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(())
     }