Limit rayon to one thread during fuzzing

This should enable more deterministic execution.

crates/fuzzing/src/lib.rs

@@ -4,3 +4,29 @@
 
 pub mod generators;
 pub mod oracles;
+
+/// One time start up initialization for fuzzing:
+///
+/// * Enables `env_logger`.
+///
+/// * Restricts `rayon` to a single thread in its thread pool, for more
+///   deterministic executions.
+///
+/// If a fuzz target is taking raw input bytes from the fuzzer, it is fine to
+/// call this function in the fuzz target's oracle or in the fuzz target
+/// itself. However, if the fuzz target takes an `Arbitrary` type, and the
+/// `Arbitrary` implementation is not derived and does interesting things, then
+/// the `Arbitrary` implementation should call this function, since it runs
+/// before the fuzz target itself.
+pub(crate) fn init_fuzzing() {
+    static INIT: std::sync::Once = std::sync::Once::new();
+
+    INIT.call_once(|| {
+        let _ = env_logger::try_init();
+
+        rayon::ThreadPoolBuilder::new()
+            .num_threads(1)
+            .build_global()
+            .expect("should only initialize the rayon thread pool once!");
+    })
+}