Add Backoff.ErrCause() (#538)

* Backoff: change Err() to return the context cause when available

Signed-off-by: Marco Pracucci <[email protected]>

* Add Backoff.ErrCause() instead

Signed-off-by: Marco Pracucci <[email protected]>

* Renamed unit test

Signed-off-by: Marco Pracucci <[email protected]>

---------

Signed-off-by: Marco Pracucci <[email protected]>

CHANGELOG.md

@@ -214,6 +214,7 @@
 * [ENHANCEMENT] SpanProfiler: do less work on unsampled traces. #528
 * [ENHANCEMENT] Log Middleware: if the trace is not sampled, log its ID as `trace_id_unsampled` instead of `trace_id`. #529
 * [EHNANCEMENT] httpgrpc: httpgrpc Server can now use error message from special HTTP header when converting HTTP response to an error. This is useful when HTTP response body contains binary data that doesn't form valid utf-8 string, otherwise grpc would fail to marshal returned error. #531
+* [CHANGE] Backoff: added `Backoff.ErrCause()` which is like `Backoff.Err()` but returns the context cause if backoff is terminated because the context has been canceled. #538
 * [BUGFIX] spanlogger: Support multiple tenant IDs. #59
 * [BUGFIX] Memberlist: fixed corrupted packets when sending compound messages with more than 255 messages or messages bigger than 64KB. #85
 * [BUGFIX] Ring: `ring_member_ownership_percent` and `ring_tokens_owned` metrics are not updated on scale down. #109

backoff/backoff.go

@@ -54,7 +54,7 @@ func (b *Backoff) Ongoing() bool {
 	return b.ctx.Err() == nil && (b.cfg.MaxRetries == 0 || b.numRetries < b.cfg.MaxRetries)
 }
 
-// Err returns the reason for terminating the backoff, or nil if it didn't terminate
+// Err returns the reason for terminating the backoff, or nil if it didn't terminate.
 func (b *Backoff) Err() error {
 	if b.ctx.Err() != nil {
 		return b.ctx.Err()
@@ -65,6 +65,15 @@ func (b *Backoff) Err() error {
 	return nil
 }
 
+// ErrCause is like Err() but returns the context cause if backoff is terminated because the
+// context has been canceled.
+func (b *Backoff) ErrCause() error {
+	if b.ctx.Err() != nil {
+		return context.Cause(b.ctx)
+	}
+	return b.Err()
+}
+
 // NumRetries returns the number of retries so far
 func (b *Backoff) NumRetries() int {
 	return b.numRetries

backoff/backoff_test.go

@@ -4,6 +4,9 @@ import (
 	"context"
 	"testing"
 	"time"
+
+	"github.com/pkg/errors"
+	"github.com/stretchr/testify/require"
 )
 
 func TestBackoff_NextDelay(t *testing.T) {
@@ -101,3 +104,68 @@ func TestBackoff_NextDelay(t *testing.T) {
 		})
 	}
 }
+
+func TestBackoff_ErrAndErrCause(t *testing.T) {
+	cause := errors.New("my cause")
+
+	tests := map[string]struct {
+		ctx              func(*testing.T) context.Context
+		expectedErr      error
+		expectedErrCause error
+	}{
+		"context deadline exceeded without cause": {
+			ctx: func(t *testing.T) context.Context {
+				ctx, cancel := context.WithDeadline(context.Background(), time.Now())
+				t.Cleanup(cancel)
+
+				return ctx
+			},
+			expectedErr:      context.DeadlineExceeded,
+			expectedErrCause: context.DeadlineExceeded,
+		},
+		"context deadline exceeded with cause": {
+			ctx: func(t *testing.T) context.Context {
+				ctx, cancel := context.WithDeadlineCause(context.Background(), time.Now(), cause)
+				t.Cleanup(cancel)
+
+				return ctx
+			},
+			expectedErr:      context.DeadlineExceeded,
+			expectedErrCause: cause,
+		},
+		"context is canceled without cause": {
+			ctx: func(_ *testing.T) context.Context {
+				ctx, cancel := context.WithCancel(context.Background())
+				cancel()
+
+				return ctx
+			},
+			expectedErr:      context.Canceled,
+			expectedErrCause: context.Canceled,
+		},
+		"context is canceled with cause": {
+			ctx: func(_ *testing.T) context.Context {
+				ctx, cancel := context.WithCancelCause(context.Background())
+				cancel(cause)
+
+				return ctx
+			},
+			expectedErr:      context.Canceled,
+			expectedErrCause: cause,
+		},
+	}
+
+	for testName, testData := range tests {
+		t.Run(testName, func(t *testing.T) {
+			b := New(testData.ctx(t), Config{})
+
+			// Wait until the backoff returns error.
+			require.Eventually(t, func() bool {
+				return b.Err() != nil
+			}, time.Second, 10*time.Millisecond)
+
+			require.Equal(t, testData.expectedErr, b.Err())
+			require.Equal(t, testData.expectedErrCause, b.ErrCause())
+		})
+	}
+}