Retry requests on a 429 that's a lock timeout (#841)

Tweaks the retry logic so that 429s which have the special status code
lock_timeout are retried automatically.

Similar to what was recently implemented in Go: stripe/stripe-go#935

lib/stripe/stripe_client.rb

@@ -88,6 +88,16 @@ def self.should_retry?(error, method:, num_retries:)
         # 409 Conflict
         return true if error.http_status == 409
 
+        # 429 Too Many Requests
+        #
+        # There are a few different problems that can lead to a 429. The most
+        # common is rate limiting, on which we *don't* want to retry because
+        # that'd likely contribute to more contention problems. However, some
+        # 429s are lock timeouts, which is when a request conflicted with
+        # another request or an internal process on some particular object.
+        # These 429s are safe to retry.
+        return true if error.http_status == 429 && error.code == "lock_timeout"
+
         # 500 Internal Server Error
         #
         # We only bother retrying these for non-POST requests. POSTs end up

test/stripe/stripe_client_test.rb

@@ -122,6 +122,12 @@ class StripeClientTest < Test::Unit::TestCase
                                           method: :post, num_retries: 0)
       end
 
+      should "retry on a 429 Too Many Requests when lock timeout" do
+        assert StripeClient.should_retry?(Stripe::StripeError.new(http_status: 429,
+                                                                  code: "lock_timeout"),
+                                          method: :post, num_retries: 0)
+      end
+
       should "retry on a 500 Internal Server Error when non-POST" do
         assert StripeClient.should_retry?(Stripe::StripeError.new(http_status: 500),
                                           method: :get, num_retries: 0)
@@ -142,6 +148,12 @@ class StripeClientTest < Test::Unit::TestCase
                                           method: :post, num_retries: 0)
       end
 
+      should "not retry on a 429 Too Many Requests when not lock timeout" do
+        refute StripeClient.should_retry?(Stripe::StripeError.new(http_status: 429,
+                                                                  code: "rate_limited"),
+                                          method: :post, num_retries: 0)
+      end
+
       should "not retry on a 500 Internal Server Error when POST" do
         refute StripeClient.should_retry?(Stripe::StripeError.new(http_status: 500),
                                           method: :post, num_retries: 0)