Fix some issues around TLS management with async (#2709)

This commit fixes a few issues around managing the thread-local state of
a wasmtime thread. We intentionally only have a singular TLS variable in
the whole world, and the problem is that when stack-switching off an
async thread we were not restoring the previous TLS state. This is
necessary in two cases:

* Futures aren't guaranteed to be polled/completed in a stack-like
  fashion. If a poll sees that a future isn't ready then we may resume
  execution in a previous wasm context that ends up needing the TLS
  information.

* Futures can also cross threads (when the whole store crosses threads)
  and we need to save/restore TLS state from the thread we're coming
  from and the thread that we're going to.

The stack switching issue necessitates some more glue around suspension
and resumption of a stack to ensure we save/restore the TLS state on
both sides. The thread issue, however, also necessitates that we use
`#[inline(never)]` on TLS access functions and never have TLS borrows
live across a function which could result in running arbitrary code (as
was the case for the `tls::set` function.

crates/runtime/src/lib.rs

@@ -48,7 +48,7 @@ pub use crate::mmap::Mmap;
 pub use crate::table::{Table, TableElement};
 pub use crate::traphandlers::{
     catch_traps, init_traps, raise_lib_trap, raise_user_trap, resume_panic, with_last_info,
-    SignalHandler, Trap, TrapInfo,
+    SignalHandler, TlsRestore, Trap, TrapInfo,
 };
 pub use crate::vmcontext::{
     VMCallerCheckedAnyfunc, VMContext, VMFunctionBody, VMFunctionImport, VMGlobalDefinition,

crates/runtime/src/traphandlers.rs

@@ -12,6 +12,8 @@ use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
 use std::sync::Once;
 use wasmtime_environ::ir;
 
+pub use self::tls::TlsRestore;
+
 extern "C" {
     fn RegisterSetjmp(
         jmp_buf: *mut *const u8,
@@ -491,6 +493,7 @@ pub struct CallThreadState<'a> {
     jmp_buf: Cell<*const u8>,
     handling_trap: Cell<bool>,
     trap_info: &'a (dyn TrapInfo + 'a),
+    prev: Cell<tls::Ptr>,
 }
 
 /// A package of functionality needed by `catch_traps` to figure out what to do
@@ -541,6 +544,7 @@ impl<'a> CallThreadState<'a> {
             jmp_buf: Cell::new(ptr::null()),
             handling_trap: Cell::new(false),
             trap_info,
+            prev: Cell::new(ptr::null()),
         }
     }
 
@@ -753,43 +757,108 @@ impl<T: Copy> Drop for ResetCell<'_, T> {
 // the caller to the trap site.
 mod tls {
     use super::CallThreadState;
-    use std::cell::Cell;
     use std::mem;
     use std::ptr;
 
-    thread_local!(static PTR: Cell<*const CallThreadState<'static>> = Cell::new(ptr::null()));
+    pub use raw::Ptr;
+
+    // An even *more* inner module for dealing with TLS. This actually has the
+    // thread local variable and has functions to access the variable.
+    //
+    // Note that this is specially done to fully encapsulate that the accessors
+    // for tls must not be inlined. Wasmtime's async support employs stack
+    // switching which can resume execution on different OS threads. This means
+    // that borrows of our TLS pointer must never live across accesses because
+    // otherwise the access may be split across two threads and cause unsafety.
+    //
+    // This also means that extra care is taken by the runtime to save/restore
+    // these TLS values when the runtime may have crossed threads.
+    mod raw {
+        use super::CallThreadState;
+        use std::cell::Cell;
+        use std::ptr;
+
+        pub type Ptr = *const CallThreadState<'static>;
+
+        thread_local!(static PTR: Cell<Ptr> = Cell::new(ptr::null()));
+
+        #[inline(never)] // see module docs for why this is here
+        pub fn replace(val: Ptr) -> Ptr {
+            PTR.with(|p| p.replace(val))
+        }
+
+        #[inline(never)] // see module docs for why this is here
+        pub fn get() -> Ptr {
+            PTR.with(|p| p.get())
+        }
+    }
+
+    /// Opaque state used to help control TLS state across stack switches for
+    /// async support.
+    pub struct TlsRestore(raw::Ptr);
+
+    impl TlsRestore {
+        /// Takes the TLS state that is currently configured and returns a
+        /// token that is used to replace it later.
+        ///
+        /// This is not a safe operation since it's intended to only be used
+        /// with stack switching found with fibers and async wasmtime.
+        pub unsafe fn take() -> TlsRestore {
+            // Our tls pointer must be set at this time, and it must not be
+            // null. We need to restore the previous pointer since we're
+            // removing ourselves from the call-stack, and in the process we
+            // null out our own previous field for safety in case it's
+            // accidentally used later.
+            let raw = raw::get();
+            assert!(!raw.is_null());
+            let prev = (*raw).prev.replace(ptr::null());
+            raw::replace(prev);
+            TlsRestore(raw)
+        }
+
+        /// Restores a previous tls state back into this thread's TLS.
+        ///
+        /// This is unsafe because it's intended to only be used within the
+        /// context of stack switching within wasmtime.
+        pub unsafe fn replace(self) {
+            // We need to configure our previous TLS pointer to whatever is in
+            // TLS at this time, and then we set the current state to ourselves.
+            let prev = raw::get();
+            assert!((*self.0).prev.get().is_null());
+            (*self.0).prev.set(prev);
+            raw::replace(self.0);
+        }
+    }
 
