Errno::EACCES upon deleting a file (on Windows 10) - ruby-on-rails

Question

There are a number of similar questions, but none of the answers worked for me.
I'm running Rails 6.0.3.2, Ruby 2.6.6, and SQLite3 on Windows 10 version 2004 (19041.388). I followed the Getting Started guide on the official Rails site to install Ruby on Rails and and everything should be up to date.
I can delete the files normally, and I'm logged in with an administrator account — not that it should be necessary.
I'm new to Ruby and Rails, so detailed answers would be appreciated.
Code
Here's what causes the error:
def destroy
book = Book.find(params[:id])
begin
File.open(book.cover_url, 'w') do |f|
File.delete(f)
end
rescue Errno::ENOENT
end
book.destroy
redirect_to books_path
end
What this does it first delete the cover image for a book and then delete the book itself from the database.
Errors
The error screen:
If the picture doesn't load, here are the error messages:
Errno::EACCES in BooksController#destroy
Permission denied # apply2files - D:/Projects/Web/RoR/ecommerce/app/assets/images/covers/circles_scaling_anim_positioning.png
File.delete(f) is the culprit.
Attempted Solutions
The only actionable answer I could find for Windows was this,
which advocated adding a 'lib' gem, but it did not work at all.
I also tried changing file mode from 'w' to 'wb+', but that didn't
work either.
EDIT 2: As per Dave Newton's suggestion in the comments (if that's what he meant), I moved the image storage directory outside the 'app' folder; to 'public/uploads/covers'. It hasn't worked either.
EDIT 3: I copied the delete code to a new script in another directory entirely and tried it on a sample file. I got the same error. In other words, the problem is not with Rails but Ruby (or my OS).
I called rm on the file from the terminal and that worked just fine, so I don't know if it's a file permissions problem.
EDIT: I checked the file in question and though it still remains, it's 0 bytes large now, so I presume it was overwritten by empty data. However, the rest of the code that should've execuded on destroy — i.e. the destruction of the object in the databse — seems not to have run, because the object is still in there.

Answer 1

The file handling enforced by the operating system is different on Windows compared to most UNIXy operating systems (such as macOS, Linux, or the various BSDs).
On the UNIXy operating systems, the file entry in the filesystem is just a pointer to the "real" file stored on disk. There are other possible pointers, such as a file handle when the file is opened by a process, or a hardlink (i.e. a different filesystem entry pointing to the the exact same file). The file exists on disk as long as there is at least one valid pointer to that file.
As such, on UNIXy systems, you can delete (or rename / move) a file from the filesystem while it is opened from a process. The file itself will only actually be deleted once the last filehandle is closed.
Windows however is more strict in this regard by default. It will not allow to delete a file as long as there is any process which has a file handle on the file (i.e. if any process has opened the file). This explains why you can't delete the file while you have a filehandle on it
With that being said, since Ruby 2.3.0 there is a flag you can set when opening the file which instructs Windows to allow deleting (or moving) the file while it is opened:
# the flags for the normal 'w' mode
file_mode = IO::WRONLY | IO::CREAT | IO::TRUNC
# Add flags to allow deleting (or moving) the opened file
file_mode |= IO::BINARY | IO::SHARE_DELETE
File.open(book.cover_url, mode: file_mode) do |f|
File.delete(f)
end
Note that the IO::SHARE_DELETE flag is only taken into account for files opened in binary mode (rather than text mode) take this into account when working with this opened file. On UNIXy systems the IO::SHARE_DELETE is ignored.
There is some documentation of the various flags available.
As a final remark: I assume that your code is a shortened example where you have left out some code actually interacting with the opened file in addition to deleting it.
If all you want is to delete the file, there is no need to opened it first. Just delete it with File.delete(book.cover_url). Alternatively, if you you don't care for any errors (such as if the file doesn't exist in the first place, you can also use FileUtils.rm_f(book.cover_url).

Answer 2

Changed the code to this...
begin
File.delete(book.cover_url)
rescue Errno::ENOENT
end
...and it works now. No opening any files; just File.delete(URL).
If anyone knows why the original version didn't work, or why it's so commonly suggested, please post an answer or comment on this answer.