feat: allow setting capacity on external sources prompting volume resize

fixes: #52

builder/libvirt/volume/external.go

@@ -70,6 +70,8 @@ func (vs *ExternalVolumeSource) PrepareVolume(pctx *PreparationContext) multiste
 		return pctx.HaltOnError(err, "preparing volume: %s", err)
 	}
 
+	storageTargetCapacity := pctx.VolumeDefinition.Capacity
+
 	pctx.VolumeDefinition.Allocation = &libvirtxml.StorageVolumeSize{
 		Value: uint64(stat.Size()),
 		Unit:  "B",
@@ -110,5 +112,25 @@ func (vs *ExternalVolumeSource) PrepareVolume(pctx *PreparationContext) multiste
 		log.Printf("Error while refreshing volume definition: %s\n", err)
 	}
 
+	if storageTargetCapacity != nil {
+		pctx.Ui.Message(fmt.Sprintf(
+			"Resizing volume %s/%s to meet capacity %d%s",
+			pctx.VolumeConfig.Pool,
+			pctx.VolumeConfig.Name,
+			storageTargetCapacity.Value,
+			storageTargetCapacity.Unit,
+		))
+		multiplier, err := unitToMultiplier(storageTargetCapacity.Unit)
+		if err != nil {
+			return pctx.HaltOnError(err, "Error during volume resize: %s", err)
+		}
+		targetCapacityInBytes := storageTargetCapacity.Value * uint64(multiplier)
+
+		err = pctx.Driver.StorageVolResize(*pctx.VolumeRef, targetCapacityInBytes, 0)
+		if err != nil {
+			return pctx.HaltOnError(err, "Error during volume resize: %s", err)
+		}
+	}
+
 	return multistep.ActionContinue
 }

builder/libvirt/volume/volume.go

@@ -275,3 +275,19 @@ func fmtReadPostfixedValue(s string) (value uint64, unit string, err error) {
 	}
 	return
 }
+
+func unitToMultiplier(unit string) (value int64, err error) {
+	switch unit {
+	case "B":
+		value = 1
+	case "KiB":
+		value = 1024
+	case "Mib":
+		value = 1024 * 1024
+	case "Gib":
+		value = 1024 * 1024 * 1024
+	default:
+		err = fmt.Errorf("unknown unit %s", unit)
+	}
+	return
+}