replay: avoid recursive call of checkpoints

This patch adds a flag which denies recursive call of replay_checkpoint
function. Checkpoints may be accompanied by the hardware events. When event
is processed, virtual device may invoke timer modification functions that
also invoke the checkpoint function. This leads to infinite loop.

Signed-off-by: Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>
Message-Id: <20180227095305.1060.56463.stgit@pasha-VirtualBox>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>

replay/replay.c

@@ -176,13 +176,24 @@ void replay_shutdown_request(ShutdownCause cause)
 bool replay_checkpoint(ReplayCheckpoint checkpoint)
 {
     bool res = false;
+    static bool in_checkpoint;
     assert(EVENT_CHECKPOINT + checkpoint <= EVENT_CHECKPOINT_LAST);
-    replay_save_instructions();
 
     if (!replay_file) {
         return true;
     }
 
+    if (in_checkpoint) {
+        /* If we are already in checkpoint, then there is no need
+           for additional synchronization.
+           Recursion occurs when HW event modifies timers.
+           Timer modification may invoke the checkpoint and
+           proceed to recursion. */
+        return true;
+    }
+    in_checkpoint = true;
+
+    replay_save_instructions();
 
     if (replay_mode == REPLAY_MODE_PLAY) {
         g_assert(replay_mutex_locked());
@@ -204,6 +215,7 @@ bool replay_checkpoint(ReplayCheckpoint checkpoint)
         res = true;
     }
 out:
+    in_checkpoint = false;
     return res;
 }