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

@@ -212,33 +212,20 @@ func (w *Watcher) runDNSChecks(ctx context.Context) {
 	}
 }
 
-// logFailedLookup logs a failed DNS lookup at error level, unless ctx
-// was cancelled: shutdown cancels it, and a lookup it cut short did not
-// fail. A lookup that ran out of time did fail, so it is logged.
-func (w *Watcher) logFailedLookup(
-	ctx context.Context,
-	msg string,
-	args ...any,
-) {
-	if errors.Is(ctx.Err(), context.Canceled) {
-		return
-	}
-
-	w.log.Error(msg, args...)
-}
-
 func (w *Watcher) checkDomain(
 	ctx context.Context,
 	domain string,
 ) {
 	nameservers, err := w.resolver.LookupNS(ctx, domain)
 	if err != nil {
-		w.logFailedLookup(
+		// Shutdown cancels ctx; a lookup it cut short did not fail.
-			ctx,
+		if !errors.Is(ctx.Err(), context.Canceled) {
+			w.log.Error(
 				"failed to lookup NS",
 				"domain", domain,
 				"error", err,
 			)
+		}
 
 		return
 	}
@@ -274,12 +261,14 @@ func (w *Watcher) checkDomain(
 	// the domain's IP addresses.
 	results, err := w.resolver.LookupAllRecords(ctx, domain)
 	if err != nil {
-		w.logFailedLookup(
+		// Shutdown cancels ctx; a lookup it cut short did not fail.
-			ctx,
+		if !errors.Is(ctx.Err(), context.Canceled) {
+			w.log.Error(
 				"failed to lookup records for domain",
 				"domain", domain,
 				"error", err,
 			)
+		}
 
 		return
 	}
@@ -355,12 +344,14 @@ func (w *Watcher) resolveNameserverAddresses(
 			continue
 		}
 
-		w.logFailedLookup(
+		// Shutdown cancels ctx; a lookup it cut short did not fail.
-			ctx,
+		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
@@ -408,12 +399,14 @@ func (w *Watcher) checkHostname(
 ) {
 	results, err := w.resolver.LookupAllRecords(ctx, hostname)
 	if err != nil {
-		w.logFailedLookup(
+		// Shutdown cancels ctx; a lookup it cut short did not fail.
-			ctx,
+		if !errors.Is(ctx.Err(), context.Canceled) {
+			w.log.Error(
 				"failed to lookup records",
 				"hostname", hostname,
 				"error", err,
 			)
+		}
 
 		return
 	}