http.writeFile API fails with non-ascii characters

[lancetarn]

Describe the bug
Using writeFile API with unicode payload results in failed to match a command error.

To Reproduce
Implement a minimal function:

import {
  Dir, writeFile,
} from 'tauri/api/fs';

function pleaseWrite() {
  writeFile({ path: 'unicodeTest.txt', contents: 'å' }, { dir: Dir.Cache });
}

Receive the subsequent error in WebView console:

failed to match a command for {"callback":"499c79fd-8dde-4720-e708-d94956083059","error":"b4aa06f4-a06f-45b1-66d5-b69bdc463d43","cmd":"writeFile","path":"unicodeTest.txt","contents":"å","options":{"dir":2}}], trailing characters at line 1 column 178

Expected behavior
The file contents are written without error.

Platform and Versions (please complete the following information):

OS: macOS 10.14.6
Node: 12.18.0
NPM: 6.14.4
Yarn: -
Rustc: 1.45.0

taurijs: 0.11.0 (also appears in 0.10.0)
tauri.rs: 0.9.0 (also appears in 0.8.0)

Additional context
I realize that I have only been trying this in dev.

[kevinkassimo]

Was about to file the exact same bug, but I think I have figured out the reason for this:

tauri/tauri/src/app/runner.rs

Lines 305 to 306 in cc67680

	// transform `[payload]` to `payload`
	let arg = arg.chars().skip(1).take(arg.len() - 2).collect::<String>();

Likely the use of .chars() breaks the behavior: per https://doc.rust-lang.org/std/string/struct.String.html#method.chars , the behavior might not be what we want here: .chars() iterates over Unicode Scalar Value instead (while arg.len() returns the byte length). Notice the example above

..."options":{"dir":2}}] // final ']' failed to be removed. This should have been removed

(I cannot try working on a fix myself due to some legal personal reasons atm, but hopefully this observation can help)

Update: I made a small mistake before, but it is still Unicode issue. Tested and seems replacing arg.len() with arg.chars().count() should work.