[C++] Do not include padding bytes in "Buffer" IPC metadata accounting

[asfimport]

At this line, we include the padding bytes into the IPC metadata

https://github.com/apache/arrow/blob/apache-arrow-0.16.0/cpp/src/arrow/ipc/writer.cc#L192

The effect of this is that buffer sizes are modified by an IPC roundtrip. According to the Format, the padding bytes do not need to be accounted for in the metadata.

https://github.com/apache/arrow/blob/master/format/Schema.fbs#L330

The Java implementation, for example, does not.

I ran into this when working on a prototype implementation of ARROW-300, where it is important to have the exact unpadded size of the original buffer that was written.

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #6513

_{Note: This issue was originally created as ARROW-7975. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 6513
#6513