Multipart HttpMessageConverter drops parts with empty bodies

[shaggyinsomniac]

Description

The new servlet MultipartHttpMessageConverter (spring-web, 7.1) silently drops every part whose body is empty, so a blank form field is indistinguishable from an absent field.

Analysis

Parts are emitted exclusively from PartListener.onBody(buffer, last=true) (PartGenerator: FormFieldState, InMemoryState, FileState). For an empty part, MultipartParser.BodyState computes len == 0 and transitions to the next headers state without ever invoking onBody, so the part is never added to the resulting MultiValueMap.

This is the servlet-side equivalent of the issue fixed for the reactive stack in #30953 (regression test: DefaultPartHttpMessageReaderTests.emptyLastPart asserts empty parts are emitted). The new servlet port did not carry the fix over; MultipartParserTests.noBody() currently pins the broken event sequence.

Impact

An HTML form with a blank text input yields "field absent" instead of "field = empty string", which can flip application logic that distinguishes the two (including auth/business-rule branches).

Reproduction

GET-style body:

--boundary
Content-Disposition: form-data; name="text1"

<empty>
--boundary
Content-Disposition: form-data; name="text2"

a
--boundary--

Reading via MultipartHttpMessageConverter returns only text2; text1 is missing.

[bclozel]

Superseded by #37202