Issues which may be related to overlapping blocks in the freelists.

1. Double free but also corrupt file, which means it is different from the stuff fixed in https://github.com/realm/realm-core/issues/6813

• #6531

1. Overlaps among blocks freed over separate transactions - this is detected by GroupWriter::recreate_freelist()

• #6071
• #6189
• #6378

OH also: #5993, was a presumably fix for several bugs with similar stacks:

• #5300

This may indicate that we did not fix the root cause, but merely changed the timing enough to hide the real problem.

A few similar old issue from differents repos either closed without fix or known cause or still presumably valid:

• https://github.com/realm/realm-java-private/issues/261
• https://github.com/Clipy/Clipy/issues/375
• https://github.com/realm/realm-swift/issues/6489
• https://github.com/realm/realm-swift/issues/7932

Root cause unknown, prime suspects:

• Mutex emulation on iOS (and possibly windows) (but if so, then the data races will need a different explanation)
• Online compaction
• Invalid changes to the m_baseline member of the allocator (this could be some interaction between online compaction and async commit)
• Invalid sharing of history objects (or part of their representation).

Cleared:

• Reclaiming intermediate versions - only changes release time stamps of already released blocks, so cannot possibly produce a free block, which was not a free block to begin with.
• Data races which may be explained by (wrong) allocation of an already allocated block:
  #6474
  #6624
  #6738
  These are just encryption specific false positives. There is no indication that they could be relevant.