fix(storages): keep volumes a VM still needs

libvirt deletes any volume it is asked to. The storage page and the API
deleted a VM's disk, the base image of another volume or the file a
snapshot was taken on without a check, breaking the VM or its
snapshots.

wvmStorage.del_volume now refuses a volume a domain on the compute uses
as a disk or a backing file of one, a file of a domain's snapshots, or
the backing file of another volume. The check is the one destroy
already used, moved to wvmConnect and extended to snapshot files, so
destroy also keeps the files of another VM's snapshots. The storage
page shows the refusal; the API answers 400 with it, as for other
refusals. Deleting a disk from a VM refuses after the detach when the
VM's own snapshots need the volume, and says the volume was kept.

Network disks count by their source name, pool volume disks by their
volume's path. A volume whose metadata cannot be read stops the check
instead of counting as unused; only one removed meanwhile is skipped.
The check and the delete hold the compute lock, so no VM operation
attaches the volume in between; a busy compute is a message on the page
and 409 in the API.

vrtManager/connection.py

@@ -11,6 +11,7 @@ import libvirt
 from django.conf import settings
 from django.utils.functional import cached_property
 from libvirt import libvirtError
+from lxml import etree
 from vrtManager import util
 from vrtManager.rwlock import ReadWriteLock
 
@@ -946,6 +947,59 @@ class wvmConnect(object):
             instances.append(name)
         return instances
 
+    def paths_in_use(self, skip_uuid=None, own=()):
+        """Paths (names for network disks) a domain uses as a disk or a
+        backing file of one, or as a file of its snapshots (overlays, memory,
+        the disks they were taken on); and the backing files of volumes.
+        skip_uuid leaves out one domain, own the volumes whose backing files
+        do not count."""
+        used = set()
+        for dom in self.wvm.listAllDomains():
+            if dom.UUIDString() == skip_uuid:
+                continue
+            for flags in (0, libvirt.VIR_DOMAIN_XML_INACTIVE):
+                used.update(self._source_paths(etree.fromstring(dom.XMLDesc(flags))))
+            for snap in dom.listAllSnapshots(0):
+                tree = etree.fromstring(snap.getXMLDesc(0))
+                used.update(tree.xpath("disks/disk/source/@file|memory/@file"))
+                for saved in tree.xpath("domain|inactiveDomain"):
+                    used.update(self._source_paths(saved))
+        # A stopped domain's XML has no backing chain; the volumes have it.
+        # Any volume backed by a path keeps that path, even an unattached one.
+        for pool in self.wvm.listAllStoragePools():
+            if not pool.isActive():
+                continue
+            with contextlib.suppress(libvirtError):
+                pool.refresh(0)
+            for vol in pool.listAllVolumes():
+                # only a volume removed meanwhile is skipped: one that cannot
+                # be read may still keep a backing file
+                try:
+                    if vol.path() not in own:
+                        used.update(etree.fromstring(vol.XMLDesc(0)).xpath("./backingStore/path/text()"))
+                except libvirtError as err:
+                    if err.get_error_code() != libvirt.VIR_ERR_NO_STORAGE_VOL:
+                        raise
+        return used
+
+    def _source_paths(self, domain):
+        """The files, devices or network names of a domain definition's
+        disks and of their backing chains ("//" reaches a running domain's
+        backingStore); a pool volume source as its volume's path."""
+        paths = []
+        for source in domain.xpath("devices/disk//source"):
+            if source.get("pool") and source.get("volume"):
+                try:
+                    pool = self.wvm.storagePoolLookupByName(source.get("pool"))
+                    paths.append(pool.storageVolLookupByName(source.get("volume")).path())
+                except libvirtError as err:
+                    # a volume that is not there cannot be the one deleted
+                    if err.get_error_code() not in (libvirt.VIR_ERR_NO_STORAGE_VOL, libvirt.VIR_ERR_NO_STORAGE_POOL):
+                        raise
+            else:
+                paths += [v for v in (source.get("file"), source.get("dev"), source.get("name")) if v]
+        return paths
+
     def get_snapshots(self):
         instance = []
         for snap_id in self.wvm.listDomainsID():