Add a null undo function to the vmbackup null provider.

If a snapshot operation times out, vmbackup can attempt
to undo quiescing.  Since no quiescing is done for the null
backup provider, no undo function was provided.  If vmbackup
attempts to call the undo function, it dereferences a garbage
pointer resulting in a segfault.

Rather than add null backup provider specific checks to vmbackup,
this change adds a null undo function to provide vmbackup with a
valid function pointer it can call.  The new undo function updates
the vmbackup state machine state with a new currentOpName, but
has no other effect.  currentOpName is set to the calling
function name, e.g. __FUNCTION__.

open-vm-tools/services/plugins/vmbackup/nullProvider.c

@@ -1,5 +1,5 @@
 /*********************************************************
- * Copyright (C) 2010-2016 VMware, Inc. All rights reserved.
+ * Copyright (C) 2010-2016, 2022 VMware, Inc. All rights reserved.
  *
  * This program is free software; you can redistribute it and/or modify it
  * under the terms of the GNU Lesser General Public License as published
@@ -243,6 +243,32 @@ VmBackupNullSnapshotDone(VmBackupState *state,
    return TRUE;
 }
 
+
+/*
+ ******************************************************************************
+ * VmBackupNullUndo --                                                  */ /**
+ *
+ * Update the state machine state with the currentOpName.
+ *
+ * Can be called when snapshot times out.  See PR2993571 and PR3003917.
+ *
+ * @param[in] state        the backup state
+ * @param[in] clientData   client data
+ *
+ * @return TRUE
+ *
+ ******************************************************************************
+ */
+
+static Bool
+VmBackupNullUndo(VmBackupState *state,
+                 void *clientData)
+{
+   g_debug("*** %s\n", __FUNCTION__);
+   VmBackup_SetCurrentOp(state, NULL, NULL, __FUNCTION__);
+   return TRUE;
+}
+
 #endif
 
 /*
@@ -281,6 +307,9 @@ VmBackup_NewNullProvider(void)
 
    provider = g_new(VmBackupSyncProvider, 1);
    provider->start = VmBackupNullStart;
+#if !defined(_WIN32)
+   provider->undo = VmBackupNullUndo;
+#endif
    provider->snapshotDone = VmBackupNullSnapshotDone;
    provider->release = VmBackupNullRelease;
    provider->clientData = NULL;