fuzzing: Add initial API call fuzzer

We only generate *valid* sequences of API calls. To do this, we keep track of
what objects we've already created in earlier API calls via the `Scope` struct.

To generate even-more-pathological sequences of API calls, we use [swarm
testing]:

> In swarm testing, the usual practice of potentially including all features
> in every test case is abandoned. Rather, a large “swarm” of randomly
> generated configurations, each of which omits some features, is used, with
> configurations receiving equal resources.

[swarm testing]: https://www.cs.utah.edu/~regehr/papers/swarm12.pdf

There are more public APIs and instance introspection APIs that we have than
this fuzzer exercises right now. We will need a better generator of valid Wasm
than `wasm-opt -ttf` to really get the most out of those currently-unexercised
APIs, since the Wasm modules generated by `wasm-opt -ttf` don't import and
export a huge variety of things.

crates/fuzzing/src/generators/api.rs

@@ -0,0 +1,182 @@
+//! Generating sequences of Wasmtime API calls.
+//!
+//! We only generate *valid* sequences of API calls. To do this, we keep track
+//! of what objects we've already created in earlier API calls via the `Scope`
+//! struct.
+//!
+//! To generate even-more-pathological sequences of API calls, we use [swarm
+//! testing]:
+//!
+//! > In swarm testing, the usual practice of potentially including all features
+//! > in every test case is abandoned. Rather, a large “swarm” of randomly
+//! > generated configurations, each of which omits some features, is used, with
+//! > configurations receiving equal resources.
+//!
+//! [swarm testing]: https://www.cs.utah.edu/~regehr/papers/swarm12.pdf
+
+use arbitrary::{Arbitrary, Unstructured};
+use std::collections::HashSet;
+
+struct Swarm {
+    config_debug_info: bool,
+    module_new: bool,
+    module_drop: bool,
+    instance_new: bool,
+    instance_drop: bool,
+    call_exported_func: bool,
+}
+
+impl Arbitrary for Swarm {
+    fn arbitrary<U>(input: &mut U) -> Result<Self, U::Error>
+    where
+        U: Unstructured + ?Sized,
+    {
+        Ok(Swarm {
+            config_debug_info: bool::arbitrary(input)?,
+            module_new: bool::arbitrary(input)?,
+            module_drop: bool::arbitrary(input)?,
+            instance_new: bool::arbitrary(input)?,
+            instance_drop: bool::arbitrary(input)?,
+            call_exported_func: bool::arbitrary(input)?,
+        })
+    }
+}
+
+/// A call to one of Wasmtime's public APIs.
+#[derive(Clone, Debug)]
+#[allow(missing_docs)]
+pub enum ApiCall {
+    ConfigNew,
+    ConfigDebugInfo(bool),
+    EngineNew,
+    StoreNew,
+    ModuleNew { id: usize, wasm: super::WasmOptTtf },
+    ModuleDrop { id: usize },
+    InstanceNew { id: usize, module: usize },
+    InstanceDrop { id: usize },
+    CallExportedFunc { instance: usize, nth: usize },
+}
+use ApiCall::*;
+
+#[derive(Default)]
+struct Scope {
+    id_counter: usize,
+    modules: HashSet<usize>,
+    instances: HashSet<usize>,
+}
+
+impl Scope {
+    fn next_id(&mut self) -> usize {
+        let id = self.id_counter;
+        self.id_counter = id + 1;
+        id
+    }
+}
+
+/// A sequence of API calls.
+#[derive(Debug)]
+pub struct ApiCalls {
+    /// The API calls.
+    pub calls: Vec<ApiCall>,
+}
+
+impl Arbitrary for ApiCalls {
+    fn arbitrary<U>(input: &mut U) -> Result<Self, U::Error>
+    where
+        U: Unstructured + ?Sized,
+    {
+        let swarm = Swarm::arbitrary(input)?;
+        let mut calls = vec![];
+
+        arbitrary_config(input, &swarm, &mut calls)?;
+        calls.push(EngineNew);
+        calls.push(StoreNew);
+
+        let mut scope = Scope::default();
+
+        for _ in 0..input.container_size()? {
+            let mut choices: Vec<fn(_, &mut Scope) -> Result<ApiCall, U::Error>> = vec![];
+
+            if swarm.module_new {
+                choices.push(|input, scope| {
+                    let id = scope.next_id();
+                    scope.modules.insert(id);
+                    let wasm = super::WasmOptTtf::arbitrary(input)?;
+                    Ok(ModuleNew { id, wasm })
+                });
+            }
+            if swarm.module_drop && !scope.modules.is_empty() {
+                choices.push(|input, scope| {
+                    let modules: Vec<_> = scope.modules.iter().cloned().collect();
+                    let id = arbitrary_choice(input, &modules)?.cloned().unwrap();
+                    scope.modules.remove(&id);
+                    Ok(ModuleDrop { id })
+                });
+            }
+            if swarm.instance_new && !scope.modules.is_empty() {
+                choices.push(|input, scope| {
+                    let modules: Vec<_> = scope.modules.iter().cloned().collect();
+                    let module = arbitrary_choice(input, &modules)?.cloned().unwrap();
+                    let id = scope.next_id();
+                    scope.instances.insert(id);
+                    Ok(InstanceNew { id, module })
+                });
+            }
+            if swarm.instance_drop && !scope.instances.is_empty() {
+                choices.push(|input, scope| {
+                    let instances: Vec<_> = scope.instances.iter().cloned().collect();
+                    let id = arbitrary_choice(input, &instances)?.cloned().unwrap();
+                    scope.instances.remove(&id);
+                    Ok(InstanceDrop { id })
+                });
+            }
+            if swarm.call_exported_func && !scope.instances.is_empty() {
+                choices.push(|input, scope| {
+                    let instances: Vec<_> = scope.instances.iter().cloned().collect();
+                    let instance = arbitrary_choice(input, &instances)?.cloned().unwrap();
+                    let nth = usize::arbitrary(input)?;
+                    Ok(CallExportedFunc { instance, nth })
+                });
+            }
+
+            if let Some(c) = arbitrary_choice(input, &choices)? {
+                calls.push(c(input, &mut scope)?);
+            } else {
+                break;
+            }
+        }
+
+        Ok(ApiCalls { calls })
+    }
+}
+
+fn arbitrary_choice<'a, T, U>(input: &mut U, choices: &'a [T]) -> Result<Option<&'a T>, U::Error>
+where
+    U: Unstructured + ?Sized,
+{
+    if choices.is_empty() {
+        Ok(None)
+    } else {
+        let i = usize::arbitrary(input)? % choices.len();
+        Ok(Some(&choices[i]))
+    }
+}
+
+fn arbitrary_config<U>(
+    input: &mut U,
+    swarm: &Swarm,
+    calls: &mut Vec<ApiCall>,
+) -> Result<(), U::Error>
+where
+    U: Unstructured + ?Sized,
+{
+    calls.push(ConfigNew);
+
+    if swarm.config_debug_info && bool::arbitrary(input)? {
+        calls.push(ConfigDebugInfo(bool::arbitrary(input)?));
+    }
+
+    // TODO: flags, features, and compilation strategy.
+
+    Ok(())
+}