Send every log line to a syslog server as well (closes #28)

With SWWAF_LOG_REMOTE_URL set (syslog+udp, syslog+tcp or syslog+tls),
every line on stdout is also sent as the message of an RFC 5424 record,
octet-counted over TCP and TLS, from a bounded buffer that drops its
oldest line when full, so a slow or unreachable server holds up nothing.
Failed connections are retried with backoff; lines sent, dropped and
waiting are metrics. At a stop the lines still waiting get at most two
seconds. SWWAF_LOG_REMOTE_APP_NAME defaults to SWWAF_INSTANCE_NAME; while
sending, an app name RFC 5424 does not allow stops the start. Standard
library only: log/syslog writes only the older format.

Model: opus-5-5

internal/smallwebwaf/smallwebwaf_internal_test.go

@@ -0,0 +1,82 @@
+package smallwebwaf
+
+import (
+	"log/slog"
+	"net/url"
+	"testing"
+	"testing/synctest"
+	"time"
+
+	"sneak.berlin/go/smallwebwaf/internal/remotelog"
+)
+
+// The stop's tests run in a synctest bubble, where the time package runs
+// on a clock of the test's own, so that how long the stop takes can be
+// told exactly. The sender is held up by its process log, not by the
+// network: a goroutine of the bubble that waits on the network keeps that
+// clock from moving on.
+
+func TestStopWaitsForTheSenderToFinish(t *testing.T) {
+	t.Parallel()
+
+	synctest.Test(t, func(t *testing.T) {
+		took := stopHeldSender(t, time.Second)
+		if took != time.Second {
+			t.Errorf("the stop took %s, want the second the sender took", took)
+		}
+	})
+}
+
+func TestStopWaitsForTheSenderAtMostTwoSeconds(t *testing.T) {
+	t.Parallel()
+
+	synctest.Test(t, func(t *testing.T) {
+		took := stopHeldSender(t, time.Minute)
+		if took != 2*time.Second {
+			t.Errorf("the stop took %s, want 2s", took)
+		}
+	})
+}
+
+// heldLog holds each line written to it until it is closed.
+type heldLog chan struct{}
+
+// Write waits until the log is closed.
+func (l heldLog) Write(p []byte) (int, error) {
+	<-l
+
+	return len(p), nil
+}
+
+// stopHeldSender starts sending to an endpoint the sender cannot connect
+// to, holds the sender as it logs that failure until release has passed,
+// stops the sending, and returns how long the stop took. It returns once
+// the sender has ended, as a bubble must.
+func stopHeldSender(t *testing.T, release time.Duration) time.Duration {
+	t.Helper()
+
+	log := make(heldLog)
+	sender := remotelog.New(remotelog.Params{
+		// No port is 65536, so each attempt to connect fails at once,
+		// before it reaches the network.
+		URL:    &url.URL{Scheme: remotelog.SchemeTCP, Host: "127.0.0.1:65536"},
+		Buffer: 1,
+	})
+
+	stopSending := startSending(t.Context(), sender,
+		slog.New(slog.NewJSONHandler(log, nil)))
+
+	synctest.Wait()
+	time.AfterFunc(release, func() { close(log) })
+
+	stopped := time.Now()
+
+	stopSending()
+
+	took := time.Since(stopped)
+
+	time.Sleep(release)
+	synctest.Wait()
+
+	return took
+}