wasi-nn: track upstream specification

In bytecodealliance#8873, we stopped tracking the wasi-nn's upstream WIT files
temporarily because it was not clear to me at the time how to implement
errors as CM resources. This PR fixes that, resuming tracking in the
`vendor-wit.sh` and implementing what is needed in the wasi-nn crate.

This leaves several threads unresolved, though:
- it looks like the `vendor-wit.sh` has a new mechanism for retrieving
  files into `wit/deps`--at some point wasi-nn should migrate to use
  that (?)
- it's not clear to me that "errors as resources" is even the best
  approach here; I've opened [bytecodealliance#75] to consider the possibility of using
  "errors as records" instead.
- this PR identifies that the constructor for errors is in fact
  unnecessary, prompting an upstream change ([bytecodealliance#76]) that should
  eventually be implemented here.

[bytecodealliance#75]: WebAssembly/wasi-nn#75
[bytecodealliance#76]: WebAssembly/wasi-nn#76

prtest:full

Author: abrown

ci/vendor-wit.sh

@@ -65,10 +65,6 @@ rm -rf $cache_dir
 # Separately (for now), vendor the `wasi-nn` WIT files since their retrieval is
 # slightly different than above.
 repo=https://raw.githubusercontent.com/WebAssembly/wasi-nn
-revision=e2310b
+revision=0.2.0-rc-2024-06-25
 curl -L $repo/$revision/wasi-nn.witx -o crates/wasi-nn/witx/wasi-nn.witx
-# TODO: the in-tree `wasi-nn` implementation does not yet fully support the
-# latest WIT specification on `main`. To create a baseline for moving forward,
-# the in-tree WIT incorporates some but not all of the upstream changes. This
-# TODO can be removed once the implementation catches up with the spec.
-# curl -L $repo/$revision/wit/wasi-nn.wit -o crates/wasi-nn/wit/wasi-nn.wit
+curl -L $repo/$revision/wit/wasi-nn.wit -o crates/wasi-nn/wit/wasi-nn.wit

crates/wasi-nn/src/wit.rs

@@ -17,6 +17,7 @@
 
 use crate::backend::Id;
 use crate::{Backend, Registry};
+use anyhow::anyhow;
 use std::collections::HashMap;
 use std::hash::Hash;
 use std::{fmt, str::FromStr};
@@ -54,29 +55,63 @@ impl<'a> WasiNnView<'a> {
     }
 }
 
-pub enum Error {
+/// A wasi-nn error; this appears on the Wasm side as a component model
+/// resource.
+#[derive(Debug)]
+pub struct Error {
+    code: ErrorCode,
+    data: anyhow::Error,
+}
+
+/// Construct an [`Error`] resource and immediately return it.
+///
+/// The WIT specification currently relies on "errors as resources;" this helper
+/// macro hides some of that complexity. If [#75] is adopted ("errors as
+/// records"), this macro is no longer necessary.
+///
+/// [#75]: https://github.com/WebAssembly/wasi-nn/pull/75
+macro_rules! bail {
+    ($self:ident, $code:expr, $data:expr) => {
+        let e = Error {
+            code: $code,
+            data: $data.into(),
+        };
+        tracing::error!("failure: {e:?}");
+        let r = $self.table.push(e)?;
+        return Ok(Err(r));
+    };
+}
+
+impl From<wasmtime::component::ResourceTableError> for Error {
+    fn from(error: wasmtime::component::ResourceTableError) -> Self {
+        Self {
+            code: ErrorCode::Trap,
+            data: error.into(),
+        }
+    }
+}
+
+/// The list of error codes available to the `wasi-nn` API; this should match
+/// what is specified in WIT.
+#[derive(Debug)]
+pub enum ErrorCode {
     /// Caller module passed an invalid argument.
     InvalidArgument,
     /// Invalid encoding.
     InvalidEncoding,
     /// The operation timed out.
     Timeout,
-    /// Runtime Error.
+    /// Runtime error.
     RuntimeError,
     /// Unsupported operation.
     UnsupportedOperation,
     /// Graph is too large.
     TooLarge,
     /// Graph not found.
     NotFound,
-    /// A runtime error occurred that we should trap on; see `StreamError`.
-    Trap(anyhow::Error),
-}
-
-impl From<wasmtime::component::ResourceTableError> for Error {
-    fn from(error: wasmtime::component::ResourceTableError) -> Self {
-        Self::Trap(error.into())
-    }
+    /// A runtime error that Wasmtime should trap on; this will not appear in
+    /// the WIT specification.
+    Trap,
 }
 
