refactor(retrypolicy): fix off-by-one in computeBackoff logging

retry-go's OnRetry callback supplies a 1-based retry attempt number,
so computeBackoff(n+1, ...) logged a delay that was one doubling
ahead of the actual backoff used by the retry loop. Pass n directly.

Also drop the unreachable attempt == 0 branch in computeBackoff
(retry-go never supplies 0) and skip that case in the test.

Addresses review feedback on PR #468 (gemini).

Signed-off-by: Zhao Chen <winters.zc@antgroup.com>

Author: aftersnow

pkg/retrypolicy/retrypolicy.go

@@ -130,7 +130,7 @@ func Do(ctx context.Context, fn func(ctx context.Context) error, opts DoOpts) er
 			return retryable
 		}),
 		retry.OnRetry(func(n uint, err error) {
-			backoff := computeBackoff(n+1, initialDelay, maxBackoff)
+			backoff := computeBackoff(n, initialDelay, maxBackoff)
 			elapsed := time.Since(startTime)
 
 			log.WithFields(log.Fields{
@@ -173,10 +173,9 @@ func computeDynamicParams(fileSize int64) (time.Duration, time.Duration) {
 
 // computeBackoff estimates the backoff duration for display purposes.
 // It mirrors the exponential backoff calculation without jitter.
+// The attempt parameter is the 1-based retry number as provided by
+// retry-go's OnRetry callback, so it is always >= 1.
 func computeBackoff(attempt uint, initial, maxDelay time.Duration) time.Duration {
-	if attempt == 0 {
-		return initial
-	}
 	backoff := time.Duration(float64(initial) * math.Pow(2, float64(attempt-1)))
 	if backoff > maxDelay {
 		backoff = maxDelay

pkg/retrypolicy/retrypolicy_test.go

@@ -523,10 +523,8 @@ func TestComputeBackoff(t *testing.T) {
 	initial := 5 * time.Second
 	maxDelay := 1 * time.Minute
 
-	// attempt 0 => initial
-	if got := computeBackoff(0, initial, maxDelay); got != initial {
-		t.Errorf("attempt 0: got %v, want %v", got, initial)
-	}
+	// retry-go's OnRetry always supplies a 1-based attempt number, so 0 is
+	// not a value the function is ever called with in production. Start from 1.
 
 	// attempt 1 => 5s * 2^0 = 5s
 	if got := computeBackoff(1, initial, maxDelay); got != 5*time.Second {