Tolerate ErrDumpInterrupted when listing tun addresses (#1842)

Backport from #1835 to release 1.11

    ## What

    Since the bump to netlink v1.3.0, `netlink.AddrList` returns `ErrDumpInterrupted` ("results may be incomplete or inconsistent") when the kernel sets `NLM_F_DUMP_INTR` on an address dump. `addIPs` treats any error from `AddrList` as fatal, so a transient interrupted dump aborts startup:

    ```
    failed to start nebula: failed to get tun address list: results may be incomplete or inconsistent
    ```

    Before v1.3.0, the pinned netlink release had no `NLM_F_DUMP_INTR` handling at all and silently returned partial results, so this condition was previously invisible.

    ## Root cause

    The dump is racing nebula's own setup. `RTM_GETADDR` dumps addresses for the whole system, and the kernel flags the dump as interrupted if any address changes between recvmsg batches. When the cert contains an IPv6 network, the `AddrReplace` a few lines above returns while the new address is still tentative; the kernel's DAD worker flips it to preferred asynchronously ~100-400us later, and that flip lands inside the immediately-following dump. The race is entirely self-contained - no concurrent restart or interface churn is required, and the hit rate scales with how many addresses are on the host (more dump batches = wider window).

    Reproduced with a standalone tool replaying this exact netlink sequence on a host with ~80 addresses: an IPv6 /80 alone interrupts 10/200 cycles and v4+v6 together 16/200, while IPv4-only runs 0/800. `ip -ts monitor addr` shows the tentative add and the async flag-clear as two events during the dump window. Hosts with DAD disabled (`net.ipv6.conf.all.accept_dad=0`, e.g. LXC containers) never reproduce, confirming the flip is the trigger. Observed in the wild as dnclient package upgrades reliably failing their post-install restart (systemd start timeout, package left half-configured).

    ## How

    Treat `ErrDumpInterrupted` as success with a warning. netlink still returns the partial result set alongside the error, and this call site only uses the list to prune addresses that are not in the certs; a missed stale address survives until the next reload, which is strictly better than failing startup. All cert addresses were already applied via `AddrReplace` above.

Co-authored-by: John Maguire <john@defined.net>

overlay/tun_linux.go

@@ -5,6 +5,7 @@ package overlay
 
 import (
 	"encoding/binary"
+	"errors"
 	"fmt"
 	"io"
 	"log/slog"
@@ -483,7 +484,16 @@ func (t *tun) addIPs(link netlink.Link) error {
 	//iterate over remainder, remove whoever shouldn't be there
 	al, err := netlink.AddrList(link, netlink.FAMILY_ALL)
 	if err != nil {
-		return fmt.Errorf("failed to get tun address list: %s", err)
+		//RTM_GETADDR dumps the whole system, so any concurrent address change
+		//interrupts it - including the kernel's async tentative->preferred
+		//flip of an IPv6 address the AddrReplace calls above just added,
+		//which makes this a race against our own setup. Partial results are
+		//still returned; the worst case is a stale address surviving until
+		//the next config reload, which beats failing startup over it.
+		if !errors.Is(err, netlink.ErrDumpInterrupted) {
+			return fmt.Errorf("failed to get tun address list: %s", err)
+		}
+		t.l.Warn("tun address list dump was interrupted, stale addresses may remain")
 	}
 
 	for i := range al {