Reactor support. (#1565)

* Reactor support.

This implements the new WASI ABI described here:

https://github.com/WebAssembly/WASI/blob/master/design/application-abi.md

It adds APIs to `Instance` and `Linker` with support for running
WASI programs, and also simplifies the process of instantiating
WASI API modules.

This currently only includes Rust API support.

* Add comments and fix a typo in a comment.

* Fix a rustdoc warning.

* Tidy an unneeded `mut`.

* Factor out instance initialization with `NewInstance`.

This also separates instantiation from initialization in a manner
similar to bytecodealliance/lucet#506.

* Update fuzzing oracles for the API changes.

* Remove `wasi_linker` and clarify that Commands/Reactors aren't connected to WASI.

* Move Command/Reactor semantics into the Linker.

* C API support.

* Fix fuzzer build.

* Update usage syntax from "::" to "=".

* Remove `NewInstance` and `start()`.

* Elaborate on Commands and Reactors and add a spec link.

* Add more comments.

* Fix wat syntax.

* Fix wat.

* Use the `Debug` formatter to format an anyhow::Error.

* Fix wat.

RELEASES.md

@@ -8,7 +8,10 @@ Unreleased
 
 ### Added
 
-### Changed
+* The [Commands and Reactors ABI] is now supported in the Rust API. `Linker::module`
+  loads a module and automatically handles Commands and Reactors semantics.
+
+[Commands and Reactors ABI]: https://github.com/WebAssembly/WASI/blob/master/design/application-abi.md#current-unstable-abi
 
 ### Fixed
 

crates/c-api/include/wasmtime.h

@@ -117,6 +117,18 @@ WASM_API_EXTERN own wasmtime_error_t* wasmtime_linker_instantiate(
     own wasm_trap_t **trap
 );
 
+WASM_API_EXTERN own wasmtime_error_t* wasmtime_linker_module(
+    const wasmtime_linker_t *linker,
+    const wasm_name_t *name,
+    const wasm_module_t *module
+);
+
+WASM_API_EXTERN own wasmtime_error_t* wasmtime_linker_get_default(
+    const wasmtime_linker_t *linker,
+    const wasm_name_t *name,
+    own wasm_func_t **func
+);
+
 ///////////////////////////////////////////////////////////////////////////////
 //
 // wasmtime_caller_t extension, binding the `Caller` type in the Rust API

crates/c-api/src/linker.rs

@@ -1,8 +1,8 @@
 use crate::{bad_utf8, handle_result, wasmtime_error_t};
 use crate::{wasm_extern_t, wasm_store_t, ExternHost};
-use crate::{wasm_instance_t, wasm_module_t, wasm_name_t, wasm_trap_t};
+use crate::{wasm_func_t, wasm_instance_t, wasm_module_t, wasm_name_t, wasm_trap_t};
 use std::str;
-use wasmtime::{Extern, Linker};
+use wasmtime::{Extern, HostRef, Linker};
 
 #[repr(C)]
 pub struct wasmtime_linker_t {
@@ -89,3 +89,33 @@ pub unsafe extern "C" fn wasmtime_linker_instantiate(
     let result = linker.linker.instantiate(&module.module.borrow());
     super::instance::handle_instantiate(result, instance_ptr, trap_ptr)
 }
+
+#[no_mangle]
+pub unsafe extern "C" fn wasmtime_linker_module(
+    linker: &mut wasmtime_linker_t,
+    name: &wasm_name_t,
+    module: &wasm_module_t,
+) -> Option<Box<wasmtime_error_t>> {
+    let linker = &mut linker.linker;
+    let name = match str::from_utf8(name.as_slice()) {
+        Ok(s) => s,
+        Err(_) => return bad_utf8(),
+    };
+    handle_result(linker.module(name, &module.module.borrow()), |_linker| ())
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn wasmtime_linker_get_default(
+    linker: &mut wasmtime_linker_t,
+    name: &wasm_name_t,
+    func: &mut *mut wasm_func_t,
+) -> Option<Box<wasmtime_error_t>> {
+    let linker = &mut linker.linker;
+    let name = match str::from_utf8(name.as_slice()) {
+        Ok(s) => s,
+        Err(_) => return bad_utf8(),
+    };
+    handle_result(linker.get_default(name), |f| {
+        *func = Box::into_raw(Box::new(HostRef::new(f).into()))
+    })
+}

