Answer the review of hq issue 348's catalogue change

- The dnsmasq watch restarts only a unit that runs, without waiting for
  its start; a unit stopped, starting or failed is held and said (C2).
- dnsmasq_resolve asks only an address the resolver listens on (C3).
- Tests for the watch loop keeping its budget, a watch that wrote nothing
  or stopped looking, and a unit not running (C4).
- A restart is remembered as said only once emitted, restarts made before
  the bundle started are said, and whether one helped is said (C5).
- docker_events reads the runtime's whole buffer: complete when it is not
  full or reaches past the window, never a quiet machine read as lost (C6).
- netcheck: refused read with errors.Is; tests for addresses and ports,
  and for a container named as docker knows it.

modules/netcheck/cmd/netcheck/ask_test.go

@@ -115,6 +115,18 @@ func TestDNSSaysARefusedPortAndRefusesANameAsServer(t *testing.T) {
 	if a, _ := serverAddress("10.10.0.1"); a != "10.10.0.1:53" {
 		t.Fatalf("default port: %s", a)
 	}
+	for server, want := range map[string]string{"10.10.0.1:5353": "10.10.0.1:5353", " 127.0.0.1 ": "127.0.0.1:53",
+		"[::1]:53": "[::1]:53", "::1": "[::1]:53"} {
+		if a, err := serverAddress(server); err != nil || a != want {
+			t.Errorf("%q: %q, %v; want %q", server, a, err, want)
+		}
+	}
+	for _, server := range []string{"resolver.example:53", "resolver.example", "10.10.0.1:0", "10.10.0.1:65536",
+		"10.10.0.1:dns", ""} {
+		if a, err := serverAddress(server); err == nil {
+			t.Errorf("%q was taken as %q", server, a)
+		}
+	}
 }
 
 func TestListeningReadsTheKernelsTablesAndSaysWhatItCouldNotSee(t *testing.T) {

modules/netcheck/cmd/netcheck/checks.go

@@ -9,6 +9,7 @@ import (
 	"sort"
 	"strconv"
 	"strings"
+	"syscall"
 	"time"
 )
 
@@ -220,7 +221,7 @@ func timeoutOf(args map[string]any) (time.Duration, error) {
 func dnsWords(err error, timeout time.Duration) string {
 	var ne net.Error
 	switch {
-	case strings.Contains(err.Error(), "connection refused"):
+	case errors.Is(err, syscall.ECONNREFUSED):
 		return "connection refused: nothing listens on that address and port"
 	case errors.As(err, &ne) && ne.Timeout():
 		return fmt.Sprintf("no answer within %s", timeout)