feat(server): give irtt-server sensible zero-argument defaults

With no --bind, the server now binds the wildcard IRTT port on both
address families ([::]:2112 then 0.0.0.0:2112) instead of requiring an
explicit address, on platforms where wildcard reply-source selection
is already supported. Any explicit --bind fully replaces the default
pair rather than adding to it; repeated explicit binds are unaffected.

If one of the two default listeners fails because its address family
has no local support on this host at all (IPv6 disabled on an
otherwise IPv4-capable host, for example), the server falls back to
serving just the surviving family instead of failing outright. Any
other failure reason -- the port already in use, permission denied, no
safe wildcard reply-source path -- is not a family-support problem and
is not silently worked around; startup fails as it would for an
explicit --bind. This distinction is made with the new
irtt_server::address_family_available, which actually binds the
wildcard address of the failed listener's family on an ephemeral port:
a bare socket can construct successfully even when every bind for that
family fails (Linux administratively disabling IPv6 per-interface
leaves AF_INET6 sockets constructible but every bind EADDRNOTAVAIL), so
only a real bind attempt is a reliable signal.

crates/irtt-app/src/cmd/server/run.rs

@@ -1,9 +1,10 @@
 use std::{
+    net::SocketAddr,
     sync::atomic::{AtomicBool, Ordering},
     time::Duration,
 };
 
-use irtt_server::ServerSet;
+use irtt_server::{address_family_available, ServerSet, ServerSetError};
 
 use super::args::ServerArgs;
 
@@ -36,7 +37,29 @@ async fn serve(
     shutdown_requested: &AtomicBool,
 ) -> Result<(), Box<dyn std::error::Error>> {
     let config = args.server_config();
-    let set = ServerSet::bind(args.bind, config).await?;
+    let binds = args.resolve_binds();
+    let is_implicit_default = args.bind.is_empty();
+    let set = match ServerSet::bind(binds.clone(), config.clone()).await {
+        Ok(set) => set,
+        Err(ServerSetError::ListenerSetup { addr, source }) => {
+            let family_available = address_family_available(addr);
+            match implicit_default_fallback(is_implicit_default, family_available, &binds, addr) {
+                Some(remaining) => {
+                    eprintln!(
+                        "irtt-server: default listener {addr} unavailable ({source}); \
+                         continuing with the remaining default listener"
+                    );
+                    // `remaining`'s lone survivor is bound exactly as it
+                    // would be alone: the address family that just failed
+                    // has no local support at all, so nothing on that family
+                    // could be competing for the surviving listener's port.
+                    ServerSet::bind(remaining, config).await?
+                }
+                None => return Err(ServerSetError::ListenerSetup { addr, source }.into()),
+            }
+        }
+        Err(err) => return Err(err.into()),
+    };
     // Only once every listener is up. Announcing one address and then failing
     // to bind the next would report a service that never started.
     for addr in set.local_addrs() {
@@ -46,6 +69,37 @@ async fn serve(
     Ok(())
 }
 
+/// Whether a listener-setup failure for `failed_addr` should be retried with
+/// a reduced bind set instead of failing the whole invocation, and if so,
+/// what to retry with.
+///
+/// Only the implicit zero-argument default gets this treatment, and only
+/// when `failed_addr`'s own address family has no local support on this host
+/// at all (`family_available` is `false`) — not merely because its specific
+/// port was unavailable for some other reason: already in use, permission
+/// denied, no safe wildcard reply-source path, or anything else
+/// `ServerSet::bind` can fail with. Those are real configuration or
+/// environment problems this invocation should fail on, not silently work
+/// around by dropping a listener the operator asked for. An explicit
+/// `--bind` list keeps the library's ordinary all-or-nothing behavior
+/// regardless.
+fn implicit_default_fallback(
+    is_implicit_default: bool,
+    family_available: bool,
+    original_binds: &[SocketAddr],
+    failed_addr: SocketAddr,
+) -> Option<Vec<SocketAddr>> {
+    if !is_implicit_default || family_available || !original_binds.contains(&failed_addr) {
+        return None;
+    }
+    let remaining: Vec<SocketAddr> = original_binds
+        .iter()
+        .copied()
+        .filter(|&addr| addr != failed_addr)
+        .collect();
+    (!remaining.is_empty()).then_some(remaining)
+}
+
 async fn shutdown_signal(shutdown_requested: &AtomicBool) {
     while !shutdown_requested.load(Ordering::Relaxed) {
         tokio::time::sleep(SHUTDOWN_POLL_INTERVAL).await;
@@ -82,4 +136,67 @@ mod tests {
 
         run_server(args, &shutdown_requested).unwrap();
     }
+
+    fn default_binds() -> Vec<SocketAddr> {
+        ServerArgs::try_parse_from(["irtt-server"])
+            .unwrap()
+            .resolve_binds()
+    }
+
+    #[test]
+    fn implicit_default_losing_an_unsupported_family_falls_back_to_the_other() {
+        let binds = default_binds();
+        let failed = binds[0];
+
+        let remaining = implicit_default_fallback(true, false, &binds, failed).unwrap();
+
+        assert_eq!(remaining, [binds[1]]);
+    }
+
+    #[test]
+    fn a_supported_familys_failure_never_falls_back() {
+        // The family itself works fine here; whatever made this specific
+        // bind fail (port in use, permission, ...) is a real problem this
+        // invocation should surface, not paper over.
+        let binds = default_binds();
+
+        assert_eq!(
+            implicit_default_fallback(true, true, &binds, binds[0]),
+            None,
+            "family_available: true means this is not a family-support problem"
+        );
+    }
+
+    #[test]
+    fn explicit_binds_never_fall_back() {
+        let binds = default_binds();
+
+        assert_eq!(
+            implicit_default_fallback(false, false, &binds, binds[0]),
+            None
+        );
+    }
+
+    #[test]
+    fn a_failure_outside_the_requested_set_does_not_fall_back() {
+        let binds = default_binds();
+        let unrelated = SocketAddr::from(([127, 0, 0, 1], 2113));
+
+        assert_eq!(
+            implicit_default_fallback(true, false, &binds, unrelated),
+            None,
+            "the failing address must actually be one this invocation asked for"
+        );
+    }
+
+    #[test]
+    fn losing_the_only_remaining_default_address_does_not_fall_back() {
+        let single = [SocketAddr::from(([0, 0, 0, 0], 2112))];
+
+        assert_eq!(
+            implicit_default_fallback(true, false, &single, single[0]),
+            None,
+            "no address would be left to retry with"
+        );
+    }
 }