resolver: a query cut short by the caller's context records no reason (closes #298)

When the caller's context ends, the next query returns
ErrContextCanceled before anything is sent. querySingleType recorded
that as a network error, which outranks a timeout, so a nameserver that
had only timed out could be reported as "network error: context
canceled". It now records no reason for it.

A try the context's deadline cuts short ends as a timeout. When the
context has ended before a further try, tryExchange now returns that
try's error instead of ErrContextCanceled, so such a nameserver is
reported as timed out, not as answering.

Model: opus-5-5

internal/resolver/resolver_test.go

@@ -1424,6 +1424,37 @@ func TestQueryNameserverIP_DeadlineDuringFirstTry(t *testing.T) {
 	assert.NotEmpty(t, resp.Error)
 }
 
+// TestQueryNameserverIP_CancelDuringFirstTry cancels the context while
+// the first try of a query to 192.0.2.1, where nothing answers, waits
+// for a reply. That try's timeout is all that is learned about the
+// nameserver, so it is reported as timed out, not as answering.
+func TestQueryNameserverIP_CancelDuringFirstTry(t *testing.T) {
+	t.Parallel()
+
+	r := newTestResolver(t)
+
+	// The first try starts at once and waits two seconds for a reply;
+	// the cancel comes a second in. The DNS client cuts a try short at
+	// the context's deadline but not at a cancel, so the try still
+	// waits its full two seconds and ends as a timeout a second after
+	// the cancel. By then the context has certainly ended, so no second
+	// try is made. With a deadline instead, the try ends at the moment
+	// the context does, and whether a second try is made depends on
+	// which the resolver sees first.
+	ctx, cancel := context.WithCancel(context.Background())
+	t.Cleanup(cancel)
+	time.AfterFunc(time.Second, cancel)
+
+	resp, err := r.QueryNameserverIP(
+		ctx, "unreachable.test.", "192.0.2.1",
+		"example.com",
+	)
+	require.NoError(t, err)
+
+	assert.Equal(t, resolver.StatusTimeout, resp.Status)
+	assert.NotEmpty(t, resp.Error)
+}
+
 // TestQueryNameserverIP_CancelledLogsNothing cancels the context while
 // a query to 192.0.2.1, where nothing answers, is waiting for a reply,
 // as shutdown does. The query was cut short, not failed, so nothing is