[jbonofre]

What's Changed

Follow-up to #1461. When no allocation manager type is set, CheckAllocator.check() used to pick the factory class by searching the resource location for memory-core, memory-unsafe or memory-netty. It now looks up each known DefaultAllocationManagerFactory resource and returns the factory it actually found:

• A Netty or Unsafe factory loaded from a location containing memory-core is no longer inferred as the memory-core test factory.
• The precedence is unchanged (Unsafe before Netty), but a warning is now logged when several allocation manager types are on the classpath, with the property / environment variable to choose one.
• The inference is logged once, with the Netty / Unsafe name accepted by arrow.allocation.manager.type and the full location:

INFO  o.apache.arrow.memory.CheckAllocator - Allocation manager type not specified, inferred Unsafe from the classpath at file:/.../memory/memory-unsafe/target/classes/org/apache/arrow/memory/unsafe/DefaultAllocationManagerFactory.class

With both Unsafe and Netty on the classpath:

WARN  o.apache.arrow.memory.CheckAllocator - Found allocation manager types [Unsafe, Netty] on the classpath, using Unsafe. Set the arrow.allocation.manager.type system property or the ARROW_ALLOCATION_MANAGER_TYPE environment variable to choose one explicitly

TestCheckAllocator covers the inference with a class loader serving resources from chosen locations.

Closes #1469.

[lidavidm]

LGTM, just needs to be formatted