fix: re-encode multipart body on request retry

lib/auth0/internal/multipart/multipart_encoder.rb

@@ -47,11 +47,18 @@ def content_type
 
         # Encode the given FormData object into a multipart/form-data payload.
         #
+        # Each call starts from an empty body and keeps the same boundary, so the
+        # payload can be encoded again when a request is retried.
+        #
         # @param form_data [FormData] The form data to encode
         # @return [String] The encoded body.
         def encode(form_data)
           return "" if form_data.parts.empty?
 
+          @body = String.new
+          @closed = false
+          @first_field = true
+
           form_data.parts.each do |part|
             write_part(part)
           end

test/unit/internal/multipart/test_multipart_encoder.rb

@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+
+require "test_helper"
+require "stringio"
+require "webmock"
+
+describe Auth0::Internal::Multipart::Encoder do
+  def build_form_data
+    form_data = Auth0::Internal::Multipart::FormData.new
+    form_data.add(name: "connection_id", value: "con_123")
+    form_data.add_file(name: "users", file: StringIO.new('[{"email":"a@example.com"}]'), filename: "users.json")
+    form_data
+  end
+
+  describe "#encode" do
+    it "returns the same payload when called more than once" do
+      form_data = build_form_data
+
+      first = form_data.encode
+      second = form_data.encode
+
+      boundary = form_data.content_type.split("boundary=").last
+
+      assert_equal first, second
+      assert_equal 1, second.scan("--#{boundary}--").size
+    end
+  end
+
+  describe "when a multipart request is retried" do
+    include WebMock::API
+
+    before do
+      WebMock.enable!
+      WebMock.disable_net_connect!
+    end
+
+    after do
+      WebMock.reset!
+      WebMock.disable!
+    end
+
+    it "sends the same body on every attempt" do
+      url = "https://example.auth0.com/api/v2/jobs/users-imports"
+      bodies = []
+      stub_request(:post, url).to_return do |request|
+        bodies << request.body
+        { status: bodies.size == 1 ? 503 : 201, body: "{}" }
+      end
+
+      form_data = build_form_data
+      request = Auth0::Internal::Multipart::Request.new(
+        base_url: "https://example.auth0.com/api/v2",
+        path: "jobs/users-imports",
+        method: "POST",
+        body: form_data
+      )
+      raw_client = Auth0::Internal::Http::RawClient.new(base_url: "https://example.auth0.com/api/v2", max_retries: 1)
+
+      response = raw_client.send(request)
+
+      assert_equal "201", response.code
+      assert_equal 2, bodies.size
+      assert_equal bodies[0], bodies[1]
+      assert_includes bodies[1], 'name="connection_id"'
+    end
+  end
+end