milestone 16: behavioral fixes for silent failures, locks, counters and the query log

src/server/handler.zig

@@ -140,6 +140,13 @@ pub const Handler = struct {
         uncloak_blocked: std.atomic.Value(u64) = .init(0),
         local_answers: std.atomic.Value(u64) = .init(0),
         forward_zone_answers: std.atomic.Value(u64) = .init(0),
+        /// The three forward-client counters, folded in after every exchange.
+        /// `ForwardClient` is built per query and dropped with the query, so
+        /// these are where its numbers survive. Its `queries` counter is not
+        /// mirrored: `forward_zone_answers` already counts the exchanges.
+        forward_udp_truncated: std.atomic.Value(u64) = .init(0),
+        forward_foreign_datagrams: std.atomic.Value(u64) = .init(0),
+        forward_failures: std.atomic.Value(u64) = .init(0),
         cache_hits: std.atomic.Value(u64) = .init(0),
         paused_queries: std.atomic.Value(u64) = .init(0),
         /// Queries answered before the first snapshot existed, so no group and
@@ -396,6 +403,11 @@ const Context = struct {
             &ctx.scratch.frame,
             ctx.handler.forward_read_timeout,
         );
+        // The client lives on this query's stack, so its counters have to move
+        // into the handler's before it goes out of scope — on the failure path
+        // too, which is the one `forward_failures` exists for.
+        defer foldForwardStats(&ctx.handler.stats, client.stats);
+
         const answer = client.exchange(ctx.io, ctx.query, ctx.response_buf) catch |err| {
             return switch (transport.group(err)) {
                 .cancellation => .drop,
@@ -879,6 +891,15 @@ fn bump(counter: *std.atomic.Value(u64)) void {
     _ = counter.fetchAdd(1, .monotonic);
 }
 
+/// Moves one forward-zone exchange's counters into the handler's. `stats` is a
+/// plain per-instance struct and stays that way (milestone-16 ruling 14); this
+/// is the one place it becomes a process-wide number.
+fn foldForwardStats(into: *Handler.Stats, from: forward_client.ForwardClient.Stats) void {
+    _ = into.forward_udp_truncated.fetchAdd(from.udp_truncated, .monotonic);
+    _ = into.forward_foreign_datagrams.fetchAdd(from.foreign_datagrams, .monotonic);
+    _ = into.forward_failures.fetchAdd(from.failures, .monotonic);
+}
+
 // ---------------------------------------------------------------------------
 // Tests
 // ---------------------------------------------------------------------------
@@ -1758,6 +1779,7 @@ fn fixtureManager(m: *manager.Manager, snapshot: *matcher.Snapshot) void {
         .total_budget = forward_timeout,
         .lock = .init,
         .writer_lock = .init,
+        .refresh_lock = .init,
         .current = snapshot,
         .generation = 1,
         .statuses = &.{},