[jira] [Created] (ARROW-494) [C++] When MemoryMappedFile is destructed, memory is unmapped even if buffer referecnes still exist

["Wes McKinney (JIRA)" <ji...@apache.org>]

Wes McKinney created ARROW-494:
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             Summary: [C++] When MemoryMappedFile is destructed, memory is unmapped even if buffer referecnes still exist
                 Key: ARROW-494
                 URL: https://issues.apache.org/jira/browse/ARROW-494
             Project: Apache Arrow
          Issue Type: Bug
          Components: C++
            Reporter: Wes McKinney


I'd like to see if there is some way to "protect" the memory map from premature destruction. This is a slight artifact of MemoryMappedFile's implementation sharing code with normal on-disk files (which read into allocated memory), i.e. the `Close` function unmaps the memory and closes the file handle. This would amount to creating a Buffer subclass that retains ownership of the file descriptor and memory map, so that if any Buffer still references the memory map, then `MemoryMappedFile::Close` will not unmap the memory or close the file. But then the unmapping / file close would need to happen when the last Buffer reference is destroyed. 



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