livemerge: prune bitmaps before measuring

Prune stale bitmaps before a live merge to avoid
failing with ENOSPC.

Fixes: oVirt#352
Signed-off-by: Albert Esteve <aesteve@redhat.com>

lib/vdsm/virt/livemerge.py

@@ -287,6 +287,16 @@ def merge(self, driveSpec, base, top, bandwidth, job_id):
             except JobExistsError as e:
                 raise exception.MergeFailed(str(e), job=job.id)
 
+            # Prune stale bitmaps if needed once before measuring the image.
+            # Currently, qemu-img ignores bitmaps in base, but future
+            # version may not.
+            if self._base_needs_prune_bitmaps(job, base_info):
+                self._dom._vm.prune_bitmaps(
+                    drive.domainID,
+                    drive.imageID,
+                    job.top,
+                    job.base)
+
             needs_extend, new_size = self._base_needs_extend(
                 drive, job, base_info)
 
@@ -402,6 +412,19 @@ def _start_commit(self, drive, job):
             self._untrack_job(job.id)
             raise exception.MergeFailed(str(e), job=job.id)
 
+    def _base_needs_prune_bitmaps(self, job, base_info):
+        if base_info['format'] != 'COW':
+            log.debug("Base volume does not support bitmaps, "
+                      "job=%r base=%r", job.id, job.base)
+            return False
+
+        if job.active_commit:
+            log.debug("Top volume is the active volume, "
+                      "job=%r top=%r", job.id, job.top)
+            return False
+
+        return True
+
     def _base_needs_extend(self, drive, job, base_info):
         """
         Return True, new_size if base needs to be extended, otherwise False,

lib/vdsm/virt/vm.py

@@ -6106,6 +6106,22 @@ def onDeviceRemoved(self, device_alias):
 
     # Accessing storage
 
+    def prune_bitmaps(self, domainID, imageID, topID, baseID):
+        """
+        Prune stale bitmaps from the base volume.
+        """
+        res = self.cif.irs.prune_bitmaps(
+            sdUUID=domainID,
+            imgUUID=imageID,
+            volUUID=topID,
+            baseUUID=baseID)
+
+        if res['status']['code'] != 0:
+            message = res['status']['message']
+            raise errors.StorageUnavailableError(
+                f"Unable to prune bitmaps in domain {domainID} "
+                f"image {imageID} top {topID} base {baseID}: {message}")
+
     def measure(self, domainID, imageID, topID, dest_format, backing=True,
                 baseID=None):
         """

tests/virt/vmfakelib.py

@@ -51,6 +51,9 @@ def appropriateDevice(self, guid, vmid):
     def inappropriateDevices(self, ident):
         pass
 
+    def prune_bitmaps(self, sdUUID, imgUUID, volUUID, baseUUID):
+        return response.success(result=None)
+
     def measure(self, sdUUID, imgUUID, volUUID, dest_format, backing=True,
                 baseUUID=None):
         # Return fake measure set up by the test. Not setting up anyting will