NullPointerException when running with remote neo4j

[lennartfricke]

First of all, thank you for all the work you put in jqassistant.

Under special conditions the plugin jqassistant crashes with the following stacktrace.
I was not able to reproduce the error with public material only.

Steps to reproduce:

• I need to launch jqassistant with limited memory and mariadb connector on classpath.
  java -Xmx200M -cp jqassistant-commandline-distribution-2.0.10/lib/com.buschmais.jqassistant.cli-jqassistant-commandline-distribution-2.0.10.jar:mariadb-java-client-2.7.12.jar scan -f jqassistant.plugin.rdbms.properties

• I use the older mariadb-client to connect to mysql. neo4j is version 5.15.0.

• jqassistant.yml:

jqassistant:
 plugins:
   - group-id: org.jqassistant.plugin
     artifact-id: jqassistant-rdbms-plugin
     version: 2.0.0
 store:
   uri: XXXXXX
   remote:
       username: neo4j
       password: XXXXXXXX
       encryption: true
       trust-strategy: trustAllCertificates

• Unfortunately I was not able to reproduce the error with any other schema than the one I cannot disclose at the moment.
  But the schema is rather large containing about 500 tables, over 1500 indices and about 6000 columns. I was not able to find some similar schema for testing fast enough.

The run throws the following exception:

