[Xen-devel] Re: [PATCH] Fix bootloader handling when empty string is being output

[Paolo Bonzini <pbonzini@xxxxxxxxxx>]

On 08/31/2010 10:47 AM, Ian Campbell wrote:
> On Mon, 2010-08-30 at 14:10 +0100, Michal Novotny wrote:
> > Hi,
> > this is the patch to fix empty string as the output value
> > of the bootloader string. If there is no output then
> > xend is being hung indefinitely unless you press Ctrl + C keys
> > to trigger SIGINT.
> I think a similar fix will be needed to libxl_bootloader.c, right?
Yes.

> Could we potentially avoid the need to use a select timeout to poll for
> bootloader exit by including the bootloader FIFO FD and/or the PTY FDs
> in the select's exceptfds array? Presumably if the process on the other
> end of such an FD exits that causes some sort of exceptional condition
> (although historically I've had trouble finding the actual specified
> behaviour in cases like this).
Using the FIFO is not possible in general because you cannot be sure 
that the bootloader is opening it at all.  In fact, in this case both 
libxl and xend will hang at "fifo_fd = open(fifo, O_RDONLY);" so you 
have to apply O_NDELAY already when you open the FIFO.
Also, in the case the bootloader is not opening the FIFO at all, doing 
the waitpid in the same thread has a race if the process exits between 
the waitpid and select.  Fixing the race requires pselect and, 
especially in Python, introduces more complications than it removes.
So, for RHEL5 xend, Michal has a better version of the patch that will 
run the waitpid in a separate thread, and use a pipe to wake up the 
select.  This does the same thing as /proc, but portably and without the 
need for polling.
However, for both current xend and libxl, using the PTY sounds better, 
and it looks like an even simpler patch will work that:
- uses O_RDONLY|O_NDELAY when opening the FIFO, and

- exits bootloader_interact when bootloader_fd was in the returned rsel 
but ret == 0.
Paolo

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