feat: try next DNS resolution result if TLS setup fails

Previously Delta Chat tried all DNS resolution results
in sequence until TCP connection is established successfully,
then tried to establish TLS on top of the TCP connection.
If establishing TLS fails, the whole
connection establishment procedure failed
without trying next DNS resolution results.

In particular, in a scenario
where DNS returns incorrect result
pointing e.g. to a server
that listens on the TCP port
but does not have correpsponding TLS certificate,
Delta Chat now will fall back to the cached result
and connect successfully.

src/net/dns.rs

@@ -22,6 +22,42 @@ async fn lookup_host_with_timeout(
     Ok(res.collect())
 }
 
+// Updates timestamp of the cached entry
+// or inserts a new one if cached entry does not exist.
+//
+// This function should be called when a successful TLS
+// connection is established with strict TLS checks.
+//
+// This increases priority of existing cached entries
+// and copies fallback addresses from built-in cache
+// into database cache on successful use.
+//
+// Unlike built-in cache,
+// database cache is used even if DNS
+// resolver returns a non-empty
+// (but potentially incorrect and unusable) result.
+pub(crate) async fn update_connect_timestamp(
+    context: &Context,
+    host: &str,
+    address: &str,
+) -> Result<()> {
+    if host == address {
+        return Ok(());
+    }
+
+    context
+        .sql
+        .execute(
+            "INSERT INTO dns_cache (hostname, address, timestamp)
+                 VALUES (?, ?, ?)
+                 ON CONFLICT (hostname, address)
+                 DO UPDATE SET timestamp=excluded.timestamp",
+            (host, address, time()),
+        )
+        .await?;
+    Ok(())
+}
+
 /// Looks up hostname and port using DNS and updates the address resolution cache.
 ///
 /// If `load_cache` is true, appends cached results not older than 30 days to the end
@@ -39,7 +75,7 @@ pub(crate) async fn lookup_host_with_cache(
         Err(err) => {
             warn!(
                 context,
-                "DNS resolution for {}:{} failed: {:#}.", hostname, port, err
+                "DNS resolution for {hostname}:{port} failed: {err:#}."
             );
             Vec::new()
         }