Play Framework

[PF-32] Netty 4 Stage B: restore disk-spooling for large request bodies

Stage A (PF-31) deleted the Netty 3-coupled StreamChunkAggregator and
defaulted the pipeline to the stock HttpObjectAggregator with a 1 MB
in-memory cap, which rejected large uploads with 413. This stage
reintroduces a Netty 4-native streaming aggregator that keeps small
bodies in memory and spools larger bodies to a temp file, so file
uploads work end-to-end again without unbounded heap usage.

New StreamChunkAggregator (ChannelInboundHandlerAdapter):
- Accumulates HttpRequest + HttpContent into a CompositeByteBuf while
  bytesReceived <= play.netty.spoolThresholdBytes (default 1 MB).
- On crossing the threshold, opens a temp file under Play.tmpDir, drains
  the in-memory buffer to disk, and writes subsequent HttpContent direct
  to the file.
- On LastHttpContent, emits a DefaultFullHttpRequest downstream:
  in-memory bodies travel as the request's content ByteBuf; spooled
  bodies use Unpooled.EMPTY_BUFFER and stash the temp File on a channel
  AttributeKey<File> SPOOLED_BODY for PlayHandler.parseRequest to pick
  up. Transfer-Encoding header is stripped and Content-Length is set to
  the aggregated length on emit.
- Hard cap via play.netty.maxContentLength (default -1 = unlimited):
  exceeded requests are rejected with 413 + Connection: close, drained
  to LastHttpContent, then the connection is closed. No further work
  reaches PlayHandler for a rejected request.
- Refcount discipline: every HttpContent.release() in finally; retain()
  on slices added to the CompositeByteBuf so they survive chunk release.
- channelInactive and exceptionCaught delete any in-flight temp file so
  a dropped connection mid-upload doesn't leak Play.tmpDir entries.

PlayHandler.parseRequest:
- Reads ctx.channel().attr(SPOOLED_BODY).getAndSet(null) atomically on
  the IO thread (safe under HTTP/1.1 keep-alive request serialization)
  and switches between FileInputStream and ByteArrayInputStream body
  paths. The spool File and the FileInputStream are stashed on the
  Http.Request.args map under __play.spoolFile / __play.spoolStream.
- New cleanupSpooledBody helper (idempotent) closes the stream and
  deletes the file; falls back to deleteOnExit() if delete fails (e.g.,
  a Windows file-still-open race).

NettyInvocation.run() finally-clause now calls cleanupSpooledBody so
disk leaks can't survive a controller throwing.

Pipeline defaults flipped from HttpObjectAggregator back to
play.server.StreamChunkAggregator in HttpServerPipelineFactory and
SslHttpServerPipelineFactory. The aggregator defines its own no-arg
constructor that reads play.netty.spoolThresholdBytes and
play.netty.maxContentLength from Play.configuration, so the existing
reflective handler-construction path needs no special-case for it.
HttpObjectAggregator's int-arg fallback in getInstance() is preserved
for users who explicitly wire it.

application-skel/conf/application.conf documented defaults updated;
adds a brief note on play.netty.spoolThresholdBytes vs maxContentLength.

Verified with `ant clean unittest`: 295 tests pass, 0 failures.

PF-33 (Stage C) will restore WebSocket support and retire the
deprecated Http.Inbound(ChannelHandlerContext) ctor.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>