* Improve wasmi differential fuzzer
* Support modules with a `start` function
* Implement trap-matching to ensure that wasmi and Wasmtime both report
the same flavor of trap.
* Support differential fuzzing where no engines match
Locally I was attempting to run against just one wasm engine with
`ALLOWED_ENGINES=wasmi` but the fuzzer quickly panicked because the
generated test case didn't match wasmi's configuration. This commit
updates engine-selection in the differential fuzzer to return `None` if
no engine is applicable, throwing out the test case. This won't be hit
at all with oss-fuzz-based runs but for local runs it'll be useful to
have.
* Improve proposal support in differential fuzzer
* De-prioritize unstable wasm proposals such as multi-memory and
memory64 by making them more unlikely with `Unstructured::ratio`.
* Allow fuzzing multi-table (reference types) and multi-memory by
avoiding setting their maximums to 1 in `set_differential_config`.
* Update selection of the pooling strategy to unconditionally support
the selected module config rather than the other way around.
* Improve handling of traps in differential fuzzing
This commit fixes an issue found via local fuzzing where engines were
reporting different results but the underlying reason for this was that
one engine was hitting stack overflow before the other. To fix the
underlying issue I updated the execution to check for stack overflow
and, if hit, it discards the entire fuzz test case from then on.
The rationale behind this is that each engine can have unique limits for
stack overflow. One test case I was looking at for example would stack
overflow at less than 1000 frames with epoch interruption enabled but
would stack overflow at more than 1000 frames with it disabled. This
means that the state after the trap started to diverge and it looked
like the engines produced different results.
While I was at it I also improved the "function call returned a trap"
case to compare traps to make sure the same trap reason popped out.
* Fix fuzzer tests
10dbb19983