Fix externalsampler interface

[penelopeysm]

v0.39.5 #2616 inadvertently changed the required interface for external samplers to include

AbstractMCMC.getparams(::DynamicPPL.Model, ::SamplerState)

rather than

Turing.Inference.getparams(::DynamicPPL.Model, ::SamplerTransition)

which is what came before it. Note that Turing.Inference.getparams serves a different purpose compared to AbstractMCMC.getparams and the external sampler interface should really use AbstractMCMC.getparams.

Furthermore, Turing.Inference.getparams is an internal function so in principle changing it should not require a breaking release --- but unfortunately I didn't catch that this internal function was part of the external sampler interface. So it was a mistake to make this change in a patch release. (Sorry.) This PR therefore reverts the change.

On top of that, the existing externalsampler tests did not catch this because they only tested on AdvancedMH and AdvancedHMC, for which both methods are overloaded.

So this PR also adds a test with a external sampler that satisfies the minimal interface as specified in the ExternalSampler docstring. This ensures that if we change the interface this test will break. See e.g. 'The Test Fake' in https://invenia.github.io/blog/2020/11/06/interfacetesting/.

[github-actions]

Turing.jl documentation for PR #2640 is available at:
https://TuringLang.github.io/Turing.jl/previews/PR2640/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.88%. Comparing base (d0510b1) to head (4b92f18).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2640   +/-   ##
=======================================
  Coverage   84.88%   84.88%           
=======================================
  Files          22       22           
  Lines        1475     1475           
=======================================
  Hits         1252     1252           
  Misses        223      223           

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[sunxd3]

LGTM (https://github.com/TuringLang/Turing.jl/actions/runs/16673713759/job/47195523882?pr=2640 is a bit worrying because on min)

[penelopeysm]

Yeahh, just noticed that too. It's been happening a few times and we're not sure what triggered it although rerunning CI tends to make it pass, so it's not fully deterministic.

[penelopeysm]

It might be this? JuliaLang/julia#54840

[penelopeysm]

I think that's the offending bug. The stack trace shows speccache_eq, and the segfault only happens with 2 threads, and it only happens sometimes, which all lines up with the fact that the issue is about data races. I checked the Julia source code and this bugfix doesn't seem to have been backported to 1.10.

In fact it wasn't backported to 1.11 either but I suppose it's possible that other changes were made to 1.11 that made the race conditions less likely to happen.

FYI @mhauru @AoifeHughes

[coveralls]

Pull Request Test Coverage Report for Build 16673713759

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage remained the same at 84.997%

---

Files with Coverage Reduction	New Missed Lines	%
src/mcmc/external_sampler.jl	2	91.49%

Totals
Change from base Build 16566258730:	0.0%
Covered Lines:	1252
Relevant Lines:	1473

---

💛 - Coveralls