watcher: a lookup cut short by shutdown is not logged as an error (closes #229)

Stopping dnswatcher during a DNS check logged every lookup the stop cut
short as an error, "context canceled". The watcher now logs a failed
lookup only when its context was not cancelled; the port and TLS checks
already return quietly. It asks the context, not the lookup's error,
because the resolver reports a cancelled lookup with its own error,
which does not wrap context.Canceled. A context whose deadline passed is
not cancelled, so a lookup that ran out of time is still logged as an
error. The tests run a check on a cancelled context and on one whose
deadline passed; neither sends a query.

Model: opus-5-5

internal/watcher/watcher.go

@@ -2,6 +2,7 @@ package watcher
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"log/slog"
 	"slices"
@@ -217,11 +218,14 @@ func (w *Watcher) checkDomain(
 ) {
 	nameservers, err := w.resolver.LookupNS(ctx, domain)
 	if err != nil {
-		w.log.Error(
-			"failed to lookup NS",
-			"domain", domain,
-			"error", err,
-		)
+		// Shutdown cancels ctx; a lookup it cut short did not fail.
+		if !errors.Is(ctx.Err(), context.Canceled) {
+			w.log.Error(
+				"failed to lookup NS",
+				"domain", domain,
+				"error", err,
+			)
+		}
 
 		return
 	}
@@ -257,11 +261,14 @@ func (w *Watcher) checkDomain(
 	// the domain's IP addresses.
 	results, err := w.resolver.LookupAllRecords(ctx, domain)
 	if err != nil {
-		w.log.Error(
-			"failed to lookup records for domain",
-			"domain", domain,
-			"error", err,
-		)
+		// Shutdown cancels ctx; a lookup it cut short did not fail.
+		if !errors.Is(ctx.Err(), context.Canceled) {
+			w.log.Error(
+				"failed to lookup records for domain",
+				"domain", domain,
+				"error", err,
+			)
+		}
 
 		return
 	}
@@ -337,11 +344,14 @@ func (w *Watcher) resolveNameserverAddresses(
 			continue
 		}
 
-		w.log.Error(
-			"no addresses found for nameserver",
-			"nameserver", ns,
-			"error", err,
-		)
+		// Shutdown cancels ctx; a lookup it cut short did not fail.
+		if !errors.Is(ctx.Err(), context.Canceled) {
+			w.log.Error(
+				"no addresses found for nameserver",
+				"nameserver", ns,
+				"error", err,
+			)
+		}
 
 		if prevIPs, ok := prev[ns]; ok {
 			addresses[ns] = prevIPs
@@ -389,11 +399,14 @@ func (w *Watcher) checkHostname(
 ) {
 	results, err := w.resolver.LookupAllRecords(ctx, hostname)
 	if err != nil {
-		w.log.Error(
-			"failed to lookup records",
-			"hostname", hostname,
-			"error", err,
-		)
+		// Shutdown cancels ctx; a lookup it cut short did not fail.
+		if !errors.Is(ctx.Err(), context.Canceled) {
+			w.log.Error(
+				"failed to lookup records",
+				"hostname", hostname,
+				"error", err,
+			)
+		}
 
 		return
 	}