test: Fix too many files open errors

[msbrogli]

Acceptance Criteria

1. Be able to run tests without increasing the maximum number of open files.
2. Properly close the file descriptor after it's been used.

[codecov]

Codecov Report

Merging #503 (60b0f33) into dev (7cba651) will increase coverage by 0.11%.
The diff coverage is n/a.

❗ Current head 60b0f33 differs from pull request most recent head d80cb63. Consider uploading reports for the commit d80cb63 to get more accurate results

@@            Coverage Diff             @@
##              dev     #503      +/-   ##
==========================================
+ Coverage   83.45%   83.56%   +0.11%     
==========================================
  Files         188      187       -1     
  Lines       17083    16829     -254     
  Branches     2642     2590      -52     
==========================================
- Hits        14256    14063     -193     
+ Misses       2339     2301      -38     
+ Partials      488      465      -23     

Impacted Files	Coverage Δ
hathor/prometheus.py	86.20% <0.00%> (-3.45%)	⬇️
hathor/debug_resources.py	56.12% <0.00%> (-3.07%)	⬇️
hathor/exception.py	100.00% <0.00%> (ø)
hathor/builder.py

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[luislhl]

The error we are trying to fix happened only when running tests? Do we have more info about it?

[jansegre]

The error we are trying to fix happened only when running tests? Do we have more info about it?

It definitely happens on my machine:

❯ uname -a
Darwin mewtwo.local 22.1.0 Darwin Kernel Version 22.1.0: Sun Oct  9 20:15:09 PDT 2022; root:xnu-8792.41.9~2/RELEASE_ARM64_T6000 arm64

Which by default has a low ulimit:

❯ ulimit -n
256

I can't set my ulimit to 1000 or higher:

❯ ulimit -n 10000
ulimit: Permission denied when changing resource of type 'Maximum number of open file descriptors'

But I can set it to 9999:

❯ ulimit -n 9999

I can't complete a large suit of tests (like make tests-lib or pytest tests/tx/) without it starting to fail nearly every test after a certain point, and eventually I get to see that all the errors are from "too many open files" when trying to open a rocksdb database.

Since this was only on my machine and not on any of the test environments I never bothered too much, but I guess this started happening for Marcelo too.

I'll test if this fix works for me, maybe it is possible to setup the CI to similar conditions (like lowering the ulimit), but I'm not sure if it's worth it. It might be good because it's a sign that we're cleaning up correctly, but most usually it is something test-environment related and dev-system specific.