Re: bug: :if-exists :append

Hello,

Thank you, I understand it.

On Mon, Feb 13, 2017 at 01:51:10PM -0500, Sam Steingold wrote:
> When I click on "Create Ticket" on https://sourceforge.net/p/clisp/bugs/,
> I am redirected to https://sourceforge.net/p/clisp/bugs/new/
> What happens to you?

I guess I don't have account there. I have account only on mailing
list. Not that I am fond of sourceforge especially for GNU projects.

OK, if you happen to know how to get file descriptor by sys:: let me
know. Or how to append to file which is chattr +a

I have tried that with SBCL and it appended to the file, the technical
stuff behind it is well explained from your side, on my side, I can
just observe what happened, cannot observe it. Now I am using shell
echo ~a >> to the file.

Jean

> IOW, to open your `chattr +a` file, we need to pass `O_APPEND` to `open(2)`.
> However, this will break this code:
> 
> --8<---------------cut here---------------start------------->8---
> (with-open-file (s "my-append-only-file" :direction :output :if-exists :append)
>   (let ((pos (file-position s)))
>     (write-string "foo" s)
>     (file-position s (- pos 3))
>     (write-string "bar" s)))
> --8<---------------cut here---------------end--------------->8---
> 
> ANSI CL mandates that this appends 3 chars "bar" to
> "my-append-only-file".
> However, if "my-append-only-file" was opened with `O_APPEND`, it will
> actually append 6 chars "foobar".
> 
> It is not clear to me what to do here:
> 
> * `file-position' can detect that the file is O_APPEND and return nil -
>   this is a functionality loss in many ways - e.g., we cannot scroll
>   back and read an O_RDWR|O_APPEND file.
> 
> * Accept that the code above will not do what the user expects for
>   ___SOME___ streams.
> 
> * Add another `:direction' option -- `:unix-append' which will use
>   `O_APPEND'.
> 
> I think the best course of action if the last one - it will be backwards
> compatible and will not violate too many expectations.
> 
> For now the only course of action is to use FFI or syscall (not sure if
> it can do that) to get a raw file descriptor and then wrap it into a
> Lisp stream using [`make-stream`](http://clisp.org/impnotes/make-stream.html).

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