`BodyTypeMismatch` is thrown for multipart requests with different boundary attributes

Given the test below, the following mismatches will be generated:

Mismatches

Mismatches: [BodyTypeMismatch { expected: "multipart/form-data;boundary=d79da11d45fff2d33b9abdc83fa82b85676b6be49783211ee847923acd11", actual: "multipart/form-data;boundary=21ab08b0e8448ae4ed94a64f0e02baaea0206dda648442d5b06d49972523", mismatch: "Expected a body of 'multipart/form-data;boundary=d79da11d45fff2d33b9abdc83fa82b85676b6be49783211ee847923acd11' but the actual content type was 'multipart/form-data;boundary=21ab08b0e8448ae4ed94a64f0e02baaea0206dda648442d5b06d49972523'", expected_body: Some(b"--d79da11d45fff2d33b9abdc83fa82b85676b6be49783211ee847923acd11\r\nContent-Disposition: form-data; name=\"baz\"\r\n\r\nbat\r\n--d79da11d45fff2d33b9abdc83fa82b85676b6be49783211ee847923acd11--\r\n"), actual_body: Some(b"--21ab08b0e8448ae4ed94a64f0e02baaea0206dda648442d5b06d49972523\r\nContent-Disposition: form-data; name=\"baz\"\r\n\r\nbat\r\n--21ab08b0e8448ae4ed94a64f0e02baaea0206dda648442d5b06d49972523--\r\n") }]
2025-01-07T04:38:48.163393Z DEBUG tokio-runtime-worker pact_mock_server::hyper_server: Request did not match: Request did not match - HTTP Request ( method: POST, path: /formpost, query: None, headers: Some({"Content-Type": ["multipart/form-data;boundary=d79da11d45fff2d33b9abdc83fa82b85676b6be49783211ee847923acd11"]}), body: Present(181 bytes) )    0) Expected a body of 'multipart/form-data;boundary=d79da11d45fff2d33b9abdc83fa82b85676b6be49783211ee847923acd11' but the actual content type was 'multipart/form-data;boundary=21ab08b0e8448ae4ed94a64f0e02baaea0206dda648442d5b06d49972523'

Test

func TestMultipart(t *testing.T) {
	log.SetLogLevel("TRACE")

	mockProvider, err := consumer.NewV2Pact(consumer.MockHTTPProviderConfig{
		Consumer: "PactGoV2Consumer",
		Provider: "V2Provider",
		Host:     "127.0.0.1",
	})

	assert.NoError(t, err)

	// Set up our expected interactions.
	mockProvider.
		AddInteraction().
		UponReceiving("A multipart request").
		WithRequestPathMatcher("POST", S("/formpost"), func(b *consumer.V2RequestBuilder) {
			var buf bytes.Buffer
			content := multipart.NewWriter(&buf)
			// content.SetBoundary("abcd")
			content.WriteField("baz", "bat")
			content.Close()

			b.Body(fmt.Sprintf("multipart/form-data;boundary=%s", content.Boundary()), buf.Bytes())
			b.Header("Content-Type", Regex(fmt.Sprintf("multipart/form-data;boundary=%s", content.Boundary()), "multipart/form-data.*"))
		}).
		WillRespondWith(200).
		ExecuteTest(t, func(config consumer.MockServerConfig) error {
			client := &http.Client{}

			var buf bytes.Buffer
			content := multipart.NewWriter(&buf)
			//content.SetBoundary("abcd")
			content.WriteField("baz", "bat")
			content.Close()

			req := &http.Request{
				Method: "POST",
				URL: &url.URL{
					Host:   fmt.Sprintf("%s:%d", "localhost", config.Port),
					Scheme: "http",
					Path:   "/formpost",
				},
				Body:   io.NopCloser(&buf),
				Header: make(http.Header),
			}

			req.Header.Set("Content-Type", fmt.Sprintf("multipart/form-data;boundary=%s", content.Boundary()))

			_, err := client.Do(req)

			return err
		})
	assert.NoError(t, err)
}

Uncommenting the SetBoundary methods will cause the test setup and HTTP calls to have the same boundary, and the tests pass.

Desired behaviour

The user should be able to specify a regex that bypasses the specific boundary attribute checking (see working JVM example with desired behaviour: https://github.com/pact-foundation/pact-jvm/blob/master/consumer/junit5/src/test/java/au/com/dius/pact/consumer/junit5/MultipartRequestTest.java#L50-L55)

Discussion

The pactffi_with_body FFI method is the underlying function used here to set the initial body and content type.

The body content matcher should only be checking the main type to see if it got the correct content type (multipart/form-data) and not the attributes. This seems to be the cause of the issue and perhaps is only present in the FFI.

Start with the Go TestMultipart example and trace how pactffi_with_body sets the initial body and content type. Inspect the body content matcher to determine where multipart attributes are compared. Done means a regex can ignore the boundary while still matching multipart/form-data, with the provided test passing.