* Added basic async capabilities for vm2.
This is a whole new Git branch, not the same one as last time
(https://github.com/status-im/nimbus-eth1/pull/1250) - there wasn't
much worth salvaging. Main differences:
I didn't do the "each opcode has to specify an async handler" junk
that I put in last time. Instead, in oph_memory.nim you can see
sloadOp calling asyncChainTo and passing in an async operation.
That async operation is then run by the execCallOrCreate (or
asyncExecCallOrCreate) code in interpreter_dispatch.nim.
In the test code, the (previously existing) macro called "assembler"
now allows you to add a section called "initialStorage", specifying
fake data to be used by the EVM computation run by that test. (In
the long run we'll obviously want to write tests that for-real use
the JSON-RPC API to asynchronously fetch data; for now, this was
just an expedient way to write a basic unit test that exercises the
async-EVM code pathway.)
There's also a new macro called "concurrentAssemblers" that allows
you to write a test that runs multiple assemblers concurrently (and
then waits for them all to finish). There's one example test using
this, in test_op_memory_lazy.nim, though you can't actually see it
doing so unless you uncomment some echo statements in
async_operations.nim (in which case you can see the two concurrently
running EVM computations each printing out what they're doing, and
you'll see that they interleave).
A question: is it possible to make EVMC work asynchronously? (For
now, this code compiles and "make test" passes even if ENABLE_EVMC
is turned on, but it doesn't actually work asynchronously, it just
falls back on doing the usual synchronous EVMC thing. See
FIXME-asyncAndEvmc.)
* Moved the AsyncOperationFactory to the BaseVMState object.
* Made the AsyncOperationFactory into a table of fn pointers.
Also ditched the plain-data Vm2AsyncOperation type; it wasn't
really serving much purpose. Instead, the pendingAsyncOperation
field directly contains the Future.
* Removed the hasStorage idea.
It's not the right solution to the "how do we know whether we
still need to fetch the storage value or not?" problem. I
haven't implemented the right solution yet, but at least
we're better off not putting in a wrong one.
* Added/modified/removed some comments.
(Based on feedback on the PR.)
* Removed the waitFor from execCallOrCreate.
There was some back-and-forth in the PR regarding whether nested
waitFor calls are acceptable:
https://github.com/status-im/nimbus-eth1/pull/1260#discussion_r998587449
The eventual decision was to just change the waitFor to a doAssert
(since we probably won't want this extra functionality when running
synchronously anyway) to make sure that the Future is already
finished.
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