feat(backups): implement differential restore

When restoring a backup, try to reuse data from an existing snapshot.
We use this snasphot and apply a reverse differential of the changes
between the backup and the snapshot

Prerequisite : a uninterrupted delta chain from the backup being
restored to a backup that still have its snapshot on the host

@xen-orchestra/backups/ImportVmBackup.mjs

@@ -4,7 +4,14 @@ import { formatFilenameDate } from './_filenameDate.mjs'
 import { importIncrementalVm } from './_incrementalVm.mjs'
 import { Task } from './Task.mjs'
 import { watchStreamSize } from './_watchStreamSize.mjs'
+import { VhdNegative, VhdSynthetic } from 'vhd-lib'
+import { decorateClass } from '@vates/decorate-with'
+import { createLogger } from '@xen-orchestra/log'
+import { dirname, join } from 'node:path'
+import pickBy from 'lodash/pickBy.js'
+import { defer } from 'golike-defer'
 
+const { debug, info, warn } = createLogger('xo:backups:importVmBackup')
 async function resolveUuid(xapi, cache, uuid, type) {
   if (uuid == null) {
     return uuid
@@ -30,8 +37,168 @@ export class ImportVmBackup {
     this._xapi = xapi
   }
 
-  async #decorateIncrementalVmMetadata(backup) {
+  async #getPathOfVdiSnapshot(snapshotUuid) {
+    const metadata = this._metadata
+    if (this._pathToVdis === undefined) {
+      const backups = await this._adapter.listVmBackups(
+        this._metadata.vm.uuid,
+        ({ mode, timestamp }) => mode === 'delta' && timestamp >= metadata.timestamp
+      )
+      const map = new Map()
+      for (const backup of backups) {
+        for (const [vdiRef, vdi] of Object.entries(backup.vdis)) {
+          map.set(vdi.uuid, backup.vhds[vdiRef])
+        }
+      }
+      this._pathToVdis = map
+    }
+    return this._pathToVdis.get(snapshotUuid)
+  }
+
+  async _reuseNearestSnapshot($defer, ignoredVdis) {
+    const metadata = this._metadata
+    const { mapVdisSrs } = this._importIncrementalVmSettings
+    const { vbds, vhds, vifs, vm, vmSnapshot } = metadata
+    const streams = {}
+    const metdataDir = dirname(metadata._filename)
+    const vdis = ignoredVdis === undefined ? metadata.vdis : pickBy(metadata.vdis, vdi => !ignoredVdis.has(vdi.uuid))
+
+    for (const [vdiRef, vdi] of Object.entries(vdis)) {
+      const vhdPath = join(metdataDir, vhds[vdiRef])
+
+      let xapiDisk
+      try {
+        xapiDisk = await this._xapi.getRecordByUuid('VDI', vdi.$snapshot_of$uuid)
+      } catch (err) {
+        // if this disk is not present anymore, fall back to default restore
+        warn(err)
+      }
+
+      let snapshotCandidate, backupCandidate
+      if (xapiDisk !== undefined) {
+        debug('found disks, wlll search its snapshots', { snapshots: xapiDisk.snapshots })
+        for (const snapshotRef of xapiDisk.snapshots) {
+          const snapshot = await this._xapi.getRecord('VDI', snapshotRef)
+          debug('handling snapshot', { snapshot })
+
+          // take only the first snapshot
+          if (snapshotCandidate && snapshotCandidate.snapshot_time < snapshot.snapshot_time) {
+            debug('already got a better candidate')
+            continue
+          }
+
+          // have a corresponding backup more recent than metadata ?
+          const pathToSnapshotData = await this.#getPathOfVdiSnapshot(snapshot.uuid)
+          if (pathToSnapshotData === undefined) {
+            debug('no backup linked to this snaphot')
+            continue
+          }
+          if (snapshot.$SR.uuid !== (mapVdisSrs[vdi.$snapshot_of$uuid] ?? this._srUuid)) {
+            debug('not restored on the same SR', { snapshotSr: snapshot.$SR.uuid, mapVdisSrs, srUuid: this._srUuid })
+            continue
+          }
+
+          debug('got a candidate', pathToSnapshotData)
+
+          snapshotCandidate = snapshot
+          backupCandidate = pathToSnapshotData
+        }
+      }
+
+      let stream
+      const backupWithSnapshotPath = join(metdataDir, backupCandidate ?? '')
+      if (vhdPath === backupWithSnapshotPath) {
+        // all the data are already on the host
+        debug('direct reuse of a snapshot')
+        stream = null
+        vdis[vdiRef].baseVdi = snapshotCandidate
+        // go next disk , we won't use this stream
+        continue
+      }
+
+      let disposableDescendants
+
+      const disposableSynthetic = await VhdSynthetic.fromVhdChain(this._adapter._handler, vhdPath)
+
+      // this will also clean if another disk of this VM backup fails
+      // if user really only need to restore non failing disks he can retry with ignoredVdis
+      let disposed = false
+      const disposeOnce = async () => {
+        if (!disposed) {
+          disposed = true
+          try {
+            await disposableDescendants?.dispose()
+            await disposableSynthetic?.dispose()
+          } catch (error) {
+            warn('openVhd: failed to dispose VHDs', { error })
+          }
+        }
+      }
+      $defer.onFailure(() => disposeOnce())
+
+      const parentVhd = disposableSynthetic.value
+      await parentVhd.readBlockAllocationTable()
+      debug('got vhd synthetic of parents', parentVhd.length)
+
+      if (snapshotCandidate !== undefined) {
+        try {
+          debug('will try to use differential restore', {
+            backupWithSnapshotPath,
+            vhdPath,
+            vdiRef,
+          })
+
+          disposableDescendants = await VhdSynthetic.fromVhdChain(this._adapter._handler, backupWithSnapshotPath, {
+            until: vhdPath,
+          })
+          const descendantsVhd = disposableDescendants.value
+          await descendantsVhd.readBlockAllocationTable()
+          debug('got vhd synthetic of descendants')
+          const negativeVhd = new VhdNegative(parentVhd, descendantsVhd)
+          debug('got vhd negative')
+
+          // update the stream with the negative vhd stream
+          stream = await negativeVhd.stream()
+          vdis[vdiRef].baseVdi = snapshotCandidate
+        } catch (err) {
+          // can be a broken VHD chain, a vhd chain with a key backup, ....
+          // not an irrecuperable error, don't dispose parentVhd, and fallback to full restore
+          warn(`can't use differential restore`, err)
+          disposableDescendants?.dispose()
+        }
+      }
+      // didn't make a negative stream : fallback to classic stream
+      if (stream === undefined) {
+        debug('use legacy restore')
+        stream = await parentVhd.stream()
+      }
+
+      stream.on('end', disposeOnce)
+      stream.on('close', disposeOnce)
+      stream.on('error', disposeOnce)
+      info('everything is ready, will transfer', stream.length)
+      streams[`${vdiRef}.vhd`] = stream
+    }
+    return {
+      streams,
+      vbds,
+      vdis,
+      version: '1.0.0',
+      vifs,
+      vm: { ...vm, suspend_VDI: vmSnapshot.suspend_VDI },
+    }
+  }
+
+  async #decorateIncrementalVmMetadata() {
     const { additionnalVmTag, mapVdisSrs } = this._importIncrementalVmSettings
+    
+    const ignoredVdis = new Set(
+      Object.entries(mapVdisSrs)
+        .filter(([_, srUuid]) => srUuid === null)
+        .map(([vdiUuid]) => vdiUuid)
+    )
+    const backup = await this._reuseNearestSnapshot(ignoredVdis)
+
     const xapi = this._xapi
 
