fix: honor raw StringIO request cursors

lib/openai/internal/util.rb

@@ -638,7 +638,10 @@ def encode_content(headers, body)
           in [_, Symbol | Numeric]
             [headers, body.to_s]
           in [_, StringIO]
-            [headers, body.string]
+            [headers, body.string.byteslice(body.pos..) || body.string.byteslice(0, 0)]
+          in [_, OpenAI::FilePart] if body.content.is_a?(StringIO)
+            content = body.content
+            [headers, content.string.byteslice(content.pos..) || content.string.byteslice(0, 0)]
           in [_, OpenAI::FilePart]
             [headers, body.content]
           else

test/openai/raw_stringio_request_test.rb

@@ -0,0 +1,99 @@
+# frozen_string_literal: true
+
+require_relative "test_helper"
+
+class OpenAI::Test::RawStringIORequestTest < Minitest::Test
+  extend Minitest::Serial
+
+  include WebMock::API
+
+  REQUEST_URL = "https://example.test/probe"
+
+  def before_all
+    super
+    WebMock.enable!
+  end
+
+  def teardown
+    WebMock.reset!
+    super
+  end
+
+  def after_all
+    WebMock.disable!
+    super
+  end
+
+  def test_request_sends_bytes_remaining_at_current_cursor_without_consuming
+    [
+      [0, "abcdef"],
+      [3, "def"],
+      [6, ""],
+      [10, ""]
+    ].each do |position, expected|
+      raw_forms.each do |wrap|
+        source = StringIO.new("abcdef")
+        source.pos = position
+
+        assert_equal([expected], request_bodies(wrap.call(source)), "byte position #{position}")
+        assert_equal(position, source.pos)
+        assert_equal("abcdef", source.string)
+        assert_equal(Encoding::UTF_8, source.string.encoding)
+        refute_predicate(source, :closed?)
+      end
+    end
+  end
+
+  def test_request_slices_at_multibyte_byte_cursor_without_consuming
+    raw_forms.each do |wrap|
+      source = StringIO.new("éx")
+      source.pos = "é".bytesize
+
+      assert_equal(["x"], request_bodies(wrap.call(source)))
+      assert_equal("é".bytesize, source.pos)
+      assert_equal(Encoding::UTF_8, source.string.encoding)
+      refute_predicate(source, :closed?)
+    end
+  end
+
+  def test_retry_replays_identical_remaining_bytes_for_each_raw_form
+    raw_forms.each do |wrap|
+      source = StringIO.new("abcdef")
+      source.pos = 3
+
+      assert_equal(%w[def def], request_bodies(wrap.call(source), retry_once: true))
+      assert_equal(3, source.pos)
+      assert_equal("abcdef", source.string)
+      assert_equal(Encoding::UTF_8, source.string.encoding)
+      refute_predicate(source, :closed?)
+    end
+  end
+
+  private def raw_forms
+    [-> (source) { source }, -> (source) { OpenAI::FilePart.new(source) }]
+  end
+
+  private def request_bodies(body, retry_once: false)
+    bodies = []
+    stub_request(:post, REQUEST_URL).to_return do |request|
+      bodies << request.body
+      status = retry_once && bodies.one? ? 500 : 200
+      {status: status, headers: {"content-type" => "application/json"}, body: "{}"}
+    end
+
+    client = OpenAI::Client.new(
+      base_url: "https://example.test",
+      api_key: "fake-test-key",
+      max_retries: retry_once ? 1 : 0,
+      initial_retry_delay: 0,
+      max_retry_delay: 0
+    )
+    client.request(
+      method: :post,
+      path: "probe",
+      headers: {"content-type" => "application/octet-stream"},
+      body: body
+    )
+    bodies
+  end
+end