Exception in thread "main" com.buschmais.jqassistant.core.scanner.impl.UnrecoverableScannerException: Unexpected problem encountered while scanning: item='jqassistant.plugin.rdbms.properties', path='....', scope='NONE', pipeline='[com.buschmais.jqassistant.plugin.common.impl.scanner.FileResourceScannerPlugin@40f35e52, com.buschmais.jqassistant.plugin.java.impl.scanner.PropertyFileScannerPlugin@7c011174, org.jqassistant.plugin.rdbms.impl.scanner.ConnectionPropertyFileScannerPlugin@480b57e2]'. Please report this error including the full stacktrace (continueOnError=false).
	at com.buschmais.jqassistant.core.scanner.impl.ScannerImpl.scan(ScannerImpl.java:105)
	at com.buschmais.jqassistant.core.scanner.impl.ScannerImpl.scan(ScannerImpl.java:64)
	at com.buschmais.jqassistant.plugin.common.impl.scanner.FileScannerPlugin.scan(FileScannerPlugin.java:35)
	at com.buschmais.jqassistant.plugin.common.impl.scanner.FileScannerPlugin.scan(FileScannerPlugin.java:21)
	at com.buschmais.jqassistant.core.scanner.impl.ScannerImpl.scan(ScannerImpl.java:125)
	at com.buschmais.jqassistant.core.scanner.impl.ScannerImpl.scan(ScannerImpl.java:85)
	at com.buschmais.jqassistant.core.scanner.impl.ScannerImpl.scan(ScannerImpl.java:64)
	at com.buschmais.jqassistant.commandline.task.ScanTask.scan(ScanTask.java:103)
	at com.buschmais.jqassistant.commandline.task.ScanTask.lambda$scanFiles$4(ScanTask.java:93)
	at java.base/java.util.Optional.ifPresent(Optional.java:178)
	at com.buschmais.jqassistant.commandline.task.ScanTask.scanFiles(ScanTask.java:83)
	at com.buschmais.jqassistant.commandline.task.ScanTask.lambda$run$0(ScanTask.java:61)
	at java.base/java.util.Optional.ifPresentOrElse(Optional.java:196)
	at com.buschmais.jqassistant.commandline.task.ScanTask.lambda$run$2(ScanTask.java:60)
	at com.buschmais.jqassistant.commandline.task.AbstractStoreTask.withStore(AbstractStoreTask.java:51)
	at com.buschmais.jqassistant.commandline.task.ScanTask.run(ScanTask.java:52)
	at com.buschmais.jqassistant.commandline.Main.executeTask(Main.java:303)
	at com.buschmais.jqassistant.commandline.Main.executeTasks(Main.java:263)
	at com.buschmais.jqassistant.commandline.Main.interpretCommandLine(Main.java:203)
	at com.buschmais.jqassistant.commandline.Main.run(Main.java:87)
	at com.buschmais.jqassistant.commandline.Main.main(Main.java:68)
Caused by: java.lang.NullPointerException: Cannot invoke "com.buschmais.xo.neo4j.remote.impl.model.AbstractRemotePropertyContainer.getId()" because "propertyContainer" is null
	at com.buschmais.xo.neo4j.remote.impl.datastore.RemoteDatastoreSessionCache.update(RemoteDatastoreSessionCache.java:94)
	at com.buschmais.xo.neo4j.remote.impl.datastore.RemoteDatastoreSessionCache.update(RemoteDatastoreSessionCache.java:89)
	at com.buschmais.xo.neo4j.remote.impl.datastore.RemoteDatastoreEntityManager.lambda$flushAddedRelationships$3(RemoteDatastoreEntityManager.java:299)
	at com.buschmais.xo.neo4j.remote.impl.datastore.StatementBatchBuilder.close(StatementBatchBuilder.java:73)
	at com.buschmais.xo.neo4j.remote.impl.datastore.RemoteDatastoreEntityManager.flush(RemoteDatastoreEntityManager.java:252)
	at com.buschmais.xo.impl.cache.CacheSynchronizationService.flush(CacheSynchronizationService.java:50)
	at com.buschmais.xo.impl.cache.CacheSynchronizationService.flush(CacheSynchronizationService.java:34)
	at com.buschmais.xo.impl.cache.CacheSynchronization.beforeCompletion(CacheSynchronization.java:15)
	at com.buschmais.xo.impl.XOTransactionImpl.lambda$beforeCompletion$0(XOTransactionImpl.java:85)
	at com.buschmais.xo.impl.XOTransactionImpl.executeSynchronizations(XOTransactionImpl.java:95)
	at com.buschmais.xo.impl.XOTransactionImpl.beforeCompletion(XOTransactionImpl.java:85)
	at com.buschmais.xo.impl.XOTransactionImpl.commit(XOTransactionImpl.java:35)
	at com.buschmais.jqassistant.core.store.impl.AbstractGraphStore.commitTransaction(AbstractGraphStore.java:204)
	at com.buschmais.jqassistant.core.store.impl.AbstractGraphStore.flush(AbstractGraphStore.java:131)
	at com.buschmais.jqassistant.core.store.impl.AbstractGraphStore.autoCommit(AbstractGraphStore.java:125)
	at com.buschmais.jqassistant.core.store.impl.AbstractGraphStore.create(AbstractGraphStore.java:95)
	at org.jqassistant.plugin.rdbms.impl.scanner.AbstractSchemaScannerPlugin.getTableDescriptor(AbstractSchemaScannerPlugin.java:349)
	at org.jqassistant.plugin.rdbms.impl.scanner.AbstractSchemaScannerPlugin.createTables(AbstractSchemaScannerPlugin.java:162)
	at org.jqassistant.plugin.rdbms.impl.scanner.AbstractSchemaScannerPlugin.createSchemas(AbstractSchemaScannerPlugin.java:136)
	at org.jqassistant.plugin.rdbms.impl.scanner.AbstractSchemaScannerPlugin.scanConnection(AbstractSchemaScannerPlugin.java:57)
	at org.jqassistant.plugin.rdbms.impl.scanner.ConnectionPropertyFileScannerPlugin.scan(ConnectionPropertyFileScannerPlugin.java:97)
	at org.jqassistant.plugin.rdbms.impl.scanner.ConnectionPropertyFileScannerPlugin.scan(ConnectionPropertyFileScannerPlugin.java:28)
	at com.buschmais.jqassistant.core.scanner.impl.ScannerImpl.scan(ScannerImpl.java:125)
	at com.buschmais.jqassistant.core.scanner.impl.ScannerImpl.scan(ScannerImpl.java:89)
	... 20 more

Debugging lead me to following hypothesis:

• The SchemaScannerPlugin creates relations, but for some reason drops strong references to them.
• During flush the values com.buschmais.xo.neo4j.remote.impl.datastore.RemoteDatastoreSessionCache are already cleaned up, so that the no value is found in cache.

I know too little about the design choices of jqassistant to work out a fix. One possibility would being prepared to get nulll when getting information from cache. The other I can imagine is a bug in this plugin.