Cross compiling a Rust application that calls Python to armv7 does not seem to work

[tropxy]

Bug Description

Hi,

I am trying to cross compile a Rust app that does a simple Hello python print call. This, however, seems to be not that easy.
Using Docker, I cross compiled python3.9 to arm and then I use this interpreter, together with cargo to build the application to the desired target, but it does not work. Here is the output:

error: failed to run custom build command for `pyo3 v0.15.1`

Caused by:
  process didn't exit successfully: `/target/debug/build/pyo3-05cbb6ee27470b67/build-script-build` (exit status: 1)
  --- stdout
  cargo:rerun-if-env-changed=PYO3_CROSS
  cargo:rerun-if-env-changed=PYO3_CROSS_LIB_DIR
  cargo:rerun-if-env-changed=PYO3_CROSS_PYTHON_VERSION
  cargo:rerun-if-env-changed=_PYTHON_SYSCONFIGDATA_NAME
  cargo:rerun-if-env-changed=PYO3_PYTHON
  cargo:rerun-if-env-changed=PYO3_PRINT_CONFIG

  --- stderr
  error: The `auto-initialize` feature is enabled, but your python installation only supports embedding the Python interpreter statically. If you are attempting to run tests, or a binary which is okay to link dynamically, install a Python distribution which ships with the Python shared library.

  Embedding the Python interpreter statically does not yet have first-class support in PyO3. If you are sure you intend to do this, disable the `auto-initialize` feature.

  For more information, see https://pyo3.rs/v0.15.1/building_and_distribution.html#embedding-python-in-rust
warning: build failed, waiting for other jobs to finish...
error: build failed
make: *** [build] Error 101

I used the PYO3_CROSS_LIB_DIR to link it to the dir where libpython is present.

For convenience, you can check the following repo, which includes my application:
https://github.com/tropxy/py2rust

Steps to Reproduce

1. cross compile Python3.9 to armv7 (make setup)
2. cross compile the rust application to arm (make build)

Backtrace

No response

Your operating system and version

Darwin Kernel Version 20.6.0 x86_64

Your Python version (python --version)

python 3.9.9

Your Rust version (rustc --version)

rustc 1.58.1 (db9d1b20b 2022-01-20)

Your PyO3 version

0.15.1

How did you install python? Did you use a virtualenv?

I have used Docker

Additional Info

No response

[tropxy]

Hi again,

I was able to find what I did wrong and what I was missing: I added the --enable-shared flag to the python cross compilation ./configure and I also removed configurations targeting MacOS from the .cargo/config and Cargo.toml (that I added by mistake).
Finally, I had to explicitly set the LD_LIBRARY_PATH to point to the same location as the PYO3_CROSS_LIB_DIR, where the libpython3.9.so.1.0 shared lib is.

Now the code successfully compiles and I was able to test it in my armv7 board ;)
I updated my repo with the end solution.

The only question that remains for me is: Why is it required to set the LD_LIBRARY_PATH? I thought the PYO3_CROSS_LIB_DIR which points to the same dir, would be enough, plus I couldnt find in the docs any reference to this issue.

[davidhewitt]

Hi, sorry it's taken me a couple of days to reply. I'd love for us to cover more documentation around this topic (see #2017). --enable-shared is briefly mentioned at https://pyo3.rs/v0.15.1/building_and_distribution.html#embedding-python-in-rust, however not very well!

As for LD_LIBRARY_PATH, what error did you get without that environment variable set? As far as I know, that wouldn't affect building your project, but could be needed to enable it to run (see https://pyo3.rs/v0.15.1/building_and_distribution.html#dynamically-embedding-the-python-interpreter)