Can't append to file on windows without blocking

[ysbaddaden]

While we can read and write to files asynchronously on Windows (by passing blocking: false so FILE_FLAG_OVERLAPPED is set), trying to append to an existing file will actually overwrite the file.

The following example is an extract from spec/std/file_spec.cr that reproduces the problem:

path = "append.txt"

File.write(path, "hello", blocking: false)
p File.read(path, blocking: false)
# => "hello"

File.write(path, " world", mode: "a", blocking: false)
p File.read(path, blocking: false)
# => " world"

[ysbaddaden]

Related to #14706 and #14321.

[ysbaddaden]

File spec failures when I try to make File read and write asynchronously by default on Windows:

Failures:

  1) File write can append to an existing file
     Failure/Error: File.read(path).should eq("hello world")

       Expected: "hello world"
            got: " world"

     # spec\std\file_spec.cr:876

  2) File supports the `b` mode flag
     Failure/Error: bytes.should eq(Bytes[1, 3, 6, 10, 13, 13, 10])

       Expected: Bytes[1, 3, 6, 10, 13, 13, 10]
            got: Bytes[13, 13, 10, 10, 0, 0, 0]

     # spec\std\file_spec.cr:942

  3) File returns the actual position with tell after append
     Failure/Error: file.tell.should eq(10)

       Expected: 10
            got: 5

     # spec\std\file_spec.cr:1021

  4) File does not overwrite existing content in append mode
     Failure/Error: File.read(filename).should eq "0123456789abcd"

       Expected: "0123456789abcd"
            got: "01234abcd9"

     # spec\std\file_spec.cr:1035

[ysbaddaden]

That's easy to fix (set the overlapped offset in the EventLoop::IOCP#write just as in FileDescriptor#write_blocking 👌

I still have the File returns the actual position with tell after append spec failing, though.

[ysbaddaden]

The file pointer reported by SetFilePointerEx only counts the bytes written. What's even weirder is that this is only happening when using async IOCP... both solutions just specify an offset through OVERLAPPED but only the async call fails 🤨