Assertion error (int overflow) in ByteSliceReader [LUCENE-9751]

New computers come with insane amounts of ram and heaps can get pretty big. If you adjust per-thread buffers to larger values strange things start happening. This happened to us today:

```Java
Caused by: java.lang.AssertionError
at org.apache.lucene.index.ByteSliceReader.init(ByteSliceReader.java:44) \~[lucene-core-8.7.0.jar:8.7.0 2dc63e901c60cda27ef3b744bc554f1481b3b067 - atrisharma - 2020-10-29 19:35:28]
at org.apache.lucene.index.TermsHashPerField.initReader(TermsHashPerField.java:88) \~[lucene-core-8.7.0.jar:8.7.0 2dc63e901c60cda27ef3b744bc554f1481b3b067 - atrisharma - 2020-10-29 19:35:28]
at org.apache.lucene.index.FreqProxFields$FreqProxPostingsEnum.reset(FreqProxFields.java:430) \~[lucene-core-8.7.0.jar:8.7.0 2dc63e901c60cda27ef3b744bc554f1481b3b067 - atrisharma - 2020-10-29 19:35:28]
at org.apache.lucene.index.FreqProxFields$FreqProxTermsEnum.postings(FreqProxFields.java:247) \~[lucene-core-8.7.0.jar:8.7.0 2dc63e901c60cda27ef3b744bc554f1481b3b067 - atrisharma - 2020-10-29 19:35:28]
at org.apache.lucene.codecs.PushPostingsWriterBase.writeTerm(PushPostingsWriterBase.java:127) \~[lucene-core-8.7.0.jar:8.7.0 2dc63e901c60cda27ef3b744bc554f1481b3b067 - atrisharma - 2020-10-29 19:35:28]
at org.apache.lucene.codecs.blocktree.BlockTreeTermsWriter$TermsWriter.write(BlockTreeTermsWriter.java:907) \~[lucene-core-8.7.0.jar:8.7.0 2dc63e901c60cda27ef3b744bc554f1481b3b067 - atrisharma - 2020-10-29 19:35:28]
at org.apache.lucene.codecs.blocktree.BlockTreeTermsWriter.write(BlockTreeTermsWriter.java:318) \~[lucene-core-8.7.0.jar:8.7.0 2dc63e901c60cda27ef3b744bc554f1481b3b067 - atrisharma - 2020-10-29 19:35:28]
at org.apache.lucene.codecs.perfield.PerFieldPostingsFormat$FieldsWriter.write(PerFieldPostingsFormat.java:170) \~[lucene-core-8.7.0.jar:8.7.0 2dc63e901c60cda27ef3b744bc554f1481b3b067 - atrisharma - 2020-10-29 19:35:28]
at org.apache.lucene.index.FreqProxTermsWriter.flush(FreqProxTermsWriter.java:120) \~[lucene-core-8.7.0.jar:8.7.0 2dc63e901c60cda27ef3b744bc554f1481b3b067 - atrisharma - 2020-10-29 19:35:28]
at org.apache.lucene.index.DefaultIndexingChain.flush(DefaultIndexingChain.java:264) \~[lucene-core-8.7.0.jar:8.7.0 2dc63e901c60cda27ef3b744bc554f1481b3b067 - atrisharma - 2020-10-29 19:35:28]
at org.apache.lucene.index.DocumentsWriterPerThread.flush(DocumentsWriterPerThread.java:350) \~[lucene-core-8.7.0.jar:8.7.0 2dc63e901c60cda27ef3b744bc554f1481b3b067 - atrisharma - 2020-10-29 19:35:28]
at org.apache.lucene.index.DocumentsWriter.doFlush(DocumentsWriter.java:480) \~[lucene-core-8.7.0.jar:8.7.0 2dc63e901c60cda27ef3b744bc554f1481b3b067 - atrisharma - 2020-10-29 19:35:28]
at org.apache.lucene.index.DocumentsWriter.postUpdate(DocumentsWriter.java:394) \~[lucene-core-8.7.0.jar:8.7.0 2dc63e901c60cda27ef3b744bc554f1481b3b067 - atrisharma - 2020-10-29 19:35:28]
at org.apache.lucene.index.DocumentsWriter.updateDocuments(DocumentsWriter.java:440) \~[lucene-core-8.7.0.jar:8.7.0 2dc63e901c60cda27ef3b744bc554f1481b3b067 - atrisharma - 2020-10-29 19:35:28]
at org.apache.lucene.index.IndexWriter.updateDocuments(IndexWriter.java:1471) \~[lucene-core-8.7.0.jar:8.7.0 2dc63e901c60cda27ef3b744bc554f1481b3b067 - atrisharma - 2020-10-29 19:35:28]
... 7 more
```

Likely an int overflow in TermsHashPerField:

```Java
reader.init(bytePool,
postingsArray.byteStarts[termID]+stream*ByteBlockPool.FIRST_LEVEL_SIZE,
streamAddressBuffer[offsetInAddressBuffer+stream]);
```

Don't know if this can be prevented somehow.

---
Migrated from [LUCENE-9751](https://issues.apache.org/jira/browse/LUCENE-9751) by Dawid Weiss (@dweiss), 1 vote, updated Feb 11 2021

Start with ByteSliceReader.init and the TermsHashPerField.java reader.init call shown in the report, then trace the byte-pool addresses and stream offsets involved when per-thread buffers are large. Reproduce the indexing failure with enlarged buffers and verify that the assertion no longer occurs during the reported flush and postings-writing path.

Written by the indexing model from the issue text.