The rename/copy operation is triggered in that order from top to bottom:

phex/download/MemoryFile.writeBuffersToDisk()
phex/download/swarming/SWDownloadFile.moveToDestinationFile()
phex/common/file/ManagedFile.renameFile( File )
phex/utils/FileUtils.renameFileMultiFallback()

Our observation shows that apparently sometimes there is a file system locking problem which seems to prevent the file operation, specially common on Windows. Or during other times, strange concurrency bugs which cause MemoryFile not detect correctly that the file has finished. Usually these problems only occur rarely. We are not sure why some people have permanent problems.

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