 /// Generate the traits and types from the `wasi-nn` WIT specification.
@@ -91,6 +126,7 @@ mod gen_ {
             "wasi:nn/graph/graph": crate::Graph,
             "wasi:nn/tensor/tensor": crate::Tensor,
             "wasi:nn/inference/graph-execution-context": crate::ExecutionContext,
+            "wasi:nn/errors/error": super::Error,
         },
         trappable_error_type: {
             "wasi:nn/errors/error" => super::Error,
@@ -131,36 +167,45 @@ impl gen::graph::Host for WasiNnView<'_> {
         builders: Vec<GraphBuilder>,
         encoding: GraphEncoding,
         target: ExecutionTarget,
-    ) -> Result<Resource<crate::Graph>, Error> {
+    ) -> wasmtime::Result<Result<Resource<Graph>, Resource<Error>>> {
         tracing::debug!("load {encoding:?} {target:?}");
         if let Some(backend) = self.ctx.backends.get_mut(&encoding) {
             let slices = builders.iter().map(|s| s.as_slice()).collect::<Vec<_>>();
             match backend.load(&slices, target.into()) {
                 Ok(graph) => {
                     let graph = self.table.push(graph)?;
-                    Ok(graph)
+                    Ok(Ok(graph))
                 }
                 Err(error) => {
-                    tracing::error!("failed to load graph: {error:?}");
-                    Err(Error::RuntimeError)
+                    bail!(self, ErrorCode::RuntimeError, error);
                 }
             }
         } else {
-            Err(Error::InvalidEncoding)
+            bail!(
+                self,
+                ErrorCode::InvalidEncoding,
+                anyhow!("unable to find a backend for this encoding")
+            );
         }
     }
 
-    fn load_by_name(&mut self, name: String) -> Result<Resource<Graph>, Error> {
+    fn load_by_name(
+        &mut self,
+        name: String,
+    ) -> wasmtime::Result<Result<Resource<Graph>, Resource<Error>>> {
         use core::result::Result::*;
         tracing::debug!("load by name {name:?}");
         let registry = &self.ctx.registry;
         if let Some(graph) = registry.get(&name) {
             let graph = graph.clone();
             let graph = self.table.push(graph)?;
-            Ok(graph)
+            Ok(Ok(graph))
         } else {
-            tracing::error!("failed to find graph with name: {name}");
-            Err(Error::NotFound)
+            bail!(
+                self,
+                ErrorCode::NotFound,
+                anyhow!("failed to find graph with name: {name}")
+            );
         }
     }
 }
@@ -169,18 +214,17 @@ impl gen::graph::HostGraph for WasiNnView<'_> {
     fn init_execution_context(
         &mut self,
         graph: Resource<Graph>,
-    ) -> Result<Resource<GraphExecutionContext>, Error> {
+    ) -> wasmtime::Result<Result<Resource<GraphExecutionContext>, Resource<Error>>> {
         use core::result::Result::*;
         tracing::debug!("initialize execution context");
         let graph = self.table.get(&graph)?;
         match graph.init_execution_context() {
             Ok(exec_context) => {
                 let exec_context = self.table.push(exec_context)?;
-                Ok(exec_context)
+                Ok(Ok(exec_context))
             }
             Err(error) => {
-                tracing::error!("failed to initialize execution context: {error:?}");
-                Err(Error::RuntimeError)
+                bail!(self, ErrorCode::RuntimeError, error);
             }
         }
     }
@@ -197,47 +241,46 @@ impl gen::inference::HostGraphExecutionContext for WasiNnView<'_> {
         exec_context: Resource<GraphExecutionContext>,
         name: String,
         tensor: Resource<Tensor>,
-    ) -> Result<(), Error> {
+    ) -> wasmtime::Result<Result<(), Resource<Error>>> {
         let tensor = self.table.get(&tensor)?;
         tracing::debug!("set input {name:?}: {tensor:?}");
         let tensor = tensor.clone(); // TODO: avoid copying the tensor
         let exec_context = self.table.get_mut(&exec_context)?;
-        if let Err(e) = exec_context.set_input(Id::Name(name), &tensor) {
-            tracing::error!("failed to set input: {e:?}");
-            Err(Error::InvalidArgument)
+        if let Err(error) = exec_context.set_input(Id::Name(name), &tensor) {
+            bail!(self, ErrorCode::InvalidArgument, error);
         } else {
-            Ok(())
+            Ok(Ok(()))
         }
     }
 
-    fn compute(&mut self, exec_context: Resource<GraphExecutionContext>) -> Result<(), Error> {
+    fn compute(
+        &mut self,
+        exec_context: Resource<GraphExecutionContext>,
+    ) -> wasmtime::Result<Result<(), Resource<Error>>> {
         let exec_context = &mut self.table.get_mut(&exec_context)?;
         tracing::debug!("compute");
         match exec_context.compute() {
-            Ok(()) => Ok(()),
+            Ok(()) => Ok(Ok(())),
             Err(error) => {
-                tracing::error!("failed to compute: {error:?}");
-                Err(Error::RuntimeError)
+                bail!(self, ErrorCode::RuntimeError, error);
             }
         }
     }
 
