error-stack support Debug hooks in no-std contexts

[indietyp]

🌟 What is the purpose of this PR?

While exploring #1399 we found the spin crate, which implements a no-std equivalent for std::sync types, like RwLock based on spin locks. This PR adds a new feature, hooks, which - if enabled - will replace the RwLock with the spin variant.

Note: while we expect a majority of calls to be read-only, there's still a chance of priority inversion, this is why if both std and hooks features are enabled, we will instead use the std RwLock.

Must read for spin locks and their application: https://matklad.github.io/2020/01/02/spinlocks-considered-harmful.html

I consider using a spinlock okay for error-stack hooks, as we expect hooks to only be initialized very, very infrequently, and most operations are reads. A read is simply a fetch_add + fetch_sub without any spinning.

An alternative approach would be using critical-section instead, which acts like a Mutex, but because we mostly only read, we do not need to hold the hooks exclusively, and critical-section is only available on single thread embedded microprocessors.

Note: this increases our test permutation count to 81!

🔗 Related links

• https://matklad.github.io/2020/01/02/spinlocks-considered-harmful.html

🚫 Blocked by

🔍 What does this change?

• new feature hook

📋 TODO

• Documentation
• Changelog

📜 Does this require a change to the docs?

We need to mention the new feature in our table and the consequences.

[codecov]

Codecov Report

Merging #1556 (81c6c68) into main (605777e) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1556      +/-   ##
==========================================
+ Coverage   56.70%   56.74%   +0.04%     
==========================================
  Files         219      219              
  Lines       14629    14644      +15     
  Branches      382      382              
==========================================
+ Hits         8295     8310      +15     
  Misses       6329     6329              
  Partials        5        5              

Flag	Coverage Δ
backend-integration-tests	2.70% <ø> (ø)
error-stack	90.89% <100.00%> (+0.09%)	⬆️
unit-tests	1.75% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/libs/error-stack/src/lib.rs	38.46% <ø> (ø)
packages/libs/error-stack/src/fmt/hook.rs	89.74% <100.00%> (+0.13%)	⬆️
packages/libs/error-stack/src/fmt/mod.rs	95.12% <100.00%> (ø)
packages/libs/error-stack/src/hook.rs	100.00% <100.00%> (ø)

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[TimDiekmann]

LGTM, thank you!

Do we need to adjust the Makefile to also test hooks on insta checks?

One other question, then I'm happy to approve

[TimDiekmann]

Does 0.9 also work for our use cases?

[indietyp]

requirement relaxed in 122a119

[TimDiekmann]

Great, thanks!

[indietyp]

Thanks! I completely forgot about the snapshot tests, I have adjusted the Makefile and test cfg guard

[TimDiekmann]

Excellent, thank you, and also for providing the link to the blog post.