perf(fs): improve `FileHandle.read` performance

package.json

@@ -31,5 +31,6 @@
   },
   "engines": {
     "pnpm": "^9.0.0"
-  }
+  },
+  "packageManager": "pnpm@9.12.1+sha512.e5a7e52a4183a02d5931057f7a0dbff9d5e9ce3161e33fa68ae392125b79282a8a8a470a51dfc8a0ed86221442eb2fb57019b0990ed24fab519bf0e1bc5ccfc4"
 }

plugins/fs/guest-js/index.ts

@@ -243,6 +243,25 @@ function parseFileInfo(r: UnparsedFileInfo): FileInfo {
   }
 }
 
+// https://mstn.github.io/2018/06/08/fixed-size-arrays-in-typescript/
+type FixedSizeArray<T, N extends number> = ReadonlyArray<T> & {
+  length: N
+}
+
+// https://gist.github.com/zapthedingbat/38ebfbedd98396624e5b5f2ff462611d
+/** Converts a big-endian eight byte array to number  */
+function fromBytes(buffer: FixedSizeArray<number, 8>): number {
+  const bytes = new Uint8ClampedArray(buffer)
+  const size = bytes.byteLength
+  let x = 0
+  for (let i = 0; i < size; i++) {
+    const byte = bytes[i]
+    x *= 0x100
+    x += byte
+  }
+  return x
+}
+
 /**
  *  The Tauri abstraction for reading and writing files.
  *
@@ -285,12 +304,20 @@ class FileHandle extends Resource {
       return 0
     }
 
-    const [data, nread] = await invoke<[number[], number]>('plugin:fs|read', {
+    const data = await invoke<ArrayBuffer | number[]>('plugin:fs|read', {
       rid: this.rid,
       len: buffer.byteLength
     })
 
-    buffer.set(data)
+    // Rust side will never return an empty array for this command and
+    // ensure there is at least 8 elements there.
+    //
+    // This is an optimization to include the number of read bytes (as bigendian bytes)
+    // at the end of returned array to avoid serialization overhead of separate values.
+    const nread = fromBytes(data.slice(-8) as FixedSizeArray<number, 8>)
+
+    const bytes = data instanceof ArrayBuffer ? new Uint8Array(data) : data
+    buffer.set(bytes.slice(0, bytes.length - 8))
 
     return nread === 0 ? null : nread
   }

plugins/fs/src/commands.rs

@@ -301,13 +301,20 @@ pub async fn read_dir<R: Runtime>(
 pub async fn read<R: Runtime>(
     webview: Webview<R>,
     rid: ResourceId,
-    len: u32,
-) -> CommandResult<(Vec<u8>, usize)> {
-    let mut data = vec![0; len as usize];
+    len: usize,
+) -> CommandResult<tauri::ipc::Response> {
+    let mut data = vec![0; len];
     let file = webview.resources_table().get::<StdFileResource>(rid)?;
     let nread = StdFileResource::with_lock(&file, |mut file| file.read(&mut data))
         .map_err(|e| format!("faied to read bytes from file with error: {e}"))?;
-    Ok((data, nread))
+
+    // This is an optimization to include the number of read bytes (as bigendian bytes)
+    // at the end of returned vector so we can use `tauri::ipc::Response`
+    // and avoid serialization overhead of separate values.
+    let nread = nread.to_be_bytes();
+    data.extend(nread);
+
+    Ok(tauri::ipc::Response::new(data))
 }
 
 #[tauri::command]