Add Engine::detect_precompiled_file() (bytecodealliance#6937)

This is handy for users of `deserialize_file()` to determine which version to call.

crates/wasmtime/src/engine.rs

@@ -647,7 +647,12 @@ impl Engine {
     /// this will return `Some(...)` indicating so. Otherwise `None` is
     /// returned.
     pub fn detect_precompiled(&self, bytes: &[u8]) -> Option<Precompiled> {
-        serialization::detect_precompiled(bytes)
+        serialization::detect_precompiled_bytes(bytes)
+    }
+
+    /// Like [`Engine::detect_precompiled`], but performs the detection on a file.
+    pub fn detect_precompiled_file(&self, path: impl AsRef<Path>) -> Result<Option<Precompiled>> {
+        serialization::detect_precompiled_file(path)
     }
 }
 

crates/wasmtime/src/engine/serialization.rs

@@ -145,8 +145,9 @@ pub fn check_compatible(engine: &Engine, mmap: &MmapVec, expected: ObjectKind) -
     bincode::deserialize::<Metadata>(data)?.check_compatible(engine)
 }
 
-pub fn detect_precompiled(bytes: &[u8]) -> Option<Precompiled> {
-    let obj = File::parse(bytes).ok()?;
+fn detect_precompiled<'data, R: object::ReadRef<'data>>(
+    obj: File<'data, R>,
+) -> Option<Precompiled> {
     match obj.flags() {
         FileFlags::Elf {
             os_abi: obj::ELFOSABI_WASMTIME,
@@ -162,6 +163,16 @@ pub fn detect_precompiled(bytes: &[u8]) -> Option<Precompiled> {
     }
 }
 
+pub fn detect_precompiled_bytes(bytes: &[u8]) -> Option<Precompiled> {
+    detect_precompiled(File::parse(bytes).ok()?)
+}
+
+pub fn detect_precompiled_file(path: impl AsRef<std::path::Path>) -> Result<Option<Precompiled>> {
+    let read_cache = object::ReadCache::new(std::fs::File::open(path)?);
+    let obj = File::parse(&read_cache)?;
+    Ok(detect_precompiled(obj))
+}
+
 #[derive(Serialize, Deserialize)]
 struct Metadata {
     target: String,