Created attachment 1448809 [details]
Attachments

Description of problem:
When trying to perform a commit to a snapshot preview we get the following error message in web admin:
"Error while executing action Revert to snapshot: Internal engine error".
Events tab (image attached) shows that the commit failed and around 20 seconds later it shows that the operation succeeds. 
The VM remains in preview-snapshot mode and Undo and Commit operations are still available.
The volumes outputs for every step (looks legit).

Version-Release number of selected component (if applicable):
4.2.4.1-0.1.e

How reproducible:
100%

Steps to Reproduce:
1. Create a VM from a template, and attach a pre-allocated disk to it.
2. Create a snapshot.
3. Start VM and create a live snapshot.
4. Stop the VM.
5. Preview first snapshot
6. Commit the first snapshot

Actual results:
1. Error in web admin:
"Error while executing action Revert to snapshot: Internal engine error".
2. Events tab shows that the commit failed and around 20 seconds later it shows that the operation succeeds. 
Jun 7, 2018, 5:23:53 PM Committing a Snapshot-Preview for VM ShirVM2 has been completed.
Jun 7, 2018, 5:23:37 PM Failed to commit Snapshot-Preview for VM ShirVM2.
3. The VM remains in preview-snapshot mode and Undo and Commit operations are still available.
4. The following errors in the engine.log:
engine log after first commit attempt:

2018-06-07 17:23:36,934+03 ERROR [org.ovirt.engine.core.bll.snapshots.RestoreFromSnapshotCommand] (Transaction Reaper Worker 0) [] Transaction rolled-back for command 'org.ovirt.engine.core.bll.snapshots.RestoreFromSnapshotCommand'.

2018-06-07 17:23:36,936+03 ERROR [org.ovirt.engine.core.bll.tasks.SPMAsyncTask] (EE-ManagedThreadFactory-engine-Thread-106601) [] SPMAsyncTask::StopTask: Error during stopping task 'd9995602-7176-4050-8092-26b8627d727a': WFLYWELD0039: Singleton not set for ModuleClassLoader for Module "org.jboss.as.transactions" from local module loader @7b1d7fff (finder: local module finder @299a06ac (roots: /usr/share/ovirt-engine/modules/common,/usr/share/ovirt-engine-extension-aaa-jdbc/modules,/usr/share/ovirt-engine-extension-aaa-ldap/modules,/opt/rh/eap7/root/usr/share/wildfly/modules,/opt/rh/eap7/root/usr/share/wildfly/modules/system/layers/base)). This means that you are trying to access a weld deployment with a Thread Context ClassLoader that is not associated with the deployment.

2018-06-07 17:23:37,080+03 ERROR [org.ovirt.engine.core.bll.job.ExecutionHandler] (default task-7) [287beb16] Exception: org.springframework.jdbc.CannotGetJdbcConnectionException: Could not get JDBC Connection; nested exception is java.sql.SQLException: javax.resource.ResourceException: IJ000457: Unchecked throwable in managedConnectionReconnected() cl=org.jboss.jca.core.connectionmanager.listener.TxConnectionListener@3f6ef092[state=NORMAL managed connection=org.jboss.jca.adapters.jdbc.local.LocalManagedConnection@4b0322bc connection handles=0 lastReturned=1528381417046 lastValidated=1528381113950 lastCheckedOut=1528381116930 trackByTx=false pool=org.jboss.jca.core.connectionmanager.pool.strategy.OnePool@1f2510bd mcp=SemaphoreConcurrentLinkedQueueManagedConnectionPool@727ab8a2[pool=ENGINEDataSource] xaResource=LocalXAResourceImpl@b669e9e[connectionListener=3f6ef092 connectionManager=6912e60f warned=false currentXid=null productName=PostgreSQL productVersion=9.5.9 jndiName=java:/ENGINEDataSource] txSync=null]

2018-06-07 17:23:37,185+03 ERROR [org.ovirt.engine.core.bll.snapshots.RestoreAllSnapshotsCommand] (default task-7) [287beb16] Failed to remove memory of snapshot '7e404501-dfbb-4067-92ae-aca7dd80d239'

2018-06-07 17:23:37,203+03 ERROR [org.ovirt.engine.core.bll.snapshots.RestoreAllSnapshotsCommand] (default task-7) [287beb16] Command 'org.ovirt.engine.core.bll.snapshots.RestoreAllSnapshotsCommand' failed: Could not get JDBC Connection; nested exception is java.sql.SQLException: javax.resource.ResourceException: IJ000457: Unchecked throwable in managedConnectionReconnected() cl=org.jboss.jca.core.connectionmanager.listener.TxConnectionListener@3f6ef092[state=NORMAL managed connection=org.jboss.jca.adapters.jdbc.local.LocalManagedConnection@4b0322bc connection handles=0 lastReturned=1528381417185 lastValidated=1528381113950 lastCheckedOut=1528381116930 trackByTx=false pool=org.jboss.jca.core.connectionmanager.pool.strategy.OnePool@1f2510bd mcp=SemaphoreConcurrentLinkedQueueManagedConnectionPool@727ab8a2[pool=ENGINEDataSource] xaResource=LocalXAResourceImpl@b669e9e[connectionListener=3f6ef092 connectionManager=6912e60f warned=false currentXid=null productName=PostgreSQL productVersion=9.5.9 jndiName=java:/ENGINEDataSource] txSync=null]

2018-06-07 17:23:37,324+03 ERROR [org.ovirt.engine.core.dal.dbbroker.auditloghandling.AuditLogDirector] (default task-7) [287beb16] EVENT_ID: USER_COMMIT_RESTORE_FROM_SNAPSHOT_FINISH_FAILURE(96), Failed to commit Snapshot-Preview for VM ShirVM2.

2018-06-07 17:23:53,069+03 ERROR [org.ovirt.engine.core.bll.snapshots.RestoreAllSnapshotsCommand] (EE-ManagedThreadFactory-engineScheduled-Thread-70) [a0d5f6ce-c0ef-4d63-afb7-70da55c93a70] Ending command 'org.ovirt.engine.core.bll.snapshots.RestoreAllSnapshotsCommand' with failure.

Second commit attempt causes the following:
2018-06-07 19:05:37,507+03 ERROR [org.ovirt.engine.core.bll.snapshots.RestoreAllSnapshotsCommand] (default task-16) [cb9004dd-9150-415f-ad84-091750bbda23] Command 'org.ovirt.engine.core.bll.snapshots.RestoreAllSnapshotsCommand' failed: null

2018-06-07 19:05:37,507+03 ERROR [org.ovirt.engine.core.bll.snapshots.RestoreAllSnapshotsCommand] (default task-16) [cb9004dd-9150-415f-ad84-091750bbda23] Exception: java.lang.NullPointerException


Expected results:
For the commit to succeed without any errors.

Additional info:
When performing the same scenario and writing to the disks there are no errors and the commit operation succeeds.