internal/lsp: avoid flake in TestDebouncer

TestDebouncer inherently depends on the timing of events. Specifically,
it can fail if the pause between two subsequent events is more than the
debouncing delay.

When this has proved flaky in the past I just increased the debouncing
delay. However, tests are still occasionally flaking. Rather than just
increase the delay arbitrarily, attempt to make the test more robust by
retrying up to three times if the base assumption (that goroutines are
scheduled within a reasonable amount of time) is not met.

Fixes golang/go#45085

Change-Id: Ifa32e695d64ae4bcfe9600a0413bf6358dff9b7a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/309276
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

internal/lsp/debounce_test.go

@@ -5,13 +5,21 @@
 package lsp
 
 import (
+	"fmt"
+	"strings"
 	"sync"
 	"testing"
 	"time"
+
+	errors "golang.org/x/xerrors"
 )
 
 func TestDebouncer(t *testing.T) {
 	t.Parallel()
+
+	const evtWait = 30 * time.Millisecond
+	const initialDelay = 100 * time.Millisecond
+
 	type event struct {
 		key       string
 		order     uint64
@@ -58,30 +66,69 @@ func TestDebouncer(t *testing.T) {
 		test := test
 		t.Run(test.label, func(t *testing.T) {
 			t.Parallel()
-			d := newDebouncer()
-			var wg sync.WaitGroup
-			for i, e := range test.events {
-				wg.Add(1)
-				go func(e *event) {
-					d.debounce(e.key, e.order, 500*time.Millisecond, func() {
-						e.fired = true
-					})
-					wg.Done()
-				}(e)
-				// For a bit more fidelity, sleep to try to make things actually
-				// execute in order. This doesn't have to be perfect, but could be done
-				// properly using fake timers.
-				if i < len(test.events)-1 {
-					time.Sleep(10 * time.Millisecond)
+
+			try := func(delay time.Duration) (bool, error) {
+				d := newDebouncer()
+				var wg sync.WaitGroup
+				valid := true
+				for i, e := range test.events {
+					wg.Add(1)
+					start := time.Now()
+					go func(e *event) {
+						if time.Since(start) > evtWait {
+							// Due to slow scheduling this event is likely to have fired out
+							// of order, so mark this attempt as invalid.
+							valid = false
+						}
+						d.debounce(e.key, e.order, delay, func() {
+							e.fired = true
+						})
+						wg.Done()
+					}(e)
+					// For a bit more fidelity, sleep to try to make things actually
+					// execute in order. This shouldn't have to be perfect: as long as we
+					// don't have extreme pauses the test should still pass.
+					if i < len(test.events)-1 {
+						time.Sleep(evtWait)
+					}
 				}
+				wg.Wait()
+				var errs []string
+				for _, event := range test.events {
+					if event.fired != event.wantFired {
+						msg := fmt.Sprintf("(key: %q, order: %d): fired = %t, want %t",
+							event.key, event.order, event.fired, event.wantFired)
+						errs = append(errs, msg)
+					}
+				}
+				var err error
+				if len(errs) > 0 {
+					err = errors.New(strings.Join(errs, "\n"))
+				}
+				// If the test took less than maxwait, no event before the
+				return valid, err
 			}
-			wg.Wait()
-			for _, event := range test.events {
-				if event.fired != event.wantFired {
-					t.Errorf("(key: %q, order: %d): fired = %t, want %t",
-						event.key, event.order, event.fired, event.wantFired)
-				}
+
+			if err := retryInvalid(100*time.Millisecond, try); err != nil {
+				t.Error(err)
 			}
 		})
 	}
 }
+
+// retryInvalid runs the try func up to three times with exponential back-off
+// in its duration argument, starting with the provided initial duration. Calls
+// to try are retried if their first result indicates that they may be invalid,
+// and their second result is a non-nil error.
+func retryInvalid(initial time.Duration, try func(time.Duration) (valid bool, err error)) (err error) {
+	delay := initial
+	for attempts := 3; attempts > 0; attempts-- {
+		var valid bool
+		valid, err = try(delay)
+		if err == nil || valid {
+			return err
+		}
+		delay += delay
+	}
+	return err
+}