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 four places in the watcher
that log a failed lookup now do it through logFailedLookup, which logs
nothing when the watcher's context was 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

@@ -212,20 +212,33 @@ 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 {
-		// 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,
-			)
-		}
+		w.logFailedLookup(
+			ctx,
+			"failed to lookup NS",
+			"domain", domain,
+			"error", err,
+		)
 
 		return
 	}
@@ -261,14 +274,12 @@ func (w *Watcher) checkDomain(
 	// the domain's IP addresses.
 	results, err := w.resolver.LookupAllRecords(ctx, domain)
 	if err != nil {
-		// 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,
-			)
-		}
+		w.logFailedLookup(
+			ctx,
+			"failed to lookup records for domain",
+			"domain", domain,
+			"error", err,
+		)
 
 		return
 	}
@@ -344,14 +355,12 @@ func (w *Watcher) resolveNameserverAddresses(
 			continue
 		}
 
-		// 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,
-			)
-		}
+		w.logFailedLookup(
+			ctx,
+			"no addresses found for nameserver",
+			"nameserver", ns,
+			"error", err,
+		)
 
 		if prevIPs, ok := prev[ns]; ok {
 			addresses[ns] = prevIPs
@@ -399,14 +408,12 @@ func (w *Watcher) checkHostname(
 ) {
 	results, err := w.resolver.LookupAllRecords(ctx, hostname)
 	if err != nil {
-		// 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,
-			)
-		}
+		w.logFailedLookup(
+			ctx,
+			"failed to lookup records",
+			"hostname", hostname,
+			"error", err,
+		)
 
 		return
 	}