feat: add demand pausing operator for Multi

[ozangunalp]

Introduces a new pauseDemand() operator that allows pausing and resuming request propagation in reactive streams.

First used in Reactive Messaging pausable channels, this implementation also handles buffering already requested items

[codecov]

Codecov Report

❌ Patch coverage is 88.53503% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.06%. Comparing base (556bc46) to head (e5ff7cf).
⚠️ Report is 39 commits behind head on main.

Files with missing lines	Patch %	Lines
...e/mutiny/operators/multi/MultiDemandPausingOp.java	84.54%	7 Missing and 10 partials ⚠️
...tation/src/main/java/io/smallrye/mutiny/Multi.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2007      +/-   ##
============================================
- Coverage     89.11%   89.06%   -0.05%     
- Complexity     3068     3101      +33     
============================================
  Files           409      412       +3     
  Lines         13099    13256     +157     
  Branches       1652     1679      +27     
============================================
+ Hits          11673    11807     +134     
- Misses          808      818      +10     
- Partials        618      631      +13     

Files with missing lines	Coverage Δ
.../io/smallrye/mutiny/groups/MultiDemandPausing.java	100.00% <100.00%> (ø)
...va/io/smallrye/mutiny/operators/AbstractMulti.java	100.00% <100.00%> (ø)
.../io/smallrye/mutiny/subscription/DemandPauser.java	100.00% <100.00%> (ø)
...tation/src/main/java/io/smallrye/mutiny/Multi.java	89.47% <0.00%> (-4.98%)	⬇️
...e/mutiny/operators/multi/MultiDemandPausingOp.java	84.54% <84.54%> (ø)

... and 10 files with indirect coverage changes

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[jponge]

This is an excellent contribution @ozangunalp 👏

See my comments, plus:

• the MultiDemandPausingTest recorded in the Git tree has some minor reformat being applied at compilation time
• I don't expect this operator to be any Reactive Streams compliant, so we should document this both in the guide and in the Javadocs.

[ozangunalp]

@jponge I am not sure about the Reactive Streams compliance of this operator. Sure, it can drop some items if it is intentional, but it doesn't emit more than what was requested from downstream.

[jponge]

That's interesting. I suppose that it won't pass the TCK when paused, but it should pass the TCK when it is not.

We should check this then 😇

[ozangunalp]

That's interesting. I suppose that it won't pass the TCK when paused, but it should pass the TCK when it is not.

I pushed a TCK test with pause/resume at every received item.

[jponge]

Let's finish the TCK discussion, then the rest of the PR is excellent!

[jponge]

I don't think this is different from passing the TCK to the upstream, since we pause then resume on the same call stack.

I think we should have this, or just have the operator as a pass-through, but if we have a TCK test where we pause for longer periods of time then I anticipate the TCK will complain.

[ozangunalp]

Yes this only verifies that a simple pause/resume doesn't break the TCK.

Another test is needed with an async consumption and an operator with prefetch.

I think if the operator is eventually resumed inside the test it should pass the TCK.

[ozangunalp]

@jponge I pushed a small update to the operator and the TCK test.
It now passes the TCK without a pass-through stream as long as the stream is not left in a paused state.
Specifically, it now handles the completion case better.

I also added a note to the javadoc.

[jponge]

Thanks, let's re-run the CI :-)

[ozangunalp]

That NPE is quite something : https://github.com/smallrye/smallrye-mutiny/actions/runs/19634508391/job/56221872235

[jponge]

All good!

Let's just not merge now, I might have a fix or two to stage in a patch release over the next 2 weeks.

[jponge]

I'm doing another patch release, then this will go to main