[Libguestfs] [nbdkit PATCH 04/10] plugins: Wire up in-memory plugin support for NBD_INFO_INIT_STATE

[Eric Blake]

On 2/11/20 4:48 AM, Richard W.M. Jones wrote:



> On Mon, Feb 10, 2020 at 03:43:57PM -0600, Eric Blake wrote:
>> +/* Does current client start with a sparse image. */
>> +static int
>> +memory_init_sparse (void *handle)
>> +{
>> +  ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&lock);
>> +  return sparse_array_is_sparse (sa);
>> +}
>> +
>> +/* Does current client start with all zeroes. */
>> +static int
>> +memory_init_zero (void *handle)
>> +{
>> +  ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&lock);
>> +  return sparse_array_is_zero (sa);
>> +}
> 
> I was going to say these are always true, but then I remembered that
> NBD_INIT_* records the state at the start of the *connection*, not the
> start of the *server* instance, and if there's been a previous
> connection then the RAM disk might indeed contain non-zero non-sparse
> data.

Yes, and in fact the added testsuite case for test-memory-init.sh proves 
this.

> 
> Do we need to define what NBD_INIT_ZERO means when disk size = 0 ?

I think it can be left ambiguous.  The NBD protocol says .pread with 
length 0 is unspecified.  Logically, returning 1 means 'every byte that 
you read is 0', and since you can't read any bytes, returning 1 is not a 
problem (you can't read any bytes to prove the bit wrong).  Requiring a 
return of 0 for that corner case doesn't seem worth it.  (The init bits 
are really more of a 'yes-or-maybe', not a 'yes-or-no'.  It is ALWAYS 
fine to return 0, even if the condition actually holds).

> 
> By the way, the data plugin can also be updated since it also uses the
> sparse array feature.

Yes, and that was done as part of this commit; from the commit message 
that you trimmed:

 > plugins, it is fairly easy to advertise several cases: data and memory
 > are usually sparse and detecting zero is easy (requires new functions
 > to the sparse_array common code), although since the sparse array is
 > reused between consecutive clients, a later client might get different
 > answers than the first client.  The null and full plugins are

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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