feat: runtime: Add read_only getter

[arajasek]

Extracted from next. Needs:

• feat: Partial implementation of FIP-0048 (f4/delegated addresses) #1118
• feat: runtime: Add send_generalized #1126

This PR simply introduces a getter to the Runtime to indicate whether an invocation is in READ_ONLY mode, and updates the test framework to consider this status as part of execution.

This PR is a pre-factor necessary for landing the changes involved in the FEVM FIPs -- it introduces new functionality, but doesn't start to use it anywhere.

[codecov-commenter]

Codecov Report

Merging #1127 (532b4b5) into asr/send-generalized (b711fb6) will decrease coverage by 0.03%.
The diff coverage is 59.09%.

Additional details and impacted files

@@                   Coverage Diff                    @@
##           asr/send-generalized    #1127      +/-   ##
========================================================
- Coverage                 89.00%   88.97%   -0.03%     
========================================================
  Files                        95       95              
  Lines                     19782    19803      +21     
========================================================
+ Hits                      17607    17620      +13     
- Misses                     2175     2183       +8     

Impacted Files	Coverage Δ
runtime/src/runtime/mod.rs	64.51% <ø> (ø)
runtime/src/test_utils.rs	81.83% <0.00%> (-0.39%)	⬇️
test_vm/src/lib.rs	85.99% <72.22%> (-0.28%)	⬇️
state/src/check.rs	87.00% <0.00%> (+0.26%)	⬆️

[anorth]

Can you point to the spec for read-only? Depending on what it says, I would expect either:

• no checks like this, but something at the end of an invocation that restores the state root to its pre-invocation value, or
• a lot more checks in create, transaction, new_actor_address, create_actor, delete_actor` (or something lower down that propagates through these) that actively prevent modifications.

[arajasek]

Spec: https://github.com/filecoin-project/FIPs/blob/322604f79c2447a4906ed2fc959e43aaedeccb46/FIPS/fip-0054.md#changed-syscalls

You're right. I've added checks to create, create_actor, transaction, set_state_root and invoke.

I'm tweaking the behaviour of the test_vm's new_actor_address to be read-only, which is how it's actually implemented. The increment in moving to create_actor which is where it belongs.

[anorth]

Thanks, the main change looks good. Ok as is, but a potential improvement:

It struck me while reviewing the test VM that a single check inside self.v.set_actor() would suffice to catch all the read-only cases (after a bit of re-ordering to ensure that was the first modification that the VM processes). But I see why we want the read_only flag to be in the invocation context rather than the self.v VM. One step better might be to implement a wrapper self.set_actor which does the check then calls through to self.v. This wouldn't prevent someone making a mistake, but make future modifications somewhat more likely to be correct. A step further would be to hide self.v even further inside the context so someone can't call directly by accident; but probably not worth it.