generator/API.ml
+++ b/generator/API.ml
@@ -1393,7 +1393,8 @@   "connect_uri", {
 =item C<nbds+vsock:>

 Connect over the C<AF_VSOCK> transport, without or with
-TLS respectively.
+TLS respectively. You can use L<nbd_supports_vsock(3)> to
+see if this build of libnbd supports C<AF_VSOCK>.

 =back

@@ -1494,7 +1495,8 @@   "connect_uri", {
     see_also = [URLLink "https://github.com/NetworkBlockDevice/nbd/blob/master/doc/uri.md";
                 Link "aio_connect_uri";
                 Link "set_export_name"; Link "set_tls";
-                Link "set_opt_mode"; Link "get_uri"];
+                Link "set_opt_mode"; Link "get_uri";
+                Link "supports_vsock"; Link "supports_uri"];
   };

   "connect_unix", {
@@ -1522,8 +1524,12 @@   "connect_vsock", {
 C<cid> should be C<2> (to connect to the host), and C<port> might be
 C<10809> or another port number assigned to you by the host
 administrator.
+
+Not all systems support C<AF_VSOCK>; to determine if libnbd was
+built on a system with vsock support, see L<nbd_supports_vsock(3)>.
 " ^ blocking_connect_call_description;
-    see_also = [Link "aio_connect_vsock"; Link "set_opt_mode"];
+    see_also = [Link "aio_connect_vsock"; Link "set_opt_mode";
+                Link "supports_vsock"];
   };

   "connect_tcp", {
@@ -3061,6 +3067,16 @@   "supports_tls", {
     see_also = [Link "set_tls"];
   };

+  "supports_vsock", {
+    default_call with
+    args = []; ret = RBool; is_locked = false; may_set_error = false;
+    shortdesc = "true if libnbd was compiled with support for AF_VSOCK";
+    longdesc = "\
+Returns true if libnbd was compiled with support for the AF_VSOCK family
+of sockets, or false if not.";
+    see_also = [Link "connect_vsock"; Link "connect_uri"];
+  };
+
   "supports_uri", {
     default_call with
     args = []; ret = RBool; is_locked = false; may_set_error = false;
@@ -3233,6 +3249,9 @@ let first_version =
   "set_request_block_size", (1, 12);
   "get_request_block_size", (1, 12);

+  (* Added in 1.13.x development cycle, will be stable and supported in 1.14. *)
+  "supports_vsock", (1, 14);
+
   (* These calls are proposed for a future version of libnbd, but
    * have not been added to any released version so far.
   "get_tls_certificates", (1, ??);
diff --git a/lib/handle.c b/lib/handle.c
index 8713e18..ac64fe9 100644
--- a/lib/handle.c
+++ b/lib/handle.c
@@ -28,6 +28,12 @@
 #include <sys/types.h>
 #include <sys/wait.h>

+#ifdef HAVE_LINUX_VM_SOCKETS_H
+#include <linux/vm_sockets.h>
+#elif HAVE_SYS_VSOCK_H
+#include <sys/vsock.h>
+#endif
+
 #include "internal.h"

 static void
@@ -483,6 +489,17 @@ nbd_unlocked_supports_tls (struct nbd_handle *h)
 #endif
 }

+/* NB: is_locked = false, may_set_error = false. */
+int
+nbd_unlocked_supports_vsock (struct nbd_handle *h)
+{
+#ifdef AF_VSOCK
+  return 1;
+#else
+  return 0;
+#endif
+}
+
 /* NB: is_locked = false, may_set_error = false. */
 int
 nbd_unlocked_supports_uri (struct nbd_handle *h)
-- 
2.37.2