Incorrect `compiler_err` field contents from `compile --format json` when compiling `.cpp` file

[NiklasKK]

Describe the problem

The compiler_err field of the machine readable output formats are incorrect when compiling a .cpp file.

I use Arduino CLI for compiling a combination of standard .cpp/.c/.h files. Because an .ino file with the same name as the sketch is required I just put a blank .ino file inside the folder.

To reproduce

Structure of the test_sketch sketch:

test_sketch/
  ├── test_sketch.ino (empty file)
  ├── main.cpp (source code)

Content of main.cpp:

int main(){
    int i;
    while(1){
        ++i // < Missing semicolon to show wrong behaviour
    }
    return 0;
}

Current behavior

C:\Users\Niklas\Desktop\arduino_cli>arduino-cli compile test_sketch -b arduino:avr:uno --format json
{
  "compiler_out": "",
  "compiler_err": "\n",
  "builder_result": {
    "build_path": "C:\\Users\\Niklas\\AppData\\Local\\Temp\\arduino-sketch-305F5EF96041948188E749EEEDCF1C2F",
    "board_platform": {
      "id": "arduino:avr",
      "version": "1.8.4"
    },
    "build_platform": {
      "id": "arduino:avr",
      "version": "1.8.4"
    }
  },
  "success": false
}
Error during build: exit status 1

🐛 The value of the compiler_err field is \n instead of the error output from the compiler.

Expected behavior

The compiler_err field of the machine readable output contains the error output from the compiler.

Arduino CLI version

Original report

0.21.1

Last verified with

b3e8f8a

Operating system

Windows

Operating system version

11

Additional context

When running the command without the --format json flag the output is correct.

C:\Users\Niklas\Desktop\arduino_cli>arduino-cli compile test_sketch -b arduino:avr:uno
C:\Users\Niklas\Desktop\arduino_cli\test_sketch\main.cpp: In function 'int main()':
C:\Users\Niklas\Desktop\arduino_cli\test_sketch\main.cpp:5:5: error: expected ';' before '}' token
     }
     ^


Error during build: exit status 1

So I suspect it must have something to to with the conversion to JSON.

Issue checklist

• I searched for previous reports in the issue tracker
• I verified the problem still occurs when using the nightly build
• My report contains all necessary details