cache: Ensure that goroutines never block

[pav-kv]

Fixes #2271

[AlCutter]

/gcbrun

[Martin2112]

The good news is that it looks like the condition that could have triggered this is very unlikely, internal errors resulting from corruption or the wrong proto type.

[pav-kv]

Yeah, this fix is mainly to make the static analysis tool happy (see the bug).

[pav-kv]

We can also assert that len(subtrees) <= len(want), otherwise it can allocate an arbitrarily large channel.

[codecov]

Codecov Report

Merging #2272 (feab950) into master (8100b63) will decrease coverage by 0.03%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #2272      +/-   ##
==========================================
- Coverage   65.06%   65.02%   -0.04%     
==========================================
  Files         117      117              
  Lines        8904     8906       +2     
==========================================
- Hits         5793     5791       -2     
- Misses       2451     2454       +3     
- Partials      660      661       +1     

Impacted Files	Coverage Δ
storage/cache/subtree_cache.go	54.16% <50.00%> (-0.58%)	⬇️
log/operation_manager.go	85.86% <0.00%> (-1.05%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8100b63...feab950. Read the comment docs.