     const cache = new Map()
@@ -55,7 +222,7 @@ export class ImportVmBackup {
     const isFull = metadata.mode === 'full'
 
     const sizeContainer = { size: 0 }
-    const { mapVdisSrs, newMacAddresses } = this._importIncrementalVmSettings
+    const { newMacAddresses } = this._importIncrementalVmSettings
     let backup
     if (isFull) {
       backup = await adapter.readFullVmBackup(metadata)
@@ -63,12 +230,7 @@ export class ImportVmBackup {
     } else {
       assert.strictEqual(metadata.mode, 'delta')
 
-      const ignoredVdis = new Set(
-        Object.entries(mapVdisSrs)
-          .filter(([_, srUuid]) => srUuid === null)
-          .map(([vdiUuid]) => vdiUuid)
-      )
-      backup = await this.#decorateIncrementalVmMetadata(await adapter.readIncrementalVmBackup(metadata, ignoredVdis))
+      backup = await this.#decorateIncrementalVmMetadata()
       Object.values(backup.streams).forEach(stream => watchStreamSize(stream, sizeContainer))
     }
 
@@ -110,3 +272,5 @@ export class ImportVmBackup {
     )
   }
 }
+
+decorateClass(ImportVmBackup, { _reuseNearestSnapshot: defer })

@xen-orchestra/backups/_incrementalVm.mjs

@@ -250,6 +250,10 @@ export const importIncrementalVm = defer(async function importIncrementalVm(
     // Import VDI contents.
     cancelableMap(cancelToken, Object.entries(newVdis), async (cancelToken, [id, vdi]) => {
       for (let stream of ensureArray(streams[`${id}.vhd`])) {
+        if (stream === null) {
+          // we restore a backup and reuse completly a local snapshot
+          continue
+        }
         if (typeof stream === 'function') {
           stream = await stream()
         }

packages/vhd-lib/_resolveRelativeFromFile.js

@@ -2,6 +2,10 @@
 
 const { dirname, resolve } = require('path')
 
-const resolveRelativeFromFile = (file, path) => resolve('/', dirname(file), path).slice(1)
-
+const resolveRelativeFromFile = (file, path) => {
+  if (file.startsWith('/')) {
+    return resolve(dirname(file), path)
+  }
+  return resolve('/', dirname(file), path).slice(1)
+}
 module.exports = resolveRelativeFromFile