Add initial OOM test infrastructure

Cargo.toml

@@ -353,6 +353,7 @@ json-from-wast = "0.241.0"
 # Non-Bytecode Alliance maintained dependencies:
 # --------------------------
 arbitrary = "1.4.2"
+backtrace = "0.3.75"
 mutatis = "0.3.2"
 cc = "1.2.41"
 object = { version = "0.37.3", default-features = false, features = ['read_core', 'elf'] }

crates/fuzzing/Cargo.toml

@@ -16,6 +16,7 @@ wasmtime-test-util = { workspace = true, features = ['wast'] }
 
 [dependencies]
 anyhow = { workspace = true }
+backtrace = { workspace = true }
 arbitrary = { workspace = true, features = ["derive"] }
 env_logger = { workspace = true }
 log = { workspace = true }

crates/fuzzing/src/lib.rs

@@ -8,6 +8,7 @@ pub use wasm_mutate;
 pub use wasm_smith;
 pub mod generators;
 pub mod mutators;
+pub mod oom;
 pub mod oracles;
 pub mod single_module_fuzzer;
 

crates/fuzzing/src/oom.rs

@@ -0,0 +1,244 @@
+//! Utilities for testing and fuzzing out-of-memory handling.
+//!
+//! Inspired by SpiderMonkey's `oomTest()` helper:
+//! https://firefox-source-docs.mozilla.org/js/hacking_tips.html#how-to-debug-oomtest-failures
+
+use anyhow::bail;
+use backtrace::Backtrace;
+use std::{alloc::GlobalAlloc, cell::Cell, mem, ptr, time};
+use wasmtime::{Error, Result};
+
+/// An allocator for use with `OomTest`.
+#[non_exhaustive]
+pub struct OomTestAllocator;
+
+impl OomTestAllocator {
+    /// Create a new OOM test allocator.
+    pub const fn new() -> Self {
+        OomTestAllocator
+    }
+}
+
+#[derive(Clone, Debug, Default, PartialEq, Eq)]
+enum OomState {
+    /// We are in code that is not part of an OOM test.
+    #[default]
+    OutsideOomTest,
+
+    /// We are inside an OOM test and should inject an OOM when the counter
+    /// reaches zero.
+    OomOnAlloc(u32),
+
+    /// We are inside an OOM test and we already injected an OOM.
+    DidOom,
+}
+
+thread_local! {
+    static OOM_STATE: Cell<OomState> = const { Cell::new(OomState::OutsideOomTest) };
+}
+
+/// Set the new OOM state, returning the old state.
+fn set_oom_state(state: OomState) -> OomState {
+    OOM_STATE.with(|s| s.replace(state))
+}
+
+/// RAII helper to set the OOM state within a block of code and reset it upon
+/// exiting that block (even if exiting via panic unwinding).
+struct ScopedOomState {
+    prev_state: OomState,
+}
+
+impl ScopedOomState {
+    fn new(state: OomState) -> Self {
+        ScopedOomState {
+            prev_state: set_oom_state(state),
+        }
+    }
+
+    /// Finish this OOM state scope early, resetting the OOM state to what it
+    /// was before this scope was created, and returning the previous state that
+    /// was just overwritten by the reset.
+    fn finish(&self) -> OomState {
+        set_oom_state(self.prev_state.clone())
+    }
+}
+
+impl Drop for ScopedOomState {
+    fn drop(&mut self) {
+        set_oom_state(mem::take(&mut self.prev_state));
+    }
+}
+
+unsafe impl GlobalAlloc for OomTestAllocator {
+    unsafe fn alloc(&self, layout: std::alloc::Layout) -> *mut u8 {
+        let old_state = set_oom_state(OomState::OutsideOomTest);
+
+        let new_state;
+        let ptr;
+        {
+            // NB: It's okay to log/backtrace/etc... in this block because the
+            // current state is `OutsideOomTest`, so any re-entrant allocations
+            // will be passed through to the system allocator.
+
+            match old_state {
+                OomState::OutsideOomTest => {
+                    new_state = OomState::OutsideOomTest;
+                    ptr = unsafe { std::alloc::System.alloc(layout) };
+                }
+                OomState::OomOnAlloc(0) => {
+                    log::trace!(
+                        "injecting OOM for allocation: {layout:?}\nAllocation backtrace:\n{:?}",
+                        Backtrace::new(),
+                    );
+                    new_state = OomState::DidOom;
+                    ptr = ptr::null_mut();
+                }
+                OomState::OomOnAlloc(c) => {
+                    new_state = OomState::OomOnAlloc(c - 1);
+                    ptr = unsafe { std::alloc::System.alloc(layout) };
+                }
+                OomState::DidOom => {
+                    panic!("OOM test attempted to allocate after OOM: {layout:?}")
+                }
+            }
+        }
+
+        set_oom_state(new_state);
+        ptr
+    }
+
+    unsafe fn dealloc(&self, ptr: *mut u8, layout: std::alloc::Layout) {
+        unsafe {
+            std::alloc::System.dealloc(ptr, layout);
+        }
+    }
+}
+
+/// A test helper that checks that some code handles OOM correctly.
+///
+/// `OomTest` will only work correctly when `OomTestAllocator` is configured as
+/// the global allocator.
+///
+/// `OomTest` does not support reentrancy, so you cannot run an `OomTest` within
+/// an `OomTest`.
+///
