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qemu-ga: add guest-sync-delimited

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guest-sync leaves it as an exercise to the user as to how to reliably
obtain the response to guest-sync if the client had previously read in a
partial response (due qemu-ga previously being restarted mid-"sentence"
due to reboot, forced restart, etc).

qemu-ga handles this situation on its end by having a client precede
their guest-sync request with a 0xFF byte (invalid UTF-8), which
qemu-ga/QEMU JSON parsers will treat as a flush event. Thus we can
reliably flush the qemu-ga parser state in preparation for receiving
the guest-sync request.

guest-sync-delimited provides the same functionality for a client: when
a guest-sync-delimited is issued, qemu-ga will precede it's response
with a 0xFF byte that the client can use as an indicator to flush its
buffer/parser state in preparation for reliably receiving the
guest-sync-delimited response.

It is also useful as an optimization for clients, since, after issuing a
guest-sync-delimited, clients can safely discard all stale data read
from the channel until the 0xFF is found.

More information available on the wiki:

http://wiki.qemu.org/Features/QAPI/GuestAgent#QEMU_Guest_Agent_Protocol

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
This commit is contained in:
Michael Roth 2012-02-07 13:56:48 -06:00
parent 3424fc9f16
commit 3cf0bed836
5 changed files with 77 additions and 9 deletions
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3
qga/commands-posix.c
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@ -35,8 +35,6 @@
#include "qemu-queue.h"
#include "host-utils.h"
static GAState *ga_state;
static void reopen_fd_to_null(int fd)
{
int nullfd;
@ -909,7 +907,6 @@ error:
/* register init/cleanup routines for stateful command groups */
void ga_command_state_init(GAState *s, GACommandState *cs)
{
ga_state = s;
#if defined(CONFIG_FSFREEZE)
ga_command_state_add(cs, guest_fsfreeze_init, guest_fsfreeze_cleanup);
#endif