Implement payment retry logic

internal/retry/retry.go

@@ -0,0 +1,38 @@
+package retry
+
+import (
+	"context"
+	"time"
+)
+
+// Policy configures automatic retry behavior for failed payment operations.
+type Policy struct {
+	MaxAttempts int
+	Backoff     time.Duration
+}
+
+// DefaultPolicy retries up to 3 times with a 2 second backoff between
+// attempts.
+var DefaultPolicy = Policy{MaxAttempts: 3, Backoff: 2 * time.Second}
+
+// Do runs fn, retrying on error according to the policy. It stops retrying
+// once ctx is done and returns the last error encountered.
+func (p Policy) Do(ctx context.Context, fn func() error) error {
+	var err error
+	for attempt := 1; attempt <= p.MaxAttempts; attempt++ {
+		if err = fn(); err == nil {
+			return nil
+		}
+
+		if attempt == p.MaxAttempts {
+			break
+		}
+
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		case <-time.After(p.Backoff):
+		}
+	}
+	return err
+}