[arp_update]: Fix IPv6 neighbor race condition (sonic-net#15583)

* [arp_update]: Fix IPv6 neighbor race condition on dualtors
Signed-off-by: Lawrence Lee <lawlee@microsoft.com>

Author: theasianpianist

files/scripts/arp_update

@@ -89,32 +89,47 @@ while /bin/true; do
       eval `eval $ip6cmd`
 
       if [[ $SUBTYPE == "dualtor" ]]; then
-        # manually set any remaining FAILED/INCOMPLETE entries to permanently INCOMPLETE
-        # this prevents any remaining INCOMPLETE entries from automatically transitioning to FAILED
-        # once these entries are incomplete, any subsequent neighbor advertisement messages
-        # are able to resolve the entry
-
-        # generates the following command for each failed or incomplete IPv6 neighbor
-        # ip neigh replace <neighbor IPv6> dev <VLAN name> nud incomplete
-        neigh_replace_template="sed -e 's/^/ip neigh replace /' -e 's/,/ dev /' -e 's/$/ nud incomplete;/'" 
-        ip_neigh_replace_cmd="ip -6 neigh show | grep -v fe80 | grep $vlan | grep -E 'FAILED|INCOMPLETE' | cut -d ' ' -f 1,3 --output-delimiter=',' | $neigh_replace_template"
-        eval `eval $ip_neigh_replace_cmd`
-
-        # on dual ToR devices, try to resolve failed neighbor entries since
-        # these entries will have tunnel routes installed, preventing normal 
-        # neighbor resolution (SWSS PR #2137)
-
-        # since ndisc6 is a userland process, the above ndisc6 commands are 
-        # insufficient to update the kernel neighbor table for failed entries
-
-        # we don't need to do this for ipv4 neighbors since arping is able to
-        # update the kernel neighbor table
-
-        # generates the following command for each failed or incomplete IPv6 neighbor
+        # capture all current failed/incomplete IPv6 neighbors in the kernel to avoid situations where new neighbors are learned
+        # in the middle of the below sequence of commands
+        unresolved_kernel_neighbors=$(ip -6 neigh show | grep -v fe80 | grep $vlan | grep -E 'FAILED|INCOMPLETE')
+        failed_kernel_neighbors=$(echo "$unresolved_kernel_neighbors" | grep FAILED | cut -d ' ' -f 1)
+
+        # it's possible for kernel neighbors to fall out of sync with the hardware
+        # this can result in failed neighbors entries that don't have corresponding zero MAC neighbor entries
+        # and therefore don't have tunnel routes installed in the hardware
+        # flush these neighbors from the kernel to force relearning and resync them to the hardware:
+        # 1. for every FAILED or INCOMPLETE neighbor in the kernel, check if there is a corresponding zero MAC neighbor in APPL_DB
+        # 2. if no zero MAC neighbor entry exists, flush the kernel neighbor entry
+        #     - generates the command 'ip neigh flush <neighbor IPv6>' for all such neighbors
+        unsync_neighbors=$(echo "$unresolved_kernel_neighbors" | cut -d ' ' -f 1 | xargs -I{} bash -c "if [[ -z \"\$(sonic-db-cli APPL_DB hget NEIGH_TABLE:$vlan:{} neigh)\" ]]; then echo '{}'; fi")
+        if [[ ! -z "$unsync_neighbors" ]]; then
+            ip_neigh_flush_cmd="echo \"$unsync_neighbors\" | sed -e 's/^/ip neigh flush /' -e 's/$/;/'"
+            eval `eval "$ip_neigh_flush_cmd"`
+            sleep 2
+        fi
+
+        # generates the following command for each FAILED or INCOMPLETE IPv6 neighbor
         # timeout 0.2 ping <neighbor IPv6> -n -q -i 0 -c 1 -W 1 -I <VLAN name> >/dev/null
-        ping6_template="sed -e 's/^/timeout 0.2 ping /' -e 's/,/ -n -q -i 0 -c 1 -W 1 -I /' -e 's/$/ >\/dev\/null;/'"
-        failed_ip6_neigh_cmd="ip -6 neigh show | grep -v fe80 | grep $vlan | grep -E 'FAILED|INCOMPLETE' | cut -d ' ' -f 1,3 --output-delimiter=',' | $ping6_template"
-        eval `eval $failed_ip6_neigh_cmd`
+        if [[ ! -z "$unresolved_kernel_neighbors" ]]; then
+            ping6_template="sed -e 's/^/timeout 0.2 ping /' -e 's/,/ -n -q -i 0 -c 1 -W 1 -I /' -e 's/$/ >\/dev\/null;/'"
+            failed_ip6_neigh_cmd="echo \"$unresolved_kernel_neighbors\" | cut -d ' ' -f 1,3 --output-delimiter=',' | $ping6_template"
+            eval `eval "$failed_ip6_neigh_cmd"`
+            # allow some time for any transient INCOMPLETE neighbors to transition to FAILED
+            sleep 5
+        fi
+
+        # manually set any remaining FAILED entries to permanently INCOMPLETE
+        # once these entries are INCOMPLETE, any subsequent neighbor advertisement messages are able to resolve the entry
+        # ignore INCOMPLETE neighbors since if they are transiently incomplete (i.e. new kernel neighbors that we are attempting to resolve for the first time),
+        # setting them to permanently incomplete here means the kernel will never generate a netlink message for that neighbor
+        # generates the following command for each FAILED IPv6 neighbor
+        # ip neigh replace <neighbor IPv6> dev <VLAN name> nud incomplete
+        failed_kernel_neighbors=$(ip -6 neigh show | grep -v fe80 | grep $vlan | grep -E 'FAILED')
+        if [[ ! -z "$failed_kernel_neighbors" ]]; then
+            neigh_replace_template="sed -e 's/^/ip neigh replace /' -e 's/,/ dev /' -e 's/$/ nud incomplete;/'" 
+            ip_neigh_replace_cmd="echo \"$failed_kernel_neighbors\" | cut -d ' ' -f 1,3 --output-delimiter=',' | $neigh_replace_template"
+            eval `eval "$ip_neigh_replace_cmd"`
+        fi
       fi
   done