src: introduce Constant class

[mmarchini]

Same idea as CheckedType: instead of relying on the default value for constants to determine if they were loaded correctly or not (default value usually was -1), have a class which will have information about the loaded constant. This can help us:

• Check if a constant was properly loaded before using it (otherwise we'd get into undefined behavior, and there are a lot of places where this happens.
• Check if a constant value was loaded or if we're looking at the default value.
• Explicitly define a constant as optional (constants which are not relevant except for specific V8 versions).

This will help us improve reliability and debugability of llnode.

[codecov-io]

Codecov Report

Merging #303 into master will decrease coverage by 1.41%. The diff coverage is 70.58%.

@@            Coverage Diff            @@
##           master    #303      +/-   ##
=========================================
- Coverage   79.02%   77.6%   -1.42%     
=========================================
  Files          33      33              
  Lines        4247    4260      +13     
=========================================
- Hits         3356    3306      -50     
- Misses        891     954      +63

Impacted Files	Coverage Δ
src/llv8.h	80.95% <ø> (-2.39%)	:arrow_down:
src/llv8-constants.h	98.48% <ø> (-1.52%)	:arrow_down:
src/error.h	75% <0%> (-10.72%)	:arrow_down:
src/llv8-constants.cc	82.05% <100%> (-1.02%)	:arrow_down:
src/llv8-inl.h	86.66% <100%> (-6.25%)	:arrow_down:
src/constants.cc	72.72% <65.78%> (-8.63%)	:arrow_down:
src/constants.h	77.77% <83.33%> (+11.11%)	:arrow_up:
src/llv8.cc	70.22% <0%> (-2.75%)	:arrow_down:
... and 3 more

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[mmarchini]

I'll update the commit message later, but the main reason I started to work on this was to reduce noise from LookupConstant and LoadConstant debug messages. We don't want to see a message if a constant wasn't loaded but the replacement constant was (we were seeing two messages for the same constant in this case). The reduced noise from this PR helped me a lot while working on #255 (PR for that will come after this one lands).

[mmarchini]

cc @nodejs/llnode

[mmarchini]

Ping @nodejs/llnode. This is the main blocker for the patches I have to fix inspection of Node.js v12 core dumps/processes.

(I can rewrite those patches in the old way if this Constant class is not something we want, but from experience the code looks cleaner and easier to debug with this patch)

[mmarchini]

Ping @nodejs/llnode @nodejs/diagnostics @nodejs/post-mortem

[mmarchini]

Ah, yes, that makes a lot more sense. I'll update the PR once I get a chance (probably later this week). Thank you for the suggestion :)

[mmarchini]

• Changed LookupConstant to return a Constant object instead of std::pair<int64_t, bool>
• Added a ConstantStatus to replace bool loaded_ and bool valid_
• Added a comment explaining how the Constant class should be interpreted and used
• Moved Constant constructors to protected and friended Constants to prevent misuse of the class (it should only be used to load constants).

As before, I kept the original LoadRawConstant and LoadConstant signatures unchanged to avoid a major refactor of llv8-constants.cc in a single PR.

[mmarchini]

Landed in 2afd59e