qemu-ga: add guest-sync-delimited

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>

3 files changed, 8 insertions, 3 deletions

diff --git a/qga/commands-posix.c b/qga/commands-posix.c
index 5b77fa9..7b2be2f 100644
--- a/qga/commands-posix.c
+++ b/qga/commands-posix.c
@@ -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
diff --git a/qga/commands.c b/qga/commands.c
index b27407d..5bcceaa 100644
--- a/qga/commands.c
+++ b/qga/commands.c
@@ -29,6 +29,12 @@ void slog(const gchar *fmt, ...)
     va_end(ap);
 }
 
+int64_t qmp_guest_sync_delimited(int64_t id, Error **errp)
+{
+    ga_set_response_delimited(ga_state);
+    return id;
+}
+
 int64_t qmp_guest_sync(int64_t id, Error **errp)
 {
     return id;
diff --git a/qga/guest-agent-core.h b/qga/guest-agent-core.h
index b5dfa5b..304525d 100644
--- a/qga/guest-agent-core.h
+++ b/qga/guest-agent-core.h
@@ -18,6 +18,7 @@
 
 typedef struct GAState GAState;
 typedef struct GACommandState GACommandState;
+extern GAState *ga_state;
 
 void ga_command_state_init(GAState *s, GACommandState *cs);
 void ga_command_state_add(GACommandState *cs,
@@ -30,3 +31,4 @@ bool ga_logging_enabled(GAState *s);
 void ga_disable_logging(GAState *s);
 void ga_enable_logging(GAState *s);
 void slog(const gchar *fmt, ...);
+void ga_set_response_delimited(GAState *s);