[FIXED] Reuse msg/hdr from pooled jsPubMsg

[MauriceVanVeen]

Both the pm.buf and pm.hdr are meant to be reused, but the pm.hdr = nil above would prevent running pm.hdr = pm.hdr[:0]. Similarly, pm.msg also wasn't reused.

Signed-off-by: Maurice van Veen github@mauricevanveen.com
Co-authored-by: Neil Twigg neil@nats.io

[MauriceVanVeen]

Seems a data race was hiding here when reusing the hdr:

WARNING: DATA RACE
Write at 0x00c0029cf560 by goroutine 81975:
  runtime.slicecopy()
      /opt/hostedtoolcache/go/1.25.6/x64/src/runtime/slice.go:355 +0x0
  github.com/nats-io/nats-server/v2/server.newJSPubMsg()
      /home/runner/work/nats-server/nats-server/server/stream.go:6777 +0x155
  github.com/nats-io/nats-server/v2/server.(*consumer).releaseAnyPendingRequests()
      /home/runner/work/nats-server/nats-server/server/consumer.go:2851 +0x288
  github.com/nats-io/nats-server/v2/server.(*consumer).stopWithFlags()
      /home/runner/work/nats-server/nats-server/server/consumer.go:6071 +0x3f3
  github.com/nats-io/nats-server/v2/server.(*consumer).delete()
      /home/runner/work/nats-server/nats-server/server/consumer.go:6008 +0x3e
  github.com/nats-io/nats-server/v2/server.TestJetStreamWorkQueueMaxWaiting.func1.deferwrap3()
      /home/runner/work/nats-server/nats-server/server/jetstream_test.go:1125 +0x17

Previous read at 0x00c0029cf560 by goroutine 82001:
  runtime.growslice()
      /opt/hostedtoolcache/go/1.25.6/x64/src/runtime/slice.go:177 +0x0
  github.com/nats-io/nats-server/v2/server.(*stream).internalLoop()
      /home/runner/work/nats-server/nats-server/server/stream.go:6971 +0x18ac
  github.com/nats-io/nats-server/v2/server.(*stream).setupSendCapabilities.gowrap2()
      /home/runner/work/nats-server/nats-server/server/stream.go:6864 +0x33

[MauriceVanVeen]

The race was due to reusing []byte("NATS/1.0 409 Consumer Deleted\r\n\r\n") for all released pull requests.
Have updated the PR to also reuse pm.msg, since only pm.buf was before, and pm.hdr was meant to be also.

[neilalexander]

LGTM