stun actor: if no address is found but an address existed <15m ago, keep it

src/fw.rs

@@ -1,5 +1,4 @@
 use std::collections::HashSet;
-use std::sync::Arc;
 
 use anyhow::{Context, Result};
 use iptables;

src/stun_actor.rs

@@ -8,6 +8,11 @@ use serde::{Deserialize, Serialize};
 use crate::config::{RuntimeConfigConsul, RuntimeConfigStun};
 use crate::consul;
 
+/// If autodiscovery returns None but an address was obtained less than
+/// this number of seconds ago (here 15 minutes), we keep that address
+/// in the Consul db instead of insterting a None.
+const PERSIST_SOME_RESULT_DURATION_SECS: u64 = 900;
+
 pub struct StunActor {
     consul: consul::Consul,
     refresh_time: Duration,
@@ -78,7 +83,7 @@ impl StunActor {
 }
 
 impl StunAutodiscovery {
-    async fn do_iteration(&self, consul: &consul::Consul) -> Result<()> {
+    async fn do_iteration(&mut self, consul: &consul::Consul) -> Result<()> {
         let binding_ip = match self.is_ipv4 {
             true => IpAddr::V4(Ipv4Addr::UNSPECIFIED),  // 0.0.0.0
             false => IpAddr::V6(Ipv6Addr::UNSPECIFIED), // [::]
@@ -96,16 +101,31 @@ impl StunAutodiscovery {
             None => None,
         };
 
-        consul
-            .kv_put(
-                &self.consul_key,
-                serde_json::to_vec(&AutodiscoverResult {
-                    timestamp: timestamp(),
+        let now = timestamp();
+
+        if discovered_addr.is_none() {
+            if let Some(last_result) = &self.last_result {
+                if last_result.address.is_some()
+                    && now - last_result.timestamp <= PERSIST_SOME_RESULT_DURATION_SECS
+                {
+                    // Keep non-None result that was obtained before by not
+                    // writing/taking into account None result.
+                    return Ok(());
+                }
+            }
+        }
+
+        let current_result = AutodiscoverResult {
+            timestamp: now,
             address: discovered_addr,
-                })?,
-            )
+        };
+
+        consul
+            .kv_put(&self.consul_key, serde_json::to_vec(&current_result)?)
             .await?;
 
+        self.last_result = Some(current_result);
+
         Ok(())
     }
 }