Convert shim-shared-memory to Rust

[sporksmith]

• Import rootedcell from https://github.com/sporksmith/objgraph/. This is a safe abstraction around the thread safety model used in Shadow, where access to a single Root object (which may be protected at a coarse level, such as by a per-Host lock) is used to prove that non-atomic operations are safe for "child" objects. This is the model used by the current shim shared memory API.

• Implement the VirtualAddressSpaceIndependent trait and Derive-macro. The former marks types as usable outside of their original virtual address space. The corresponding Derive macro is a safer way of implementing the trait for more-complex structs, by validating that all fields of that struct are VirtualAddressSpaceIndependent.

• Convert the shim shared memory structs to Rust. For now this is a fairly direct translation into Rust, using the above to help ensure safety. We'll probably want more safe Rust APIs for accessing these, but for now the focus was just on reimplementing the C APIs.

[codecov]

Codecov Report

Base: 42.48% // Head: 43.39% // Increases project coverage by +0.91% 🎉

Coverage data is based on head (21e3f01) compared to base (5442bfb).
Patch coverage: 85.09% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2462      +/-   ##
==========================================
+ Coverage   42.48%   43.39%   +0.91%     
==========================================
  Files         180      185       +5     
  Lines       26710    27246     +536     
  Branches     5379     5414      +35     
==========================================
+ Hits        11348    11824     +476     
- Misses      12736    12762      +26     
- Partials     2626     2660      +34     

Flag	Coverage Δ
tests	43.39% <85.09%> (+0.91%)	⬆️

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

Impacted Files	Coverage Δ
src/lib/shadow-shim-helper-rs/src/scmutex.rs	86.74% <ø> (-0.61%)	⬇️
src/lib/shim/shim_rdtsc.c	62.71% <ø> (ø)
src/lib/shim/shim_signals.c	67.96% <ø> (ø)
src/lib/shim/shim_syscall.c	80.80% <ø> (ø)
...rc/lib/shadow-shim-helper-rs/src/rootedcell/mod.rs	56.00% <56.00%> (ø)
src/lib/shadow-shim-helper-rs/src/shim_shmem.rs	82.79% <82.79%> (ø)
src/lib/shadow-shim-helper-rs/src/signals.rs	70.06% <85.71%> (-4.04%)	⬇️
...ib/shadow-shim-helper-rs/src/rootedcell/refcell.rs	89.28% <89.28%> (ø)
src/lib/shadow-shim-helper-rs/src/rootedcell/rc.rs	94.11% <94.11%> (ø)
src/lib/shadow-shim-helper-rs/src/emulated_time.rs	71.68% <100.00%> (+2.45%)	⬆️
... and 26 more

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[sporksmith]

Just started a benchmark: https://github.com/shadow/benchmark/actions/runs/3237922848

I don't expect this to change performance, but good to double-check since this is somewhat in the hot path

[stevenengler]

Looks cool! While I think this approach takes a little more mental effort to make sure everything is safe, I think it's a lot easier to reason about and understand than the rkyv approach.