The version of the `arbitrary` crate used in fuzz targets needs to be
the same as the version used in `libfuzzer-sys`. That's why the latter
crate re-exports the former.
But we need to make sure to consistently use the re-exported version.
That's most easily done if that's the only version we have available.
However, `fuzz/Cargo.toml` declared a direct dependency on `arbitrary`,
making it available for import, and leading to that version being used
in a couple places.
There were two copies of `arbitrary` built before, even though they were
the same version: one with the `derive` feature turned on, through the
direct dependency, and one with it turned off when imported through
`libfuzzer-sys`. So I haven't specifically tested this but fuzzer builds
might be slightly faster now.
I have not removed the build-dep on `arbitrary`, because `build.rs` is
not invoked by libFuzzer and so it doesn't matter what version of
`arbitrary` it uses.
Our other crates, like `cranelift-fuzzgen` and `wasmtime-fuzzing`, can
still accidentally use a different version of `arbitrary` than the fuzz
targets which rely on them. This commit only fixes the direct cases
within `fuzz/**`.
This addresses #4307.
For the static API we generate 100 arbitrary test cases at build time, each of
which includes 0-5 parameter types, a result type, and a WAT fragment containing
an imported function and an exported function. The exported function calls the
imported function, which is implemented by the host. At runtime, the fuzz test
selects a test case at random and feeds it zero or more sets of arbitrary
parameters and results, checking that values which flow host-to-guest and
guest-to-host make the transition unchanged.
The fuzz test for the dynamic API follows a similar pattern, the only difference
being that test cases are generated at runtime.
Signed-off-by: Joel Dice <joel.dice@fermyon.com>
