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

[Anthony Liguori <anthony@codemonkey.ws>]

Avi Kivity wrote:
> Uri Lublin wrote:
>> Currently qemu_fopen_ops accepts both get_buffer and put_buffer, but
>> if both are given (non NULL) we encounter problems:
>>   1. There is only one buffer and index, which may mean data corruption.
>>   2. qemu_flush (which is also called by qemu_fclose) is writing 
>> ("flushing")
>>      some of the data that was read (for the reader part).
>>
>> Currently qemu_fopen_fd registers both get_buffer and put_buffer 
>> functions.
>>
>> This breaks migration for tcp and ssh migration protocols.
>>
>> The following patch fix the above by:
>>   1. It makes sure that at most one of get_buffer and put_buffer is
>>      given to qemu_fopen_ops.
>>   2. It changes qemu_fopen_fd to register only get_buffer for a reader
>>      and only put_buffer for a writer (adding a 'reader' parameter).
>>   3. The incoming fd migration code calls qemu_fopen_fd as a reader 
>> only.
>>
>>   
>
> 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 would think a better solution would introduce two buffers, one for 
read and one for write.  That way, you can have a proper bidirectional 
stream.

Regards,

Anthony Liguori