fix(vhd-lib/merge): dont fail on invalid state file

Fixes zammad#4227

CHANGELOG.unreleased.md

@@ -18,6 +18,7 @@
 - [Tables/actions] Fix collapsed actions being clickable despite being disabled (PR [#6023](https://github.com/vatesfr/xen-orchestra/pull/6023))
 - [Continuous Replication] Fix `could not find the base VM`
 - [Backup/Smart mode] Always ignore replicated VMs created by the current job
+- [Backup] Fix `Unexpected end of JSON input` during merge step
 
 ### Packages to release
 
@@ -36,6 +37,7 @@
 >
 > In case of conflict, the highest (lowest in previous list) `$version` wins.
 
+- vhd-lib patch
 - @xen-orchestra/backups minor
 - @xen-orchestra/proxy minor
 - xo-server minor

packages/vhd-lib/src/merge.js

@@ -25,12 +25,23 @@ export default limitConcurrency(2)(async function merge(
   const mergeStatePath = dirname(parentPath) + '/' + '.' + basename(parentPath) + '.merge.json'
 
   return await Disposable.use(async function* () {
-    let mergeState = await parentHandler.readFile(mergeStatePath).catch(error => {
-      if (error.code !== 'ENOENT') {
-        throw error
-      }
-      // no merge state in case of missing file
-    })
+    let mergeState = await parentHandler
+      .readFile(mergeStatePath)
+      .then(content => {
+        const state = JSON.parse(content)
+
+        // ensure the correct merge will be continued
+        assert.strictEqual(parentVhd.header.checksum, state.parent.header)
+        assert.strictEqual(childVhd.header.checksum, state.child.header)
+
+        return state
+      })
+      .catch(error => {
+        if (error.code !== 'ENOENT') {
+          warn('problem while checking the merge state', { error })
+        }
+      })
+
     // during merging, the end footer of the parent can be overwritten by new blocks
     // we should use it as a way to check vhd health
     const parentVhd = yield openVhd(parentHandler, parentPath, {
@@ -38,13 +49,7 @@ export default limitConcurrency(2)(async function merge(
       checkSecondFooter: mergeState === undefined,
     })
     const childVhd = yield openVhd(childHandler, childPath)
-    if (mergeState !== undefined) {
-      mergeState = JSON.parse(mergeState)
-
-      // ensure the correct merge will be continued
-      assert.strictEqual(parentVhd.header.checksum, mergeState.parent.header)
-      assert.strictEqual(childVhd.header.checksum, mergeState.child.header)
-    } else {
+    if (mergeState === undefined) {
       assert.strictEqual(childVhd.header.blockSize, parentVhd.header.blockSize)
 
       const parentDiskType = parentVhd.footer.diskType