[jira] [Assigned] (ARROW-2790) [C++] Buffers contain uninitialized memory

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

     [ https://issues.apache.org/jira/browse/ARROW-2790?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Wes McKinney reassigned ARROW-2790:
-----------------------------------

    Assignee: Dimitri Vorona

> [C++] Buffers contain uninitialized memory
> ------------------------------------------
>
>                 Key: ARROW-2790
>                 URL: https://issues.apache.org/jira/browse/ARROW-2790
>             Project: Apache Arrow
>          Issue Type: New Feature
>          Components: C++
>    Affects Versions: 0.9.0
>            Reporter: Dimitri Vorona
>            Assignee: Dimitri Vorona
>            Priority: Major
>              Labels: pull-request-available
>             Fix For: 0.10.0
>
>          Time Spent: 13h 10m
>  Remaining Estimate: 0h
>
> Hi,
> currently, buffers can contain uninitalized memory when built with buffer builders. This happens in two ways:
>  # The most common implementation of AppendNull(s) adds a null-bit to the null-map and increments the lengths, leaving chunks of data values untouched
>  # The padding is never initialized
> This can lead to a number of problems, some of more or less theoretical nature, but it's worth fixing just to make valgrind happy again and allow it to run in CI :) Also, leaking uninitialized memory over a wire is never nice, and currently problem 1. ist never mitigated by the ipc writers (although the problem 2 ist). 
> This PR changes the behaviour of AppendNull(s) and FinishInternal for the affected builders and adds a lot of checks to ensure the correct initialization. Most of the test succeed the current master, but cause valgrind warnings, if the buffer builders aren't patched.
> I decided not to test explicitly if the nulled is really 0, just that it can be read without errors, since the specs don't specify exact values. I check the padded memory values explicitly, but I'm not completely sure, what the best course of action is there: we could also ignore the values in padding just checking if they are initialized, or we could adjust the spec in this point. 



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