plugin/shed: add UDP overload protection plugin (#8312)

* plugin/shed: add UDP overload protection plugin

UDP responses written back through one listener socket serialize on the
Go runtime's internal fdMutex, which allows at most 2^20-1 concurrent
operations per file descriptor and panics the process when exceeded.
CoreDNS serves UDP with one goroutine per query, all writing through the
shared packet connection, so a sustained overload parks every excess
in-flight query in that wait queue until the process dies with
"too many concurrent operations on a single file or socket". Observed
in production: ~2.8M goroutines and 60GiB RSS before the panic.

The shed plugin makes the panic structurally unreachable. It installs,
via Config.UDPDecorateWriterFunc, a per-socket bounded evict-oldest
stack drained newest-first by a single writer goroutine, so the fd
never sees more than one writer and residual capacity under overload
always goes to the freshest response. While a socket's stack is full,
arriving queries are dropped before any plugin runs. Drops are silent
(the client's resolver retries elsewhere) and counted in
coredns_shed_dropped_total{server, reason}.

plugin/shed/fdmutex_test.go demonstrates the failure and the fix with
one shared flood harness. Two subprocess tests reproduce the exact
runtime panic without the plugin's write discipline - one deterministic
(a held write plus >2^20 queued writers), one with nothing held or
mocked; both exercise the Go runtime rather than the plugin, so they
are gated behind SHED_FLOOD_TEST=1. The counterfactual - the same load
through the plugin's stack, completing with every response accounted
for as written or dropped - runs in every test invocation, including
-race, at 50k responders, and at the full 1.5M with SHED_FLOOD_TEST=1:

    SHED_FLOOD_TEST=1 go test ./plugin/shed/

Signed-off-by: Ryan Brewster <rpb@anthropic.com>

* test: add shed e2e test

Query a shed-enabled server over UDP (the plugin's deferred
single-writer path) and TCP (which shed passes through), and check
that coredns_shed_dropped_total is exported with its reason label.

No-Verification-Needed: test-only change
Signed-off-by: Ryan Brewster <rpb@anthropic.com>

---------

Signed-off-by: Ryan Brewster <rpb@anthropic.com>

test/shed_test.go

@@ -0,0 +1,47 @@
+package test
+
+import (
+	"testing"
+
+	"github.com/coredns/coredns/plugin/metrics"
+	"github.com/coredns/coredns/plugin/test"
+
+	"github.com/miekg/dns"
+)
+
+// TestShed checks that with the shed plugin installed a query is answered
+// over both UDP (through the plugin's deferred single-writer path) and
+// TCP (which shed passes through), and that its drop counter is exported.
+func TestShed(t *testing.T) {
+	corefile := `.:0 {
+		shed
+		prometheus localhost:0
+		whoami
+	}`
+
+	i, udp, tcp, err := CoreDNSServerAndPorts(corefile)
+	if err != nil {
+		t.Fatalf("Could not get CoreDNS serving instance: %s", err)
+	}
+	defer i.Stop()
+
+	m := new(dns.Msg)
+	m.SetQuestion("whoami.example.org.", dns.TypeA)
+
+	if r, err := dns.Exchange(m, udp); err != nil || r.Rcode != dns.RcodeSuccess {
+		t.Fatalf("Expected UDP reply, got %v: %v", r, err)
+	}
+	c := &dns.Client{Net: "tcp"}
+	if r, _, err := c.Exchange(m, tcp); err != nil || r.Rcode != dns.RcodeSuccess {
+		t.Fatalf("Expected TCP reply, got %v: %v", r, err)
+	}
+
+	data := test.Scrape("http://" + metrics.ListenAddr + "/metrics")
+	got, labels := test.MetricValue("coredns_shed_dropped_total", data)
+	if got != "0" {
+		t.Errorf("Expected coredns_shed_dropped_total 0, but got %s", got)
+	}
+	if labels["reason"] == "" {
+		t.Errorf("Expected coredns_shed_dropped_total to carry a reason label, got %v", labels)
+	}
+}