[PATCH] Initialize IO_STATUS_BLOCK for pread, pwrite

[Mark Geisert]

On Wed, 29 Nov 2017, Corinna Vinschen wrote:
> On Nov 29 13:36, Corinna Vinschen wrote:
>> On Nov 29 13:04, Corinna Vinschen wrote:
>>> - If you do async IO, you have to handle STATUS_PENDING gracefully:
>>>
>>>   - The IO_STATUS_BLOCK given to NtWriteFile *must* exist for the
>>>     entire time the operation takes after NtWriteFile returned
>>>     STATUS_PENDING.  An IO_STATUS_BLOCK fhandler member comes to mind,
>>>     maybe fhandler_base_overlapped::io_status can be reused.
>>
>> No, wait.  There can be more than one outstanding aio operations on the
>> same descriptor.  Therefore the IO_STATUS_BLOCK must be connected to the
>> aiocb struct one way or the other, becasue only that struct is local to
>> the aio operation.
>
> I guess that's what the Linux man page aio(7) subsumes under
>
>  struct aiocb {
>    [...]
>
>    /* Various implementation-internal fields not shown */
>  };

Yes, I believe that's correct.  But in my aio implementation for Cygwin, 
I'm not using overlapped I/O or any kind of async or nonblocking write. 
I'm using separate threads to do plain vanilla blocking writes (via pwrite 
if able).  I'm doing this because I'm operating with user-supplied file 
descriptors that might or might not be underlain with async-capable 
Windows handles.

(It's my understanding that if one wants to do overlapped I/O on a Windows 
handle, one has to request that explicitly when creating the handle.  I 
don't think Cygwin does this by default.  Corrections welcome.)

So in this environment seeing pwrite() return with a short write count, 
even though it's understandable that the underlying Windows write might 
still be underway, is a real problem.  Because, if I use the short count 
to determine that I need to write the remainder of a buffer, it'll have 
been written twice: once by the late-finishing pwrite and again by the
recovery code.

A blocking pwrite() (i.e., not overlapped or async in any way) has to wait 
for the underlying NtWriteFile() to complete in order to get a correct 
write count and/or correct final status code, doesn't it?

..mark

P.S. I'll look into IRC clients.  You've suggested it before and I just 
recall the wild IRC days in the 90s with egg drops and bots and bans and 
it seemed like a time sink I couldn't afford.  Maybe #cygwin-developers
on freenode is more civilized :-)).