Refactor Ubuntu Focal gcc-10 setup and update related usage instructions.

[kirkrodrigues]

Description

In #131, we made GCC v10+ a requirement. Since the default version of GCC on Ubuntu Focal is v9, #131 also added set up steps to configure v10 as the compiler on Focal. However, these steps were flawed:

• They were configuring gcc and g++ as the names for the alternatives whereas Ubuntu usually sets cc and c++ as the names.
  • This is why we had to add cc and c++ in Reduce the size of the clp container images. #288.
• Adding the alternatives was done as a configuration step rather than as part of the installation.
• The priority of the added alternatives was set to 10/20 rather than something low that wouldn't affect any alternatives the user already had installed and prioritized.
• They were not configuring slave links for the man pages (something the Ubuntu GCC package maintainers usually do).
  • To investigate the alternatives created by the package maintainers, you can use update-alternatives --query <program> where <program> is one of cc, c++, or cpp.

This PR addresses these limitations:

• We create low-priority alternatives for cc, c++, and cpp (the same alternatives created by the Ubuntu GCC package maintainers, except for GCC v10) as part of the dependency install scripts.
  • We no longer create alternatives for gcc and g++.
• The configuration setup steps only switch to the alternative.

Validation performed

• Validated that workflow for building and testing the Ubuntu core deps container image succeeded.
• Built and ran the container image locally.
• Validated that cc --version, c++ --version and /lib/cpp --version all print a v10 version string.
  • We use /lib/cpp based on the alternative that the Ubuntu gcc package usually creates (see ls /etc/alternatives).
• Validated that building clp showed that it was using v10.

[junhaoliao]

In a Ubuntu Focal distribution where package gcc is not installed, /usr/bin/x86_64-linux-gnu-gcc does not exist, which can cause task "package" to fail when it pip installs mariadb.

Building wheels for collected packages: mariadb
  Building wheel for mariadb (pyproject.toml) ... error
  error: subprocess-exited-with-error

  × Building wheel for mariadb (pyproject.toml) did not run successfully.
  │ exit code: 1
  ╰─> [20 lines of output]
      running bdist_wheel
      running build
      running build_py
      creating build
      creating build/lib.linux-x86_64-cpython-38
      creating build/lib.linux-x86_64-cpython-38/mariadb
      copying mariadb/__init__.py -> build/lib.linux-x86_64-cpython-38/mariadb
      copying mariadb/__init__.py -> build/lib.linux-x86_64-cpython-38/mariadb
      creating build/lib.linux-x86_64-cpython-38/mariadb/constants
      copying mariadb/constants/__init__.py -> build/lib.linux-x86_64-cpython-38/mariadb/constants
      copying mariadb/constants/CLIENT.py -> build/lib.linux-x86_64-cpython-38/mariadb/constants
      copying mariadb/constants/INDICATOR.py -> build/lib.linux-x86_64-cpython-38/mariadb/constants
      copying mariadb/constants/CURSOR.py -> build/lib.linux-x86_64-cpython-38/mariadb/constants
      copying mariadb/constants/FIELD_TYPE.py -> build/lib.linux-x86_64-cpython-38/mariadb/constants
      running build_ext
      building 'mariadb._mariadb' extension
      creating build/temp.linux-x86_64-cpython-38
      creating build/temp.linux-x86_64-cpython-38/mariadb
      x86_64-linux-gnu-gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -g -fwrapv -O2 -fPIC -DPY_MARIADB_MAJOR_VERSION=1 -DPY_MARIADB_MINOR_VERSION=0 -DPY_MARIADB_PATCH_VERSION=11 -I/usr/include/mariadb -I/usr/include/mariadb/mysql -I./include -I/home/junhao/workspace/clp/build/package-venv/include -I/usr/include/python3.8 -c mariadb/mariadb.c -o build/temp.linux-x86_64-cpython-38/mariadb/mariadb.o -DDEFAULT_PLUGINS_SUBDIR=\"/usr/lib/x86_64-linux-gnu/mariadb19/plugin\"
      error: command 'x86_64-linux-gnu-gcc' failed: No such file or directory
      [end of output]

  note: This error originates from a subprocess, and is likely not a problem with pip.
  ERROR: Failed building wheel for mariadb
Failed to build mariadb
ERROR: Could not build wheels for mariadb, which is required to install pyproject.toml-based projects

After we install gcc-10, only /usr/bin/x86_64-linux-gnu-gcc-10 exists. Doing sudo ln -s /usr/bin/x86_64-linux-gnu-gcc-10 /usr/bin/x86_64-linux-gnu-gcc can get the pip install to succeed without seeing any compilation error; however, it might not be the best approach, given gcc-10 is not the version used in the Ubuntu Focal toolchain (i.e. Python 3.8 + GCC 9 + ...). Maybe in some future PR, we can add sudo apt install gcc into the non-core dep setup scripts.

[kirkrodrigues]

After we install gcc-10, only /usr/bin/x86_64-linux-gnu-gcc-10 exists. Doing sudo ln -s /usr/bin/x86_64-linux-gnu-gcc-10 /usr/bin/x86_64-linux-gnu-gcc can get the pip install to succeed without seeing any compilation error; however, it might not be the best approach, given gcc-10 is not the version used in the Ubuntu Focal toolchain (i.e. Python 3.8 + GCC 9 + ...). Maybe in some future PR, we can add sudo apt install gcc into the non-core dep setup scripts.

Yep, I agree. In the upcoming PR to automate the package build, we'll have a container + scripts which install the normal gcc toolchain so that Python packages with C extensions can be installed without issue.

[junhaoliao]

Tested on a clean WSL Ubuntu 20.04.

How about

Refactor Ubuntu Focal gcc-10 setup and update related usage instructions.