perf(fs): improve FileHandle.read performance

.changes/fs-perf.md

@@ -0,0 +1,6 @@
+---
+"fs": patch
+"fs-js": patch
+---
+
+Improve performance of the `FileHandle.read` and `writeTextFile` APIs.

plugins/fs/guest-js/index.ts

@@ -243,6 +243,25 @@
   }
 }
 
+// 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 @@
       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
   }
@@ -1041,10 +1068,13 @@
     throw new TypeError('Must be a file URL.')
   }
 
-  await invoke('plugin:fs|write_text_file', {
-    path: path instanceof URL ? path.toString() : path,
-    data,
-    options
+  const encoder = new TextEncoder()
+
+  await invoke('plugin:fs|write_text_file', encoder.encode(data), {
+    headers: {
+      path: path instanceof URL ? path.toString() : path,
+      options: JSON.stringify(options)
+    }
   })
 }