+/// # Example
+///
+/// ```no_run
+/// use std::time::Duration;
+/// use wasmtime::Result;
+/// use wasmtime_fuzzing::oom::{OomTest, OomTestAllocator};
+///
+/// #[global_allocator]
+/// static GLOBAL_ALOCATOR: OomTestAllocator = OomTestAllocator::new();
+///
+/// #[test]
+/// fn my_oom_test() -> Result<()> {
+///     OomTest::new()
+///         .max_iters(1_000_000)
+///         .max_duration(Duration::from_secs(5))
+///         .test(|| {
+///             todo!("insert code here that should handle OOM here...")
+///         })
+/// }
+/// ```
+pub struct OomTest {
+    max_iters: Option<u32>,
+    max_duration: Option<time::Duration>,
+}
+
+impl OomTest {
+    /// Create a new OOM test.
+    ///
+    /// By default there is no iteration or time limit, tests will be executed
+    /// until the pass (or fail).
+    pub fn new() -> Self {
+        let _ = env_logger::try_init();
+        OomTest {
+            max_iters: None,
+            max_duration: None,
+        }
+    }
+
+    /// Configure the maximum number of times to run an OOM test.
+    pub fn max_iters(&mut self, max_iters: u32) -> &mut Self {
+        self.max_iters = Some(max_iters);
+        self
+    }
+
+    /// Configure the maximum duration of time to run an OOM text.
+    pub fn max_duration(&mut self, max_duration: time::Duration) -> &mut Self {
+        self.max_duration = Some(max_duration);
+        self
+    }
+
+    /// Repeatedly run the given test function, injecting OOMs at different
+    /// times and checking that it correctly handles them.
+    ///
+    /// The test function should not use threads, or else allocations may not be
+    /// tracked correctly and OOM injection may be incorrect.
+    ///
+    /// The test function should return an `Err(_)` if and only if it encounters
+    /// an OOM.
+    ///
+    /// Returns early once the test function returns `Ok(())` before an OOM has
+    /// been injected.
+    pub fn test(&self, test_func: impl Fn() -> Result<()>) -> Result<()> {
+        let start = time::Instant::now();
+
+        for i in 0.. {
+            if self.max_iters.is_some_and(|n| i >= n)
+                || self.max_duration.is_some_and(|d| start.elapsed() >= d)
+            {
+                break;
+            }
+
+            log::trace!("=== Injecting OOM after {i} allocations ===");
+            let (result, old_state) = {
+                let guard = ScopedOomState::new(OomState::OomOnAlloc(i));
+                assert_eq!(guard.prev_state, OomState::OutsideOomTest);
+
+                let result = test_func();
+
+                (result, guard.finish())
+            };
+
+            match (result, old_state) {
+                (_, OomState::OutsideOomTest) => unreachable!(),
+
+                // The test function completed successfully before we ran out of
+                // allocation fuel, so we're done.
+                (Ok(()), OomState::OomOnAlloc(_)) => break,
+
+                // We injected an OOM and the test function handled it
+                // correctly; continue to the next iteration.
+                (Err(e), OomState::DidOom) if self.is_oom_error(&e) => {}
+
+                // Missed OOMs.
+                (Ok(()), OomState::DidOom) => {
+                    bail!("OOM test function missed an OOM: returned Ok(())");
+                }
+                (Err(e), OomState::DidOom) => {
+                    return Err(
+                        e.context("OOM test function missed an OOM: returned non-OOM error")
+                    );
+                }
+
+                // Unexpected error.
+                (Err(e), OomState::OomOnAlloc(_)) => {
+                    return Err(
+                        e.context("OOM test function returned an error when there was no OOM")
+                    );
+                }
+            }
+        }
+
+        Ok(())
+    }
+
+    fn is_oom_error(&self, _: &Error) -> bool {
+        // TODO: We don't have an OOM error yet. Will likely need to make it so
+        // that `wasmtime::Error != anyhow::Error` as a first step here.
+        false
+    }
+}

crates/fuzzing/tests/oom.rs

@@ -0,0 +1,27 @@
+use std::alloc::{Layout, alloc};
+use wasmtime::Result;
+use wasmtime_fuzzing::oom::{OomTest, OomTestAllocator};
+
+#[global_allocator]
+static GLOBAL_ALOCATOR: OomTestAllocator = OomTestAllocator::new();
+
+#[test]
+fn smoke_test_ok() -> Result<()> {
+    OomTest::new().test(|| Ok(()))
+}
+
+#[test]
+fn smoke_test_missed_oom() -> Result<()> {
+    let err = OomTest::new()
+        .test(|| {
+            let _ = unsafe { alloc(Layout::new::<u64>()) };
+            Ok(())
+        })
+        .unwrap_err();
+    let err = format!("{err:?}");
+    assert!(
+        err.contains("OOM test function missed an OOM"),
+        "should have missed an OOM, got: {err}"
+    );
+    Ok(())
+}