Gulp sass unable to override already existing file - sass

Question

Gulp sass/gulp watch command is unable to override the already existing/generated .css file.
So when there is no generated css file in the folder, the command works fine. But, if the file exists on the folder I am getting below error.
[12:17:39] Error: EPERM: operation not permitted, open 'C:\My folder\project\components\site-products\demo\app\css\style.css'
var gulp = require('gulp');
var sass = require('gulp-sass');
gulp.task('sass', function () {
return gulp.src('./demo/app/scss/**/*.scss')
.pipe(sass().on('error', sass.logError))
.pipe(gulp.dest('./demo/app/css'));
});
gulp.task('sass:watch', function () {
gulp.watch('./demo/app/scss/**/*.scss', ['sass']);
});
Not sure its relevant but I am not using gulp-compass. I am using node-sass instead.
Thanks in advance for help.

Answer 1

Resolved it finally, it was because webpack was locking files.
Used gulp-chmod to bypass it :)
Here is the final script:
var gulp = require('gulp');
var sass = require('gulp-sass');
var chmod = require('gulp-chmod');
gulp.task('sass', function () {
return gulp.src('./demo/app/scss/**/*.scss')
.pipe(chmod(0o755))
.pipe(sass({outputStyle: 'compressed'}).on('error', sass.logError))
.pipe(gulp.dest('./dist/demo/css'));
});
gulp.task('sass:watch', function () {
gulp.watch('./demo/app/scss/**/*.scss', ['sass']);
});

Answer 2

I got the same and resolved by using gulp cache control.
I don't understand why, but it works!
For you, it would be something like that:
var cache = require('gulp-cached');
gulp.task('cache:docsource', function(){
return gulp.src('./demo/app/**/*.*').pipe(cache('cacheDocSource'));
});
gulp.task('sass', function () {
return gulp.src('./demo/app/scss/**/*.scss')
.pipe(cache('cacheDocSource'))
.pipe(sass({outputStyle: 'compressed'}).on('error', sass.logError))
.pipe(gulp.dest('./dist/demo/css'));
});
gulp.task('sass:watch', ['cache:docsource'], function () {
gulp.watch('./demo/app/scss/**/*.scss', ['sass']);
});

Answer 3

You can use the clean task:
// Path
var paths = {
scss: {
input: 'dev/assets/scss/**/*.{scss,sass}',
output: 'public/assets/css'
}
};
// SCSS
gulp.task('scss', function() {
return sass(paths.scss.input)
.pipe(autoprefixer('last 2 version'))
.pipe(cssnano())
.pipe(gulp.dest(paths.scss.output))
.pipe(browserSync.reload({stream: true}));
});
// Clean
gulp.task('clean', function() {
return del(['public/**/*']);
});
// Default
gulp.task('default', ['clean'], function() {
gulp.start('scss');
});