crates/runtime/src/instance.rs

@@ -1330,8 +1330,4 @@ pub enum InstantiationError {
     /// A trap ocurred during instantiation, after linking.
     #[error("Trap occurred during instantiation")]
     Trap(Trap),
-
-    /// A trap occurred while running the wasm start function.
-    #[error("Trap occurred while invoking start function")]
-    StartTrap(Trap),
 }

crates/test-programs/tests/wasm_tests/runtime.rs

@@ -1,8 +1,8 @@
-use anyhow::{bail, Context};
+use anyhow::Context;
 use std::fs::File;
 use std::path::Path;
 use wasi_common::VirtualDirEntry;
-use wasmtime::{Instance, Module, Store};
+use wasmtime::{Linker, Module, Store};
 
 #[derive(Clone, Copy, Debug)]
 pub enum PreopenType {
@@ -48,36 +48,19 @@ pub fn instantiate(
     let (reader, _writer) = os_pipe::pipe()?;
     builder.stdin(reader_to_file(reader));
     let snapshot1 = wasmtime_wasi::Wasi::new(&store, builder.build()?);
-    let module = Module::new(&store, &data).context("failed to create wasm module")?;
-    let imports = module
-        .imports()
-        .map(|i| {
-            let field_name = i.name();
-            if let Some(export) = snapshot1.get_export(field_name) {
-                Ok(export.clone().into())
-            } else {
-                bail!(
-                    "import {} was not found in module {}",
-                    field_name,
-                    i.module()
-                )
-            }
-        })
-        .collect::<Result<Vec<_>, _>>()?;
 
-    let instance = Instance::new(&module, &imports).context(format!(
-        "error while instantiating Wasm module '{}'",
-        bin_name,
-    ))?;
+    let mut linker = Linker::new(&store);
 
-    instance
-        .get_export("_start")
-        .context("expected a _start export")?
-        .into_func()
-        .context("expected export to be a func")?
-        .call(&[])?;
+    snapshot1.add_to_linker(&mut linker)?;
+
+    let module = Module::new(&store, &data).context("failed to create wasm module")?;
 
-    Ok(())
+    linker
+        .module("", &module)
+        .and_then(|m| m.get_default(""))
+        .and_then(|f| f.get0::<()>())
+        .and_then(|f| f().map_err(Into::into))
+        .context(format!("error while testing Wasm module '{}'", bin_name,))
 }
 
 #[cfg(unix)]

crates/wasi-common/wig/src/wasi.rs

@@ -209,6 +209,7 @@ pub fn define_struct(args: TokenStream) -> TokenStream {
                             let memory = match caller.get_export("memory") {
                                 Some(wasmtime::Extern::Memory(m)) => m,
                                 _ => {
+                                    log::warn!("callee does not export a memory as \"memory\"");
                                     let e = wasi_common::old::snapshot_0::wasi::__WASI_ERRNO_INVAL;
                                     #handle_early_error
                                 }
@@ -463,6 +464,7 @@ pub fn define_struct_for_wiggle(args: TokenStream) -> TokenStream {
                             let mem = match caller.get_export("memory") {
                                 Some(wasmtime::Extern::Memory(m)) => m,
                                 _ => {
+                                    log::warn!("callee does not export a memory as \"memory\"");
                                     let e = wasi_common::wasi::Errno::Inval;
                                     #handle_early_error
                                 }

