qga: vss-win32: Fix interference with snapshot creation by other VSS requesters

When a VSS requester such as vshadow.exe or diskshadow.exe requests to
create disk snapshots, Windows may choose qemu-ga VSS provider if it is
only provider registered on the system. However, because it provides only a
function to freeze the filesystem, the snapshotting fails.

This patch adds a check into CQGAVssProvider::IsVolumeSupported() to reject
the request from other VSS requesters, so that the other provider is chosen.

The check of requester is done by confirming event channels between
qemu-ga's requester and provider established. To ensure that the events are
initialized when CQGAVssProvider::IsVolumeSupported() is called, it moves
the initialization earlier.

Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
Reviewed-by: Gal Hammer <ghammer@redhat.com>
Reviewed-by: Yan Vugenfirer <yvugenfi@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

1 files changed, 10 insertions, 1 deletions

diff --git a/qga/vss-win32/provider.cpp b/qga/vss-win32/provider.cpp
index c3030d8..b233646 100644
--- a/qga/vss-win32/provider.cpp
+++ b/qga/vss-win32/provider.cpp
@@ -291,8 +291,17 @@ STDMETHODIMP CQGAVssProvider::BeginPrepareSnapshot(
 STDMETHODIMP CQGAVssProvider::IsVolumeSupported(
     VSS_PWSZ pwszVolumeName, BOOL *pbSupportedByThisProvider)
 {
-    *pbSupportedByThisProvider = TRUE;
+    HANDLE hEventFrozen;
+
+    /* Check if a requester is qemu-ga by whether an event is created */
+    hEventFrozen = OpenEvent(EVENT_ALL_ACCESS, FALSE, EVENT_NAME_FROZEN);
+    if (!hEventFrozen) {
+        *pbSupportedByThisProvider = FALSE;
+        return S_OK;
+    }
+    CloseHandle(hEventFrozen);
 
+    *pbSupportedByThisProvider = TRUE;
     return S_OK;
 }