Exit with a more severe error code if the program traps. (#1274)

* Exit with a more severe error code if the program traps.

Previously, the wasmtime CLI would return with a regular failure
error code, such as 1 on Unix. However, a program trap indicates a bug
in the program, which can be useful to distinguish from a simple error
status. Check for the trap case, and return an appropriate OS-specific
exit status.

* Use a loop to iterate over the error causes to find Traps.

* Use anyhow's `chain()` iterator.

* For completeness, handle non-Unix and non-Windows platforms too.

* Add a CLI test for a trapping program.

* Replace a manual `.cause` loop with a `.is` call.

* Correct the expected exit status on Windows.

* Use assert_eq/assert_ne so that if these fail, it prints the output.

src/commands/run.rs

@@ -6,10 +6,11 @@ use std::{
     ffi::{OsStr, OsString},
     fs::File,
     path::{Component, Path, PathBuf},
+    process,
 };
 use structopt::{clap::AppSettings, StructOpt};
 use wasi_common::preopen_dir;
-use wasmtime::{Engine, Instance, Module, Store};
+use wasmtime::{Engine, Instance, Module, Store, Trap};
 use wasmtime_interface_types::ModuleData;
 use wasmtime_wasi::{old::snapshot_0::Wasi as WasiSnapshot0, Wasi};
 
@@ -113,8 +114,31 @@ impl RunCommand {
         }
 
         // Load the main wasm module.
-        self.handle_module(&store, &module_registry)
-            .with_context(|| format!("failed to run main module `{}`", self.module.display()))?;
+        match self
+            .handle_module(&store, &module_registry)
+            .with_context(|| format!("failed to run main module `{}`", self.module.display()))
+        {
+            Ok(()) => (),
+            Err(e) => {
+                // 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>() {
+                    // Print the error message in the usual way.
+                    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);
+                    }
+                }
+                return Err(e);
+            }
+        }
 
         Ok(())
     }

tests/cli_tests.rs

@@ -1,15 +1,22 @@
 use anyhow::{bail, Result};
 use std::io::Write;
 use std::path::Path;
-use std::process::Command;
+use std::process::{Command, Output};
 use tempfile::NamedTempFile;
 
-fn run_wasmtime(args: &[&str]) -> Result<String> {
+// Run the wasmtime CLI with the provided args and return the `Output`.
+fn run_wasmtime_for_output(args: &[&str]) -> Result<Output> {
     let mut me = std::env::current_exe()?;
     me.pop(); // chop off the file name
     me.pop(); // chop off `deps`
     me.push("wasmtime");
-    let output = Command::new(&me).args(args).output()?;
+    Command::new(&me).args(args).output().map_err(Into::into)
+}
+
+// Run the wasmtime CLI with the provided args and, if it succeeds, return
+// the standard output in a `String`.
+fn run_wasmtime(args: &[&str]) -> Result<String> {
+    let output = run_wasmtime_for_output(args)?;
     if !output.status.success() {
         bail!(
             "Failed to execute wasmtime with: {:?}\n{}",
@@ -74,3 +81,27 @@ fn run_wasmtime_simple_wat() -> Result<()> {
     ])?;
     Ok(())
 }
+
+// Running a wat that traps.
+#[test]
+fn run_wasmtime_unreachable_wat() -> Result<()> {
+    let wasm = build_wasm("tests/wasm/unreachable.wat")?;
+    let output = run_wasmtime_for_output(&[wasm.path().to_str().unwrap(), "--disable-cache"])?;
+
+    assert_ne!(output.stderr, b"");
+    assert_eq!(output.stdout, b"");
+    assert!(!output.status.success());
+
+    let code = output
+        .status
+        .code()
+        .expect("wasmtime process should exit normally");
+
+    // Test for the specific error code Wasmtime uses to indicate a trap return.
+    #[cfg(unix)]
+    assert_eq!(code, 128 + libc::SIGABRT);
+    #[cfg(windows)]
+    assert_eq!(code, 3);
+
+    Ok(())
+}

tests/wasm/unreachable.wat

@@ -0,0 +1,5 @@
+(module
+    (func (export "_start")
+        unreachable
+    )
+)