Re: [PATCH] qemu: qemu_fopen_fd: differentiate between reader and writer user

[Anthony Liguori <anthony@codemonkey.ws>]

Uri Lublin wrote:
>>
>> Anthony, this is a problem with qemu-upstream so I'd like to solve it 
>> in a way that's acceptable for upstream.
>>
>> The proposed patch is less that ideal IMO as it introduces 
>> limitations on what you can do with a file.  An alternative 
>> implementation would add a read/write mode to the buffer, based on 
>> the last access type.  When switching from read to write, we drop the 
>> buffer, and when switching from write to read, we flush it and then 
>> drop it.  This is more complex but results in a cleaner API.
>>
>
> I am not sure we are allowed to drop the buffer a on read-to-write 
> switch, especially if there are unread bytes in it. We may need to 
> pause the write that causes the switch, similar to pausing the read in 
> order to flush the buffer on a write-to-read switch.

Yeah, I tried to implement the read/write mode buffer and it ends up not 
to work very nicely.  I don't remember the precise issue, but I hit some 
sort of wall that I thought was a fundamental limitation.  I ended up 
forcing the QEMUFile to be in either read or write mode similar to your 
original patch without introducing a new option as an argument.

Regards,

Anthony Liguori