[Python] Add unit tests for ProxyMemoryPool, enable new default MemoryPool to be constructed

[asfimport]

I could not find unit tests for ProxyMemoryPool

Reporter: Wes McKinney / @wesm
Assignee: Antoine Pitrou / @pitrou

PRs and other links:

• GitHub Pull Request #2725

_{Note: This issue was originally created as ARROW-2808. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
There is a single test (ProxyMemoryPool::Logging) in memory_pool-test.cc.

[asfimport]

Antoine Pitrou / @pitrou:
As for changing the default MemoryPool, unfortunately this may be dangerous. Objects may hold a non-owned pointer to the old default MemoryPool (a MemoryPool*). Imagine the following sequence:

• default MemoryPool is changed once

• an object A is created which holds a pointer to the default MemoryPool

• default MemoryPool is changed a second time

  What has the MemoryPool pointer held by A become? Presumably the underlying object is destroyed.

[asfimport]

Wes McKinney / @wesm:
There are Python bindings for the proxy pool but no tests to check whether they work. There are no APIs to construct a new MemoryPool in Python AFAICT

[asfimport]

Uwe Korn / @xhochy:
Issue resolved by pull request 2725
#2725