     /// Configures thread local state such that for the duration of the
     /// execution of `closure` any call to `with` will yield `ptr`, unless this
     /// is recursively called again.
-    pub fn set<R>(ptr: &CallThreadState<'_>, closure: impl FnOnce() -> R) -> R {
-        struct Reset<'a, T: Copy>(&'a Cell<T>, T);
+    pub fn set<R>(state: &CallThreadState<'_>, closure: impl FnOnce() -> R) -> R {
+        struct Reset<'a, 'b>(&'a CallThreadState<'b>);
 
-        impl<T: Copy> Drop for Reset<'_, T> {
+        impl Drop for Reset<'_, '_> {
             fn drop(&mut self) {
-                self.0.set(self.1);
+                raw::replace(self.0.prev.replace(ptr::null()));
             }
         }
 
-        PTR.with(|p| {
         // Note that this extension of the lifetime to `'static` should be
         // safe because we only ever access it below with an anonymous
         // lifetime, meaning `'static` never leaks out of this module.
         let ptr = unsafe {
-                mem::transmute::<*const CallThreadState<'_>, *const CallThreadState<'static>>(ptr)
+            mem::transmute::<*const CallThreadState<'_>, *const CallThreadState<'static>>(state)
         };
-            let _r = Reset(p, p.replace(ptr));
+        let prev = raw::replace(ptr);
+        state.prev.set(prev);
+        let _reset = Reset(state);
         closure()
-        })
     }
 
     /// Returns the last pointer configured with `set` above. Panics if `set`
     /// has not been previously called.
     pub fn with<R>(closure: impl FnOnce(Option<&CallThreadState<'_>>) -> R) -> R {
-        PTR.with(|ptr| {
-            let p = ptr.get();
+        let p = raw::get();
         unsafe { closure(if p.is_null() { None } else { Some(&*p) }) }
-        })
     }
 }
 

crates/wasmtime/src/store.rs

@@ -717,7 +717,10 @@ impl Store {
                 Poll::Pending => {}
             }
             unsafe {
-                (*suspend).suspend(())?;
+                let before = wasmtime_runtime::TlsRestore::take();
+                let res = (*suspend).suspend(());
+                before.replace();
+                res?;
             }
         }
     }

