[Packaging][Python] Python macOS wheels have dynamic dependency on libcares

[asfimport]

I'm afraid while #4685 fixed the macOS wheels for python 3, but the python 2.7 wheel is still broken (with a different error):

ImportError: dlopen(/Users/pcmoritz/anaconda3/lib/python3.6/site-packages/pyarrow/lib.cpython-36m-darwin.so, 2): Library not loaded: /usr/local/opt/c-ares/lib/libcares.2.dylib

  Referenced from: /Users/pcmoritz/anaconda3/lib/python3.6/site-packages/pyarrow/libarrow_python.14.dylib

  Reason: image not found

I tried the same hack as in #4685 for libcares but it doesn't work (removing the .dylib fails one of the earlier build steps). I think the only way to go forward on this is to compile grpc ourselves. My attempt to do this in master...pcmoritz:mac-wheels-py2 fails because OpenSSL is not found even though I'm specifying the OPENSSL_ROOT_DIR (see https://travis-ci.org/pcmoritz/crossbow/builds/550603543). Let me know if you have any ideas how to fix this!

Reporter: Philipp Moritz / @pcmoritz
Assignee: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #4725

_{Note: This issue was originally created as ARROW-5751. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Try building with -Dc-ares_SOURCE=BUNDLED. That will force using of the static library during linking

[asfimport]

Antoine Pitrou / @pitrou:
The answer is simple: Flight, like Gandiva, isn't supported on Python 2.

[asfimport]

Wes McKinney / @wesm:
You're right, forgot about that.

Also the output is weird:

ImportError: dlopen(/Users/pcmoritz/anaconda3/lib/python3.6/site-packages/pyarrow/lib.cpython-36m-darwin.so, 2): Library not loaded: /usr/local/opt/c-ares/lib/libcares.2.dylib

  Referenced from: /Users/pcmoritz/anaconda3/lib/python3.6/site-packages/pyarrow/libarrow_python.14.dylib

  Reason: image not found

This says Python 3.6, did you actually get the error on py2.7 or py3.6?

[asfimport]

Wes McKinney / @wesm:
I'm taking a brief look at this

[asfimport]

Wes McKinney / @wesm:
I confirmed the problem

$ otool -L /Users/wesm/miniconda/envs/wheel-test-3.7/lib/python3.7/site-packages/pyarrow/libarrow_flight.14.dylib 
/Users/wesm/miniconda/envs/wheel-test-3.7/lib/python3.7/site-packages/pyarrow/libarrow_flight.14.dylib:
	@rpath/libarrow_flight.14.dylib (compatibility version 14.0.0, current version 14.0.0)
	@rpath/libarrow.14.dylib (compatibility version 14.0.0, current version 14.0.0)
	/usr/local/opt/c-ares/lib/libcares.2.dylib (compatibility version 6.0.0, current version 6.0.0)
	/usr/local/opt/openssl/lib/libssl.1.0.0.dylib (compatibility version 1.0.0, current version 1.0.0)
	/usr/local/opt/openssl/lib/libcrypto.1.0.0.dylib (compatibility version 1.0.0, current version 1.0.0)
	/usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 307.5.0)
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1238.50.2)

This can be easily confirmed by installing the wheel from https://github.com/ursa-labs/crossbow/releases/tag/nightly-489-travis-wheel-osx-cp37m

I'll work on a patch

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 4725
#4725