zebra: default ns->info should be default zebra_ns

We were not connecting the default zebra_ns to the default
ns->info at namespace initialization in zebra. Thus, when
we tried to use the `ns_walk_func()` it would ignore the
default zebra_ns since there is no pointer to it from the
ns struct.

Fix this by connecting them in `zebra_ns_init()` and,
if the default ns is not found, exit with failure
since this is not recoverable.

This was found during a crash where we fail to cancel the kernel_read
thread at termination (via the `ns_walk_func()`) and then we
get a netlink notification trying to use the zns struct that has
already been freed.

```
(gdb) bt
\#0  0x00007fc1134dc7bb in raise () from /lib/x86_64-linux-gnu/libc.so.6
\#1  0x00007fc1134c7535 in abort () from /lib/x86_64-linux-gnu/libc.so.6
\#2  0x00007fc113996f8f in core_handler (signo=11, siginfo=0x7ffe5429d070, context=<optimized out>) at lib/sigevent.c:254
\#3  <signal handler called>
\#4  0x0000561880e15449 in if_lookup_by_index_per_ns (ns=0x0, ifindex=174) at zebra/interface.c:269
\#5  0x0000561880e1642c in if_up (ifp=ifp@entry=0x561883076c50) at zebra/interface.c:1043
\#6  0x0000561880e10723 in netlink_link_change (h=0x7ffe5429d8f0, ns_id=<optimized out>, startup=<optimized out>) at zebra/if_netlink.c:1384
\#7  0x0000561880e17e68 in netlink_parse_info (filter=filter@entry=0x561880e17680 <netlink_information_fetch>, nl=nl@entry=0x561882497238, zns=zns@entry=0x7ffe542a5940,
    count=count@entry=5, startup=startup@entry=0) at zebra/kernel_netlink.c:932
\#8  0x0000561880e186a5 in kernel_read (thread=<optimized out>) at zebra/kernel_netlink.c:406
\#9  0x00007fc1139a4416 in thread_call (thread=thread@entry=0x7ffe542a5b70) at lib/thread.c:1599
\FRRouting#10 0x00007fc113974ef8 in frr_run (master=0x5618823c9510) at lib/libfrr.c:1024
\FRRouting#11 0x0000561880e0b916 in main (argc=8, argv=0x7ffe542a5f78) at zebra/main.c:483
```

Ticket: CM-27294
Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>
(cherry picked from commit 257b245)

zebra/zebra_errors.c

@@ -316,6 +316,12 @@ static struct log_ref ferr_zebra_err[] = {
 		.description = "Zebra attempted to look up a interface for a particular vrf_id and interface index, but didn't find anything.",
 		.suggestion = "If you entered a command to trigger this error, make sure you entered the arguments correctly. Check your config file for any potential errors. If these look correct, seek help.",
 	},
+	{
+		.code = EC_ZEBRA_NS_NO_DEFAULT,
+		.title = "Zebra NameSpace failed to find Default",
+		.description = "Zebra NameSpace subsystem failed to find a Default namespace during initialization.",
+		.suggestion = "Open an Issue with all relevant log files and restart FRR",
+	},
 	/* Warnings */
 	{
 		.code = EC_ZEBRAING_LM_PROTO_MISMATCH,

zebra/zebra_errors.h

@@ -72,6 +72,7 @@ enum zebra_log_refs {
 	EC_ZEBRA_NHG_SYNC,
 	EC_ZEBRA_NHG_FIB_UPDATE,
 	EC_ZEBRA_IF_LOOKUP_FAILED,
+	EC_ZEBRA_NS_NO_DEFAULT,
 	/* warnings */
 	EC_ZEBRA_NS_NOTIFY_READ,
 	EC_ZEBRAING_LM_PROTO_MISMATCH,

zebra/zebra_ns.c

@@ -39,6 +39,7 @@
 #include "zebra_pbr.h"
 #include "rib.h"
 #include "table_manager.h"
+#include "zebra_errors.h"
 
 extern struct zebra_privs_t zserv_privs;
 
@@ -67,6 +68,9 @@ static int zebra_ns_new(struct ns *ns)
 {
 	struct zebra_ns *zns;
 
+	if (!ns)
+		return -1;
+
 	if (IS_ZEBRA_DEBUG_EVENT)
 		zlog_info("ZNS %s with id %u (created)", ns->name, ns->ns_id);
 
@@ -185,11 +189,10 @@ int zebra_ns_final_shutdown(struct ns *ns)
 
 int zebra_ns_init(const char *optional_default_name)
 {
+	struct ns *default_ns;
 	ns_id_t ns_id;
 	ns_id_t ns_id_external;
 
-	dzns = zebra_ns_alloc();
-
 	frr_elevate_privs(&zserv_privs) {
 		ns_id = zebra_ns_id_get_default();
 	}
@@ -198,8 +201,16 @@ int zebra_ns_init(const char *optional_default_name)
 
 	logicalrouter_init(logicalrouter_config_write);
 
+	default_ns = ns_lookup(ns_get_default_id());
+	if (!default_ns) {
+		flog_err(EC_ZEBRA_NS_NO_DEFAULT,
+			 "%s: failed to find default ns", __func__);
+		exit(EXIT_FAILURE); /* This is non-recoverable */
+	}
+
 	/* Do any needed per-NS data structure allocation. */
-	dzns->if_table = route_table_init();
+	zebra_ns_new(default_ns);
+	dzns = default_ns->info;
 
 	/* Register zebra VRF callbacks, create and activate default VRF. */
 	zebra_vrf_init();