crates/wasmtime/src/instance.rs

@@ -26,9 +26,25 @@ fn instantiate(
     sig_registry: &SignatureRegistry,
     host: Box<dyn Any>,
 ) -> Result<StoreInstanceHandle, Error> {
+    // For now we have a restriction that the `Store` that we're working
+    // with is the same for everything involved here.
+    for import in imports {
+        if !import.comes_from_same_store(store) {
+            bail!("cross-`Store` instantiation is not currently supported");
+        }
+    }
+
+    if imports.len() != compiled_module.module().imports.len() {
+        bail!(
+            "wrong number of imports provided, {} != {}",
+            imports.len(),
+            compiled_module.module().imports.len()
+        );
+    }
+
     let mut resolver = SimpleResolver { imports };
-    unsafe {
-        let config = store.engine().config();
+    let config = store.engine().config();
+    let instance = unsafe {
         let instance = compiled_module.instantiate(
             &mut resolver,
             sig_registry,
@@ -51,31 +67,38 @@ fn instantiate(
             )
             .map_err(|e| -> Error {
                 match e {
-                    InstantiationError::StartTrap(trap) | InstantiationError::Trap(trap) => {
-                        Trap::from_runtime(trap).into()
-                    }
+                    InstantiationError::Trap(trap) => Trap::from_runtime(trap).into(),
                     other => other.into(),
                 }
             })?;
 
-        // If a start function is present, now that we've got our compiled
-        // instance we can invoke it. Make sure we use all the trap-handling
-        // configuration in `store` as well.
-        if let Some(start) = instance.module().start_func {
-            let f = match instance.lookup_by_declaration(&EntityIndex::Function(start)) {
-                wasmtime_runtime::Export::Function(f) => f,
-                _ => unreachable!(), // valid modules shouldn't hit this
-            };
-            super::func::catch_traps(instance.vmctx_ptr(), store, || {
+        instance
+    };
+
+    let start_func = instance.handle.module().start_func;
+
+    // If a start function is present, invoke it. Make sure we use all the
+    // trap-handling configuration in `store` as well.
+    if let Some(start) = start_func {
+        let f = match instance
+            .handle
+            .lookup_by_declaration(&EntityIndex::Function(start))
+        {
+            wasmtime_runtime::Export::Function(f) => f,
+            _ => unreachable!(), // valid modules shouldn't hit this
+        };
+        let vmctx_ptr = instance.handle.vmctx_ptr();
+        unsafe {
+            super::func::catch_traps(vmctx_ptr, store, || {
                 mem::transmute::<
                     *const VMFunctionBody,
                     unsafe extern "C" fn(*mut VMContext, *mut VMContext),
-                >(f.address)(f.vmctx, instance.vmctx_ptr())
+                >(f.address)(f.vmctx, vmctx_ptr)
             })?;
         }
-
-        Ok(instance)
     }
+
+    Ok(instance)
 }
 
 /// An instantiated WebAssembly module.
@@ -111,6 +134,15 @@ impl Instance {
     /// automatically run (if provided) and then the [`Instance`] will be
     /// returned.
     ///
+    /// Per the WebAssembly spec, instantiation includes running the module's
+    /// start function, if it has one (not to be confused with the `_start`
+    /// function, which is not run).
+    ///
+    /// Note that this is a low-level function that just performance an
+    /// instantiation. See the `Linker` struct for an API which provides a
+    /// convenient way to link imports and provides automatic Command and Reactor
+    /// behavior.
+    ///
     /// ## Providing Imports
     ///
     /// The `imports` array here is a bit tricky. The entries in the list of
@@ -147,23 +179,6 @@ impl Instance {
     /// [`ExternType`]: crate::ExternType
     pub fn new(module: &Module, imports: &[Extern]) -> Result<Instance, Error> {
         let store = module.store();
-
-        // For now we have a restriction that the `Store` that we're working
-        // with is the same for everything involved here.
-        for import in imports {
-            if !import.comes_from_same_store(store) {
-                bail!("cross-`Store` instantiation is not currently supported");
-            }
-        }
-
-        if imports.len() != module.imports().len() {
-            bail!(
-                "wrong number of imports provided, {} != {}",
-                imports.len(),
-                module.imports().len()
-            );
-        }
-
         let info = module.register_frame_info();
         let handle = instantiate(
             store,