tests/all/async_functions.rs

@@ -1,4 +1,5 @@
 use std::cell::Cell;
+use std::cell::RefCell;
 use std::future::Future;
 use std::pin::Pin;
 use std::rc::Rc;
@@ -493,3 +494,144 @@ fn async_host_func_with_pooling_stacks() {
     run_smoke_test(&func);
     run_smoke_get0_test(&func);
 }
+
+fn execute_across_threads<F: Future + 'static>(future: F) {
+    struct UnsafeSend<T>(T);
+    unsafe impl<T> Send for UnsafeSend<T> {}
+
+    impl<T: Future> Future for UnsafeSend<T> {
+        type Output = T::Output;
+        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T::Output> {
+            unsafe { self.map_unchecked_mut(|p| &mut p.0).poll(cx) }
+        }
+    }
+
+    let mut future = Pin::from(Box::new(UnsafeSend(future)));
+    let poll = future
+        .as_mut()
+        .poll(&mut Context::from_waker(&dummy_waker()));
+    assert!(poll.is_pending());
+
+    std::thread::spawn(move || {
+        let poll = future
+            .as_mut()
+            .poll(&mut Context::from_waker(&dummy_waker()));
+        assert!(!poll.is_pending());
+    })
+    .join()
+    .unwrap();
+}
+
+#[test]
+fn resume_separate_thread() {
+    // This test will poll the following future on two threads. Simulating a
+    // trap requires accessing TLS info, so that should be preserved correctly.
+    execute_across_threads(async {
+        let store = async_store();
+        let module = Module::new(
+            store.engine(),
+            "
+            (module
+                (import \"\" \"\" (func))
+                (start 0)
+            )
+            ",
+        )
+        .unwrap();
+        let func = Func::wrap0_async(&store, (), |_, _| {
+            Box::new(async {
+                PendingOnce::default().await;
+                Err::<(), _>(wasmtime::Trap::new("test"))
+            })
+        });
+        let result = Instance::new_async(&store, &module, &[func.into()]).await;
+        assert!(result.is_err());
+    });
+}
+
+#[test]
+fn resume_separate_thread2() {
+    // This test will poll the following future on two threads. Catching a
+    // signal requires looking up TLS information to determine whether it's a
+    // trap to handle or not, so that must be preserved correctly across threads.
+    execute_across_threads(async {
+        let store = async_store();
+        let module = Module::new(
+            store.engine(),
+            "
+            (module
+                (import \"\" \"\" (func))
+                (func $start
+                    call 0
+                    unreachable)
+                (start $start)
+            )
+            ",
+        )
+        .unwrap();
+        let func = Func::wrap0_async(&store, (), |_, _| {
+            Box::new(async { PendingOnce::default().await })
+        });
+        let result = Instance::new_async(&store, &module, &[func.into()]).await;
+        assert!(result.is_err());
+    });
+}
+
+#[test]
+fn resume_separate_thread3() {
+    // This test doesn't actually do anything with cross-thread polls, but
+    // instead it deals with scheduling futures at "odd" times.
+    //
+    // First we'll set up a *synchronous* call which will initialize TLS info.
+    // This call is simply to a host-defined function, but it still has the same
+    // "enter into wasm" semantics since it's just calling a trampoline. In this
+    // situation we'll set up the TLS info so it's in place while the body of
+    // the function executes...
+    let store = Store::default();
+    let storage = Rc::new(RefCell::new(None));
+    let storage2 = storage.clone();
+    let f = Func::wrap(&store, move || {
+        // ... and the execution of this host-defined function (while the TLS
+        // info is initialized), will set up a recursive call into wasm. This
+        // recursive call will be done asynchronously so we can suspend it
+        // halfway through.
+        let f = async {
+            let store = async_store();
+            let module = Module::new(
+                store.engine(),
+                "
+            (module
+                (import \"\" \"\" (func))
+                (start 0)
+            )
+            ",
+            )
+            .unwrap();
+            let func = Func::wrap0_async(&store, (), |_, _| {
+                Box::new(async { PendingOnce::default().await })
+            });
+            drop(Instance::new_async(&store, &module, &[func.into()]).await);
+            unreachable!()
+        };
+        let mut future = Pin::from(Box::new(f));
+        let poll = future
+            .as_mut()
+            .poll(&mut Context::from_waker(&dummy_waker()));
+        assert!(poll.is_pending());
+
+        // ... so at this point our call into wasm is suspended. The call into
+        // wasm will have overwritten TLS info, and we sure hope that the
+        // information is restored at this point. Note that we squirrel away the
+        // future somewhere else to get dropped later. If we were to drop it
+        // here then we would reenter the future's suspended stack to clean it
+        // up, which would do more alterations of TLS information we're not
+        // testing here.
+        *storage2.borrow_mut() = Some(future);
+
+        // ... all in all this function will need access to the original TLS
+        // information to raise the trap. This TLS information should be
+        // restored even though the asynchronous execution is suspended.
+        Err::<(), _>(wasmtime::Trap::new(""))
+    });
+    assert!(f.call(&[]).is_err());
+}