Fix potential leaks from Mono.just(dataBuffer) or Flux.just(dataBuffer) [SPR-17575]

[spring-projects-issues]

Rossen Stoyanchev opened SPR-17575

When using Mono.just or Flux.just data buffers can be allocated before subscription and consequently won't even see a cancellation signal in case the connection is closed.

The second issue is that if Mono.just or Flux.just is passed into response#writeWith those will not see a cancellation signal because cancellation is not propagated to them (i.e. can't cancel what's already been materialized).

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Affects: 5.1.3

[rstoyanchev]

We do have a few instances of Mono.just or Flux.just with an allocated DataBuffer, e.g. in form writer, multipart writer, EncoderHttpMessageWriter, Jaxb2 encoder. While in the SSE writer the same is wrapped with Mono.defer.

Rescheduling for 5.1.6 to confirm those are issues and confirm the preferred fix, e.g. via defer? There is probably a small chance for the things that could go wrong before those are written. Nevertheless we should address it.

[poutsma]

We do have a few instances of Mono.just or Flux.just with an allocated DataBuffer

most of these examples do not have any memory leaks:

• the form writer decorates the ByteBuffer into a DataBuffer using DataBufferFactory.wrap(), which does not allocate.
• multipart writer does indeed appear to leak: all generate* methods should probably be returning Mono<DataBuffer> and use Mono.defer.
• EncoderHttpMessageWriter does not allocate buffers, so there is no need to release.
• Jaxb2 encoder has a boolean release flag that is used in the finally block.

[rstoyanchev]

EncoderHttpMessageWriter does not allocate buffers, so there is no need to release.

EncoderHttpMessageWriter itself does not but the encoder it delegates to does, and the use of flatMap will prefetch eagerly and would cache before it reaches Reactor Netty.

Jaxb2 encoder has a boolean release flag that is used in the finally block.

The boolean releases in case of an error. The scenario being discussed in this ticket is that the DataBuffer is produced eagerly before subscription, and a cancellation may occur before it is written.