[PATCH storage v3 2/2] nfs: use rpcinfo as a showmounts fallback for nfs3

[Maximiliano Sandoval <m.sandoval@proxmox.com>]

The check_connection call must succeed in order for the NFS storage to
be activated. In a setup in which the NFS server does not list exports
and which only supports NFSv3 this would result in the storage not being
usable (not without manually mounting the NFS share and using a
Directory storage instead). We add a fallback check with rpcinfo for
this case.

The rpcinfo check was replaced with showmounts in f8b0d82fe. At the time
the rationale was that rpcinfo did not have support for ipv6
connections, which does not hold anymore.

In order to not break compatibility with setups that already activate
correctly, we add a rpcinfo check as a fallback for NFSv3-only setups
only after the showmounts command fails.

Potential pitfalls:
- Some NFSv3 servers might only support UDP [RFC 1813]
- The two timeouts add up to 20 seconds
- Some NFS servers might not have rpcbind as described in acff89540

[RFC 1813]: https://datatracker.ietf.org/doc/html/rfc1813#section-2.3

Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
---
 src/PVE/Storage/NFSPlugin.pm | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/PVE/Storage/NFSPlugin.pm b/src/PVE/Storage/NFSPlugin.pm
index 56dbd9ec..78f27c33 100644
--- a/src/PVE/Storage/NFSPlugin.pm
+++ b/src/PVE/Storage/NFSPlugin.pm
@@ -198,6 +198,7 @@ sub check_connection {
         # no exports are listed
         $cmd = $nfs_get_rpcinfo_command->($server, '4');
     } else {
+        # TODO PVE 10 Consider switching to rpcinfo altogether.
         $cmd = ['/sbin/showmount', '--no-headers', '--exports', $server];
     }
 
@@ -214,6 +215,12 @@ sub check_connection {
             return 0 if $port == 0;
 
             return PVE::Network::tcp_ping($server, $port, 2);
+        } else {
+            $cmd = $nfs_get_rpcinfo_command->($server, '3');
+            eval {
+                run_command($cmd, timeout => 10, outfunc => sub { }, errfunc => sub { });
+            };
+            return 1 if !$@;
         }
         return 0;
     }
-- 
2.47.3