perf: flush Record when deadline is exceeded

Watermark and WakeupEvents allow reducing the rate at which user
space is woken up. This creates an edge case where Read() returns
os.ErrDeadlineExceeded even though there is data in one of the rings.

This is also a problem in the ringbuf package, where we've solved this
by checking whether there is any data when encountering
an ErrDeadlineExceeded. Adopt the same approach for the perf reader.

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>

perf/reader.go

@@ -357,7 +357,7 @@ func (pr *Reader) ReadInto(rec *Record) error {
 			pr.pauseMu.Unlock()
 			_, err := pr.poller.Wait(events, pr.deadline)
 			pr.pauseMu.Lock()
-			if err != nil {
+			if err != nil && !errors.Is(err, os.ErrDeadlineExceeded) {
 				return err
 			}
 
@@ -374,6 +374,11 @@ func (pr *Reader) ReadInto(rec *Record) error {
 					pr.epollRings = append(pr.epollRings, ring)
 				}
 			}
+
+			if len(pr.epollRings) == 0 {
+				// Woken up but no data, deadline exceeded.
+				return fmt.Errorf("read: %w", os.ErrDeadlineExceeded)
+			}
 		}
 
 		// Start at the last available event. The order in which we

perf/reader_test.go

@@ -531,6 +531,27 @@ func TestPerfReaderWakeupEvents(t *testing.T) {
 	}
 }
 
+func TestReadWithoutWakeup(t *testing.T) {
+	t.Parallel()
+
+	events := perfEventArray(t)
+
+	rd, err := NewReaderWithOptions(events, 1, ReaderOptions{WakeupEvents: 2})
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer rd.Close()
+
+	prog := outputSamplesProg(t, events, 5)
+	ret, _, err := prog.Test(internal.EmptyBPFContext)
+	testutils.SkipIfNotSupported(t, err)
+	qt.Assert(t, qt.IsNil(err))
+	qt.Assert(t, qt.Equals(ret, 0))
+
+	rd.SetDeadline(time.Now())
+	checkRecord(t, rd)
+}
+
 func BenchmarkReader(b *testing.B) {
 	events := perfEventArray(b)
 	prog := outputSamplesProg(b, events, 80)