feat: lookup_host_with_cache(): Don't return empty address list (#7596)

All users of this function expect a nonempty list to be returned, otherwise they fail with hard to
understand errors like "No connection attempts were made", so it's better to fail early if DNS
resolution failed and the cache doesn't contain resolutions as well.

Author: iequidoo

python/tests/test_1_online.py

@@ -1295,16 +1295,17 @@ def test_configure_error_msgs_invalid_server(acfactory):
         ev = ac2._evtracker.get_matching("DC_EVENT_CONFIGURE_PROGRESS")
         if ev.data1 == 0:
             break
+    err_lower = ev.data2.lower()
     # Can't connect so it probably should say something about "internet"
     # again, should not repeat itself
     # If this fails then probably `e.msg.to_lowercase().contains("could not resolve")`
     # in configure.rs returned false because the error message was changed
     # (i.e. did not contain "could not resolve" anymore)
-    assert (ev.data2.count("internet") + ev.data2.count("network")) == 1
+    assert (err_lower.count("internet") + err_lower.count("network")) == 1
     # Should mention that it can't connect:
-    assert ev.data2.count("connect") == 1
+    assert err_lower.count("connect") == 1
     # The users do not know what "configuration" is
-    assert "configuration" not in ev.data2.lower()
+    assert "configuration" not in err_lower
 
 
 def test_status(acfactory):

src/net/dns.rs

@@ -40,7 +40,7 @@
 //! used for successful connection timestamp of
 //! retrieving them from in-memory cache is used.
 
-use anyhow::{Context as _, Result};
+use anyhow::{Context as _, Result, ensure};
 use std::collections::HashMap;
 use std::net::{IpAddr, Ipv4Addr, Ipv6Addr, SocketAddr};
 use std::str::FromStr;
@@ -850,7 +850,7 @@ pub(crate) async fn lookup_host_with_cache(
         }
     };
 
-    if load_cache {
+    let addrs = if load_cache {
         let mut cache = lookup_cache(context, hostname, port, alpn, now).await?;
         if let Some(ips) = DNS_PRELOAD.get(hostname) {
             for ip in ips {
@@ -861,10 +861,15 @@ pub(crate) async fn lookup_host_with_cache(
             }
         }
 
-        Ok(merge_with_cache(resolved_addrs, cache))
+        merge_with_cache(resolved_addrs, cache)
     } else {
-        Ok(resolved_addrs)
-    }
+        resolved_addrs
+    };
+    ensure!(
+        !addrs.is_empty(),
+        "Could not find DNS resolutions for {hostname}:{port}. Check server hostname and your network"
+    );
+    Ok(addrs)
 }
 
 /// Merges results received from DNS with cached results.