Cherry-pick #3063 to the stable branch

[ulan]

• Restore POSIX signal handling on MacOS behind a feature flag

As described in Issue #3052, the switch to Mach Exception handling
removed unix::StoreExt from the public API of crate on MacOS.
That is a breaking change and makes it difficult for some
application to upgrade to the current stable Wasmtime.

As a workaround this PR introduces a feature flag called
posix-signals-on-macos that restores the old behaviour on MacOS.
The flag is disabled by default.

• Fix test guard

• Fix formatting in the test

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[alexcrichton]

Thanks for this! AFAIK though we don't really have any guidelines about what's backported and what historical versions are maintained or not. So far we have only backported and done a point release once, and that was for a security fix.

Is upgrading to the current wasmtime main branch possible for you? It should be easier to make a new release for us than to make another older release.

[ulan]

Right, I was also wondering if there is a process for backporting.

Is upgrading to the current wasmtime main branch possible for you?

Yep, we can update to it although it will take some time to adjust our code to the API redesign that happened in 0.28.0.

My only concern is the stability of that version. My understanding was that 0.26.1 is the current stable version suitable for production whereas the newer versions may have less coverage and may be unstable. Is this not correct?

[alexcrichton]

If anything this is the process of backporting, we just haven't written it down anywhere or really exercised it that much. (we also don't have any process yet to determine what changes should be backported, a security fix was just sort of "obviously correct" to backport)

In terms of stability and production-ready-ness all recent Wasmtime versions should suffice. We mostly make new releases for releasing new features. Currently we don't have a notion of a "stable release" or similar. We hope to signal this soon with a 1.0 release that additionally signals a level of API stability, but in the meantime the quality of the implementation of 0.26 should be the same as 0.28 and beyond.

[ulan]

Thank you for the clarification! That sounds good. I assumed too much based on the branch name stable-v0.26.

I'll start working on updating our codebase to the main branch of wasmtime.

[alexcrichton]

FWIW if this is urgent and updating to 0.28 in a reasonable timeframe isn't feasible I don't think we have a policy against backports like this, we'd just probably need to discuss it with a few more folks other than just me

[ulan]

@alexcrichton thanks, I'll keep that in mind and may come back to it if other options do not work out.