-    #[doc = r" Extract the outputs after inference."]
     fn get_output(
         &mut self,
         exec_context: Resource<GraphExecutionContext>,
         name: String,
-    ) -> Result<Resource<Tensor>, Error> {
+    ) -> wasmtime::Result<Result<Resource<Tensor>, Resource<Error>>> {
         let exec_context = self.table.get_mut(&exec_context)?;
         tracing::debug!("get output {name:?}");
         match exec_context.get_output(Id::Name(name)) {
             Ok(tensor) => {
                 let tensor = self.table.push(tensor)?;
-                Ok(tensor)
+                Ok(Ok(tensor))
             }
             Err(error) => {
-                tracing::error!("failed to get output: {error:?}");
-                Err(Error::RuntimeError)
+                bail!(self, ErrorCode::RuntimeError, error);
             }
         }
     }
@@ -285,21 +328,51 @@ impl gen::tensor::HostTensor for WasiNnView<'_> {
     }
 }
 
-impl gen::tensor::Host for WasiNnView<'_> {}
+impl gen::errors::HostError for WasiNnView<'_> {
+    fn new(
+        &mut self,
+        _code: gen::errors::ErrorCode,
+        _data: String,
+    ) -> wasmtime::Result<Resource<Error>> {
+        unimplemented!("this should be removed; see https://github.com/WebAssembly/wasi-nn/pull/76")
+    }
+
+    fn code(&mut self, error: Resource<Error>) -> wasmtime::Result<gen::errors::ErrorCode> {
+        let error = self.table.get(&error)?;
+        match error.code {
+            ErrorCode::InvalidArgument => Ok(gen::errors::ErrorCode::InvalidArgument),
+            ErrorCode::InvalidEncoding => Ok(gen::errors::ErrorCode::InvalidEncoding),
+            ErrorCode::Timeout => Ok(gen::errors::ErrorCode::Timeout),
+            ErrorCode::RuntimeError => Ok(gen::errors::ErrorCode::RuntimeError),
+            ErrorCode::UnsupportedOperation => Ok(gen::errors::ErrorCode::UnsupportedOperation),
+            ErrorCode::TooLarge => Ok(gen::errors::ErrorCode::TooLarge),
+            ErrorCode::NotFound => Ok(gen::errors::ErrorCode::NotFound),
+            ErrorCode::Trap => Err(anyhow!(error.data.to_string())),
+        }
+    }
+
+    fn data(&mut self, error: Resource<Error>) -> wasmtime::Result<String> {
+        let error = self.table.get(&error)?;
+        Ok(error.data.to_string())
+    }
+
+    fn drop(&mut self, error: Resource<Error>) -> wasmtime::Result<()> {
+        self.table.delete(error)?;
+        Ok(())
+    }
+}
+
 impl gen::errors::Host for WasiNnView<'_> {
-    fn convert_error(&mut self, err: Error) -> wasmtime::Result<gen::errors::Error> {
-        match err {
-            Error::InvalidArgument => Ok(gen::errors::Error::InvalidArgument),
-            Error::InvalidEncoding => Ok(gen::errors::Error::InvalidEncoding),
-            Error::Timeout => Ok(gen::errors::Error::Timeout),
-            Error::RuntimeError => Ok(gen::errors::Error::RuntimeError),
-            Error::UnsupportedOperation => Ok(gen::errors::Error::UnsupportedOperation),
-            Error::TooLarge => Ok(gen::errors::Error::TooLarge),
-            Error::NotFound => Ok(gen::errors::Error::NotFound),
-            Error::Trap(e) => Err(e),
+    fn convert_error(&mut self, err: Error) -> wasmtime::Result<Error> {
+        if matches!(err.code, ErrorCode::Trap) {
+            Err(err.data)
+        } else {
+            Ok(err)
         }
     }
 }
+
+impl gen::tensor::Host for WasiNnView<'_> {}
 impl gen::inference::Host for WasiNnView<'_> {}
 
 impl Hash for gen::graph::GraphEncoding {

crates/wasi-nn/wit/wasi-nn.wit

@@ -1,4 +1,4 @@
-package wasi:nn;
+package wasi:nn@0.2.0-rc-2024-06-25;
 
 /// `wasi-nn` is a WASI API for performing machine learning (ML) inference. The API is not (yet)
 /// capable of performing ML training. WebAssembly programs that want to use a host's ML
@@ -134,7 +134,7 @@ interface inference {
 
 /// TODO: create function-specific errors (https://github.com/WebAssembly/wasi-nn/issues/42)
 interface errors {
-    enum error {
+    enum error-code {
         // Caller module passed an invalid argument.
         invalid-argument,
         // Invalid encoding.
@@ -148,6 +148,21 @@ interface errors {
         // Graph is too large.
         too-large,
         // Graph not found.
-        not-found
+        not-found,
+        // The operation is insecure or has insufficient privilege to be performed.
+        // e.g., cannot access a hardware feature requested
+        security,
+        // The operation failed for an unspecified reason.
+        unknown
+    }
+
+    resource error {
+        constructor(code: error-code, data: string);
+
+        /// Return the error code.
+        code: func() -> error-code;
+
+        /// Errors can propagated with backend specific status through a string value.
+        data: func() -> string;
     }
 }