[Libguestfs] [libnbd PATCH v2 2/5] states: Wire in a read callback

[Richard W.M. Jones]

On Tue, Jun 25, 2019 at 09:57:22AM -0500, Eric Blake wrote:
> On 6/25/19 4:13 AM, Richard W.M. Jones wrote:
> > On Thu, Jun 20, 2019 at 08:45:05PM -0500, Eric Blake wrote:
> >> If any callback fails, and if no prior error was set, then the
> >> callback's failure becomes the failure reason for the overall
> >> read. (Note that this is different from the block status callback,
> >> which for now quits using the callback on subsequent chunks if an
> >> earlier chunk failed - we may decide to change one or the other for
> >> consistency before the API is stable.)
> >>
> >> Nothing actually passes a callback function yet, so for now this is no
> >> functional change; but this will make it possible for the next patch
> >> to add an 'nbd_aio_pread_structred' API.
> > 
> > I don't think non-C callbacks will be able to reliably set errno, but
> > that's something we can solve in future.  Therefore ACK.
> 
> Perhaps we could change the API to:
> 
> cmd->cb.fn.read (cmd->cb.opaque, cmd->data, cmd->count, cmd->offset,
>                  &error, LIBNBD_READ_XXX)
> 
> where the caller is not only passed the current error, but also can
> store into that variable to affect the return error (rather than the
> return error being implied from whatever is left in errno).
>
> Of course, that necessitates passing a pointer to an integer to the
> callback, rather than a bare integer, so it requires a bit more surgery
> to the generator.  And it may still not be the best interface for other
> language bindings - for example, I'm not sure how python would represent
> an in/out pointer to integer.  In nbdkit, the solution was that we had a
> utility function nbdkit_set_error() which was more easily bound to other
> languages; the various plugins then declare whether they can reliably
> set errno, or whether errno is unreliable and nbdkit_set_error() should
> be the sole source of requested error status instead.  So the similar
> interface here is that callbacks are always passed just a bare integer
> on input, and then call a utility function to request a specific error
> on output (where we then have a way to decide whether to use the value
> in errno, or just a hard-coded default, if the utility function was not
> called).

In OCaml this would map to a reference (‘int ref’ specifically).  In
Python I think we can use ‘ctypes.c_int’ which is a mutable int (the
normal Python int is not mutable).

I'll see if I can make a patch to the generator to add this feature.

Rich.

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