src: improve Check() reliability

[mmarchini]

This is a first step towards making llnode more reliable. Right now, llnode doesn't allow us to deal with partially-loaded objects (some fields working, some not working). As a postmortem tool, llnode should be able to handle partial loading of objects, as this is the proper way to deal with corrupt memory. If llnode can handle corrupted memory, it will also be able to understand the memory of unsupported Node.js versons, although some fields and objects won't load.

There are two problems regarding reliability in llnode: first, we have several attempts to access fields from the crashed/paused process memory without validating if we have enough information (and correct informaton) to do so. Second, we use Error::Fail() as the primary tool to verify if there was an error loading a field/objects/etc., but Error::Fail usually propagates and will cause us to prematurely stop processing the memory.

This commit introduces a few things to help us improve reliability in the future:

• Value classes now have a valid_ member, and Check() will take this member into account.
• A new class, CheckedType, will let us wrap primitive C++ types which were loaded from the paused/crashed process memory, so we can have a Check() method for those values as well to verify if it was possible to load the value successfuly.
• Two new macros, RETURN_IF_INVALID and RETURN_IF_THIS_INVALID, to make it easier to return from a function (with a default value) when a object/value was not loaded properly.

The goals in the future are:

• Replace all uses of Error::Fail as a loading validation tool with RETURN_IF_INVALID, keeping Error::Fail only for unrecoverable errors (instead of the double semantic it has today).
• Ensure all methods in llv8.h return either a Value subclass, or a primitive type wrapped in CheckedType
• Ensure all calls to methods which will load from the target process memory are followed by RETURN_IF_INVALID.
• Ensure all methods in llv8.h start with RETURN_IF_THIS_INVALID.

We could make all those changes in a single PR, but it would take a huge amount of work and the PR would be extremely long, making it harder to review. Instead, I suggest we make incremental changes as we see fit, until we achieve the goals described above.

The method of choice to start was String::Representation, because by making this method more robust we fix a crash on Node.js v12 after running v8 bt if there's an optimized function on the stack (the function won't be translated, but it's better than a hard crash).

[codecov-io]

Codecov Report

Merging #294 into master will decrease coverage by 0.38%. The diff coverage is 61.11%.

@@            Coverage Diff             @@
##           master     #294      +/-   ##
==========================================
- Coverage   79.84%   79.45%   -0.39%     
==========================================
  Files          33       33              
  Lines        4232     4216      -16     
==========================================
- Hits         3379     3350      -29     
- Misses        853      866      +13

Impacted Files	Coverage Δ
src/llv8.h	97.61% <100%> (-2.39%)	:arrow_down:
src/llv8-inl.h	92.58% <100%> (-0.1%)	:arrow_down:
src/llscan.cc	61.33% <27.77%> (ø)	:arrow_up:
src/llv8.cc	73.98% <87.5%> (-1.49%)	:arrow_down:
src/llv8-constants.h	98.48% <0%> (-1.52%)	:arrow_down:
src/llv8-constants.cc	82.52% <0%> (-0.98%)	:arrow_down:
src/llnode_module.cc	89.87% <0%> (-0.64%)	:arrow_down:
... and 1 more

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[mmarchini]

Landed in a501635