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>

1 files changed, 22 insertions, 5 deletions

diff --git a/qemu-ga.c b/qemu-ga.c
index 1c90e6ef0d..d6f786e50d 100644
--- a/qemu-ga.c
+++ b/qemu-ga.c
@@ -41,6 +41,7 @@
 #define QGA_VIRTIO_PATH_DEFAULT "\\\\.\\Global\\org.qemu.guest_agent.0"
 #endif
 #define QGA_PIDFILE_DEFAULT "/var/run/qemu-ga.pid"
+#define QGA_SENTINEL_BYTE 0xFF
 
 struct GAState {
     JSONMessageParser parser;
@@ -54,9 +55,10 @@ struct GAState {
 #ifdef _WIN32
     GAService service;
 #endif
+    bool delimit_response;
 };
 
-static struct GAState *ga_state;
+struct GAState *ga_state;
 
 #ifdef _WIN32
 DWORD WINAPI service_ctrl_handler(DWORD ctrl, DWORD type, LPVOID data,
@@ -198,6 +200,11 @@ static void ga_log(const gchar *domain, GLogLevelFlags level,
     }
 }
 
+void ga_set_response_delimited(GAState *s)
+{
+    s->delimit_response = true;
+}
+
 #ifndef _WIN32
 static void become_daemon(const char *pidfile)
 {
@@ -254,7 +261,7 @@ fail:
 static int send_response(GAState *s, QObject *payload)
 {
     const char *buf;
-    QString *payload_qstr;
+    QString *payload_qstr, *response_qstr;
     GIOStatus status;
 
     g_assert(payload && s->channel);
@@ -264,10 +271,20 @@ static int send_response(GAState *s, QObject *payload)
         return -EINVAL;
     }
 
-    qstring_append_chr(payload_qstr, '\n');
-    buf = qstring_get_str(payload_qstr);
+    if (s->delimit_response) {
+        s->delimit_response = false;
+        response_qstr = qstring_new();
+        qstring_append_chr(response_qstr, QGA_SENTINEL_BYTE);
+        qstring_append(response_qstr, qstring_get_str(payload_qstr));
+        QDECREF(payload_qstr);
+    } else {
+        response_qstr = payload_qstr;
+    }
+
+    qstring_append_chr(response_qstr, '\n');
+    buf = qstring_get_str(response_qstr);
     status = ga_channel_write_all(s->channel, buf, strlen(buf));
-    QDECREF(payload_qstr);
+    QDECREF(response_qstr);
     if (status != G_IO_STATUS_NORMAL) {
         return -EIO;
     }