wasi-threads: factor out process exit logic

As is being discussed [elsewhere], either calling `proc_exit` or
trapping in any thread should halt execution of all threads. The
Wasmtime CLI already has logic for adapting a WebAssembly error code to
a code expected in each OS. This change factors out this logic to a new
function, `maybe_exit_on_error`, for use within the `wasi-threads`
implementation.

This will work reasonably well for CLI users of Wasmtime +
`wasi-threads`, but embedders will want something better in the future:
when a `wasi-threads` threads fails, they may not want their application
to exit. Handling this is tricky, because it will require cancelling the
threads spawned by the `wasi-threads` implementation, something that is
not trivial to do in Rust. With this change, we defer that work until
later in order to provide a working implementation of `wasi-threads` for
experimentation.

[elsewhere]: WebAssembly/wasi-threads#17

Cargo.toml

@@ -35,7 +35,6 @@ wasmtime-wasi-threads = { workspace = true, optional = true }
 clap = { workspace = true, features = ["color", "suggestions", "derive"] }
 anyhow = { workspace = true }
 target-lexicon = { workspace = true }
-libc = "0.2.60"
 humantime = "2.0.0"
 once_cell = { workspace = true }
 listenfd = "1.0.0"

crates/wasi-threads/Cargo.toml

@@ -17,6 +17,7 @@ log = { workspace = true }
 rand = "0.8"
 wasi-common = { workspace = true }
 wasmtime = { workspace = true }
+wasmtime-wasi = { workspace = true }
 
 [badges]
 maintenance = { status = "experimental" }

crates/wasi-threads/src/lib.rs

@@ -5,8 +5,8 @@
 use anyhow::{anyhow, Result};
 use rand::Rng;
 use std::thread;
-use wasi_common::I32Exit;
 use wasmtime::{Caller, Linker, Module, SharedMemory, Store};
+use wasmtime_wasi::maybe_exit_on_error;
 
 // This name is a function export designated by the wasi-threads specification:
 // https://github.com/WebAssembly/wasi-threads/#detailed-design-discussion
@@ -63,15 +63,11 @@ impl<T: Clone + Send + 'static> WasiThreadsCtx<T> {
             );
             match thread_entry_point.call(&mut store, (wasi_thread_id, thread_start_arg)) {
                 Ok(_) => {}
-                Err(trap) => {
-                    if let Some(I32Exit(0)) = trap.downcast_ref::<I32Exit>() {
-                        log::trace!(
-                            "exited thread id = {} via `wasi::proc_exit` call",
-                            wasi_thread_id
-                        )
-                    } else {
-                        panic!("{}", fail(trap.to_string()))
-                    }
+                Err(e) => {
+                    log::trace!("exiting thread id = {} due to error", wasi_thread_id);
+                    let e = maybe_exit_on_error(e);
+                    eprintln!("Error: {:?}", e);
+                    std::process::exit(1);
                 }
             }
         })?;

crates/wasi/Cargo.toml

@@ -13,6 +13,7 @@ include = ["src/**/*", "README.md", "LICENSE", "build.rs"]
 build = "build.rs"
 
 [dependencies]
+libc = "0.2.60"
 wasi-common = { workspace = true }
 wasi-cap-std-sync = { workspace = true, optional = true }
 wasi-tokio = { workspace = true, optional = true }

crates/wasi/src/lib.rs

@@ -84,3 +84,45 @@ pub mod snapshots {
 }
 }
 }
+
+/// Exit the process with a conventional OS error code as long as Wasmtime
+/// understands the error. If the error is not an `I32Exit` or `Trap`, return
+/// the error back to the caller for it to decide what to do.
+///
+/// Note: this function is designed for CLI usage of Wasmtime; embedders of
+/// Wasmtime may want different error handling.
+pub fn maybe_exit_on_error(e: anyhow::Error) -> anyhow::Error {
+    use std::process;
+    use wasmtime::Trap;
+
+    // If a specific WASI error code was requested then that's
+    // forwarded through to the process here without printing any
+    // extra error information.
+    if let Some(exit) = e.downcast_ref::<I32Exit>() {
+        // Print the error message in the usual way.
+        // On Windows, exit status 3 indicates an abort (see below),
+        // so return 1 indicating a non-zero status to avoid ambiguity.
+        if cfg!(windows) && exit.0 >= 3 {
+            process::exit(1);
+        }
+        process::exit(exit.0);
+    }
+
+    // If the program exited because of a trap, return an error code
+    // to the outside environment indicating a more severe problem
+    // than a simple failure.
+    if e.is::<Trap>() {
+        eprintln!("Error: {:?}", e);
+
+        if cfg!(unix) {
+            // On Unix, return the error code of an abort.
+            process::exit(128 + libc::SIGABRT);
+        } else if cfg!(windows) {
+            // On Windows, return 3.
+            // https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/abort?view=vs-2019
+            process::exit(3);
+        }
+    }
+
+    e
+}

src/commands/run.rs

@@ -3,18 +3,15 @@
 use anyhow::{anyhow, bail, Context as _, Result};
 use clap::Parser;
 use once_cell::sync::Lazy;
+use std::ffi::OsStr;
+use std::path::{Component, Path, PathBuf};
 use std::sync::Arc;
 use std::thread;
 use std::time::Duration;
-use std::{
-    ffi::OsStr,
-    path::{Component, Path, PathBuf},
-    process,
-};
-use wasmtime::{Engine, Func, Linker, Module, Store, Trap, Val, ValType};
+use wasmtime::{Engine, Func, Linker, Module, Store, Val, ValType};
 use wasmtime_cli_flags::{CommonOptions, WasiModules};
+use wasmtime_wasi::maybe_exit_on_error;
 use wasmtime_wasi::sync::{ambient_authority, Dir, TcpListener, WasiCtxBuilder};
-use wasmtime_wasi::I32Exit;
 
 #[cfg(feature = "wasi-nn")]
 use wasmtime_wasi_nn::WasiNnCtx;
@@ -222,38 +219,10 @@ impl RunCommand {
         {
             Ok(()) => (),
             Err(e) => {
-                // If a specific WASI error code was requested then that's
-                // forwarded through to the process here without printing any
-                // extra error information.
-                if let Some(exit) = e.downcast_ref::<I32Exit>() {
-                    // Print the error message in the usual way.
-                    // On Windows, exit status 3 indicates an abort (see below),
-                    // so return 1 indicating a non-zero status to avoid ambiguity.
-                    if cfg!(windows) && exit.0 >= 3 {
-                        process::exit(1);
-                    }
-                    process::exit(exit.0);
-                }
-
-                // If the program exited because of a trap, return an error code
-                // to the outside environment indicating a more severe problem
-                // than a simple failure.
-                if e.is::<Trap>() {
-                    eprintln!("Error: {:?}", e);
-
-                    if cfg!(unix) {
-                        // On Unix, return the error code of an abort.
-                        process::exit(128 + libc::SIGABRT);
-                    } else if cfg!(windows) {
-                        // On Windows, return 3.
-                        // https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/abort?view=vs-2019
-                        process::exit(3);
-                    }
-                }
-
-                // Otherwise fall back on Rust's default error printing/return
+                // Exit the process if Wasmtime understands the error;
+                // otherwise, fall back on Rust's default error printing/return
                 // code.
-                return Err(e);
+                return Err(maybe_exit_on_error(e));
             }
         }