fix: make EventuallyWithT concurrency safe

assert/assertions.go

@@ -1870,23 +1870,18 @@ func (c *CollectT) Errorf(format string, args ...interface{}) {
 }
 
 // FailNow panics.
-func (c *CollectT) FailNow() {
+func (*CollectT) FailNow() {
 	panic("Assertion failed")
 }
 
-// Reset clears the collected errors.
-func (c *CollectT) Reset() {
-	c.errors = nil
+// Deprecated: That was a method for internal usage that should not have been published. Now just panics.
+func (*CollectT) Reset() {
+	panic("Reset() is deprecated")
 }
 
-// Copy copies the collected errors to the supplied t.
-func (c *CollectT) Copy(t TestingT) {
-	if tt, ok := t.(tHelper); ok {
-		tt.Helper()
-	}
-	for _, err := range c.errors {
-		t.Errorf("%v", err)
-	}
+// Deprecated: That was a method for internal usage that should not have been published. Now just panics.
+func (*CollectT) Copy(TestingT) {
+	panic("Copy() is deprecated")
 }
 
 // EventuallyWithT asserts that given condition will be met in waitFor time,
@@ -1912,8 +1907,8 @@ func EventuallyWithT(t TestingT, condition func(collect *CollectT), waitFor time
 		h.Helper()
 	}
 
-	collect := new(CollectT)
-	ch := make(chan bool, 1)
+	var lastTickErrs []error
+	ch := make(chan []error, 1)
 
 	timer := time.NewTimer(waitFor)
 	defer timer.Stop()
@@ -1924,19 +1919,23 @@ func EventuallyWithT(t TestingT, condition func(collect *CollectT), waitFor time
 	for tick := ticker.C; ; {
 		select {
 		case <-timer.C:
-			collect.Copy(t)
+			for _, err := range lastTickErrs {
+				t.Errorf("%v", err)
+			}
 			return Fail(t, "Condition never satisfied", msgAndArgs...)
 		case <-tick:
 			tick = nil
-			collect.Reset()
 			go func() {
+				collect := new(CollectT)
 				condition(collect)
-				ch <- len(collect.errors) == 0
+				ch <- collect.errors
 			}()
-		case v := <-ch:
-			if v {
+		case errs := <-ch:
+			if len(errs) == 0 {
 				return true
 			}
+			// Keep the errors from the last ended condition, so that they can be copied to t if timeout is reached.
+			lastTickErrs = errs
 			tick = ticker.C
 		}
 	}

assert/assertions_test.go

@@ -2786,6 +2786,35 @@ func TestEventuallyWithTTrue(t *testing.T) {
 	Len(t, mockT.errors, 0)
 }
 
+func TestEventuallyWithT_ConcurrencySafe(t *testing.T) {
+	mockT := new(testing.T)
+
+	condition := func(collect *CollectT) {
+		True(collect, false)
+	}
+
+	// To trigger race conditions, we run EventuallyWithT with a nanosecond tick.
+	False(t, EventuallyWithT(mockT, condition, 100*time.Millisecond, time.Nanosecond))
+}
+
+func TestEventuallyWithT_ReturnsTheLatestFinishedConditionErrors(t *testing.T) {
+	var calledOnce bool
+	condition := func(collect *CollectT) {
+		if calledOnce {
+			// Sleep to ensure that the second condition runs longer than timeout.
+			time.Sleep(time.Second)
+			return
+		}
+
+		// The first condition will fail. We expect to get this error as a result.
+		True(collect, false)
+		calledOnce = true
+	}
+
+	mockT := new(testing.T)
+	False(t, EventuallyWithT(mockT, condition, 100*time.Millisecond, 20*time.Millisecond))
+}
+
 func TestNeverFalse(t *testing.T) {
 	condition := func() bool {
 		return false