vircgroup: add support for hybrid configuration

This enables to use both cgroup v1 and v2 at the same time together
with libvirt.  It is supported by kernel and there is valid use-case,
not all controllers are implemented in cgroup v2 so there might be
configurations where administrator would enable these missing
controllers in cgroup v1.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

src/util/vircgroupbackend.c

@@ -20,6 +20,9 @@
 #include <config.h>
 
 #include "vircgroupbackend.h"
+#define __VIR_CGROUP_ALLOW_INCLUDE_PRIV_H__
+#include "vircgrouppriv.h"
+#undef __VIR_CGROUP_ALLOW_INCLUDE_PRIV_H__
 #include "vircgroupv1.h"
 #include "vircgroupv2.h"
 #include "virerror.h"
@@ -67,3 +70,20 @@ virCgroupBackendGetAll(void)
     }
     return virCgroupBackends;
 }
+
+
+virCgroupBackendPtr
+virCgroupBackendForController(virCgroupPtr group,
+                              unsigned int controller)
+{
+    size_t i;
+
+    for (i = 0; i < VIR_CGROUP_BACKEND_TYPE_LAST; i++) {
+        if (group->backends[i] &&
+            group->backends[i]->hasController(group, controller)) {
+            return group->backends[i];
+        }
+    }
+
+    return NULL;
+}

src/util/vircgroupbackend.h

@@ -436,12 +436,22 @@ virCgroupBackendRegister(virCgroupBackendPtr backend);
 virCgroupBackendPtr *
 virCgroupBackendGetAll(void);
 
-# define VIR_CGROUP_BACKEND_CALL(group, func, ret, ...) \
-    if (!group->backend->func) { \
+virCgroupBackendPtr
+virCgroupBackendForController(virCgroupPtr group,
+                              unsigned int controller);
+
+# define VIR_CGROUP_BACKEND_CALL(group, controller, func, ret, ...) \
+    virCgroupBackendPtr backend = virCgroupBackendForController(group, controller); \
+    if (!backend) { \
+        virReportError(VIR_ERR_INTERNAL_ERROR, \
+                       _("failed to get cgroup backend for '%s'"), #func); \
+        return ret; \
+    } \
+    if (!backend->func) { \
         virReportError(VIR_ERR_OPERATION_UNSUPPORTED, \
                        _("operation '%s' not supported"), #func); \
         return ret; \
     } \
-    return group->backend->func(group, ##__VA_ARGS__);
+    return backend->func(group, ##__VA_ARGS__);
 
 #endif /* __VIR_CGROUP_BACKEND_H__ */

src/util/vircgrouppriv.h

@@ -56,7 +56,7 @@ typedef virCgroupV2Controller *virCgroupV2ControllerPtr;
 struct _virCgroup {
     char *path;
 
-    virCgroupBackendPtr backend;
+    virCgroupBackendPtr backends[VIR_CGROUP_BACKEND_TYPE_LAST];
 
     virCgroupV1Controller legacy[VIR_CGROUP_CONTROLLER_LAST];
     virCgroupV2Controller unified;