short read/write and error code

[David Xu]

On 2012/8/2 18:02, Konstantin Belousov wrote:
> diff --git a/sys/kern/sys_pipe.c b/sys/kern/sys_pipe.c
> index 338256c..1a61b93 100644
> --- a/sys/kern/sys_pipe.c
> +++ b/sys/kern/sys_pipe.c
> @@ -1286,13 +1286,13 @@ pipe_write(fp, uio, active_cred, flags, td)
>   	}
>   
>   	/*
> -	 * Don't return EPIPE if I/O was successful
> +	 * Don't return EPIPE if any byte was written.
> +	 * EINTR and other interrupts are handled by generic I/O layer.
> +	 * Do not pretend that I/O succeeded for obvious user error
> +	 * like EFAULT.
>   	 */
> -	if ((wpipe->pipe_buffer.cnt == 0) &&
> -	    (uio->uio_resid == 0) &&
> -	    (error == EPIPE)) {
> +	if (uio->uio_resid != orig_resid && error == EPIPE)
>   		error = 0;
> -	}
>   
>   	if (error == 0)
>   		vfs_timestamp(&wpipe->pipe_mtime);

I dislike the patch, if I thought it is right, I would have already 
submit such
a patch. Now let us see why your patch is wore than my version (attached):
-current:
     short write is done, some bytes were sent
     dofilewrite returns -1, errno is EPIPE
     dofilewrite sends SIGPIPE
     application is killed by SIGPIPE
-my attached version:
    short write is done, some bytes were sent
    dofilewrite return number of bytes sent, errno is zero
    dofilewrite sends SIGPIPE.
   application is killed by SIGPIPE
-you version:
   short write is done, some bytes were sent.
   dofilewrite returns number of bytes sent,  errno is zero.
   dofilewrite does not send SIGPIPE signal
   application is not killed
   application does not check return value from write()
   application thinks it is successful, application does other things,
   application might begin a bank account transaction.
   ...
   application never comes back...

my patch is more compatible with -current. if application does not
setup a signal handler for SIGPIPE, when short write happens, it is killed,
it is same as -current. if the application set up a signal handler for 
the signal,
it always should check the return value from write(), this is how 
traditional
code works.

in your patch, short write does not kill application,  you can not assume
that the application will request a next write() on the pipe, and you hope
the second write to kill the application, but there is always exception,
the next write does not happen,  application works on other things.
This is too incompatible with -current.