Race condition in FileContainer#allocCache when reopening RAFContainer after interrupt

The symptom is an ArrayIndexOutOfBoundsException:

java.lang.ArrayIndexOutOfBoundsException: -1
at org.apache.derby.impl.store.raw.data.AllocationCache.validate(AllocationCache.java:581)
at org.apache.derby.impl.store.raw.data.AllocationCache.getLastPageNumber(AllocationCache.java:122)
at org.apache.derby.impl.store.raw.data.FileContainer.pageValid(FileContainer.java:2067)
at org.apache.derby.impl.store.raw.data.FileContainer.getUserPage(FileContainer.java:2522)
at org.apache.derby.impl.store.raw.data.FileContainer.getInsertablePage(FileContainer.java:2867)
at org.apache.derby.impl.store.raw.data.FileContainer.getPageForInsert(FileContainer.java:3017)
at org.apache.derby.impl.store.raw.data.BaseContainerHandle.getPageForInsert(BaseContainerHandle.java:372)
at org.apache.derby.impl.store.access.heap.HeapController.doInsert(HeapController.java:244)
at org.apache.derby.impl.store.access.heap.HeapController.insertAndFetchLocation(HeapController.java:599)
at org.apache.derby.impl.sql.execute.RowChangerImpl.insertRow(RowChangerImpl.java:452)
at org.apache.derby.impl.sql.execute.InsertResultSet.normalInsertCore(InsertResultSet.java:1028)
at org.apache.derby.impl.sql.execute.InsertResultSet.open(InsertResultSet.java:505)
at org.apache.derby.impl.sql.GenericPreparedStatement.executeStmt(GenericPreparedStatement.java:436)
at org.apache.derby.impl.sql.GenericPreparedStatement.execute(GenericPreparedStatement.java:317)
at org.apache.derby.impl.jdbc.EmbedStatement.executeStatement(EmbedStatement.java:1241)
at org.apache.derby.impl.jdbc.EmbedPreparedStatement.executeStatement(EmbedPreparedStatement.java:1686)
at org.apache.derby.impl.jdbc.EmbedPreparedStatement.executeUpdate(EmbedPreparedStatement.java:308)
at InterruptTest$WorkerThread.run(InterruptTest.java:261

This can only happen if another thread has called allocCache.reset while the thread above is in the loop in validate, so as to set numExtents to 0.
The synchronization of allocCache is documented in the Javadoc of the FileContainer class: all accesses to allocCache should synchronize.
This is omitted when we reopen: FileContainer#openContainer calls readHeader -> readHeaderFromArray -> allocCache.reset