Add function and example program detecting WASI runability (#250)

examples/detect-wasi.rs

@@ -0,0 +1,13 @@
+use std::process::exit;
+
+use cargo_binstall::wasi::detect_wasi_runability;
+
+fn main() {
+    if detect_wasi_runability().unwrap() {
+        println!("WASI is runnable!");
+        exit(0);
+    } else {
+        println!("WASI is not runnable");
+        exit(1);
+    }
+}

src/lib.rs

@@ -15,6 +15,7 @@ pub mod bins;
 pub mod binstall;
 pub mod fetchers;
 pub mod metafiles;
+pub mod wasi;
 
 mod target;
 pub use target::*;

src/miniwasi.wast

@@ -0,0 +1,10 @@
+(module
+ (memory $0 0)
+ (export "memory" (memory $0))
+ (export "_start" (func $0))
+ (import "wasi_snapshot_preview1" "proc_exit" (func $exit (param i32)))
+ (func $0
+  (call $exit (i32.const 0))
+  (unreachable)
+ )
+)

src/wasi.rs

@@ -0,0 +1,40 @@
+use std::{fs::File, io::Write, process::Command};
+#[cfg(unix)]
+use std::{fs::Permissions, os::unix::fs::PermissionsExt};
+
+use tempfile::tempdir;
+
+use crate::errors::BinstallError;
+
+const WASI_PROGRAM: &[u8] = include_bytes!("miniwasi.wasm");
+
+/// Detect the ability to run WASI
+///
+/// This attempts to run a small embedded WASI program, and returns true if no errors happened.
+/// Errors returned by the `Result` are I/O errors from the establishment of the context, not
+/// errors from the run attempt.
+///
+/// On Linux, you can configure your system to run WASI programs using a binfmt directive. Under
+/// systemd, write the below to `/etc/binfmt.d/wasi.conf`, with `/usr/bin/wasmtime` optionally
+/// replaced with the path to your WASI runtime of choice:
+///
+/// ```plain
+/// :wasi:M::\x00asm::/usr/bin/wasmtime:
+/// ```
+pub fn detect_wasi_runability() -> Result<bool, BinstallError> {
+    let progdir = tempdir()?;
+    let prog = progdir.path().join("miniwasi.wasm");
+
+    {
+        let mut progfile = File::create(&prog)?;
+        progfile.write_all(WASI_PROGRAM)?;
+
+        #[cfg(unix)]
+        progfile.set_permissions(Permissions::from_mode(0o777))?;
+    }
+
+    match Command::new(prog).output() {
+        Ok(out) => Ok(out.status.success() && out.stdout.is_empty() && out.stderr.is_empty()),
+        Err(_) => Ok(false),
+    }
+}