Build fails with empty pg_config --cppflags output

[jchampio]

Before opening this ticket, please confirm that:

• I am running the latest version of pip, i.e. typing pip --version you get this version.
• I have read the installation documentation and the frequently asked questions
• If install failed, I typed pg_config on the command line and I obtained an output instead of an error.

Please complete the following information:

• OS: FreeBSD, macOS
• Psycopg version: 2.9.6
• Python version: 3.11
• PostgreSQL version: 16beta
• pip version: 23.2

---

I'm building psycopg2 against a Meson build of PG16 beta. On the BSDs, this results in the following error:

Collecting psycopg2~=2.9.6 (from -r /Users/admin/pgsql/src/test/python/requirements.txt (line 7))
  Downloading psycopg2-2.9.6.tar.gz (383 kB)
     ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 384.0/384.0 kB 13.7 MB/s eta 0:00:00
  Preparing metadata (setup.py): started
  Preparing metadata (setup.py): finished with status 'error'
stderr:
  error: subprocess-exited-with-error
  
  × python setup.py egg_info did not run successfully.
  │ exit code: 1
  ╰─> [21 lines of output]
      /Users/admin/pgsql/build/python_venv/lib/python3.11/site-packages/setuptools/config/setupcfg.py:293: _DeprecatedConfig: Deprecated config in `setup.cfg`
      !!
      
              ********************************************************************************
              The license_file parameter is deprecated, use license_files instead.
      
              By 2023-Oct-30, you need to update your project and remove deprecated calls
              or your builds will no longer be supported.
      
              See https://setuptools.pypa.io/en/latest/userguide/declarative_config.html for details.
              ********************************************************************************
      
      !!
        parsed = self.parsers.get(option_name, lambda x: x)(value)
      running egg_info
      creating /private/var/folders/s0/b8snf7qj1479hhmzr90n5s4m0000gn/T/pip-pip-egg-info-u8mn3a2m/psycopg2.egg-info
      writing /private/var/folders/s0/b8snf7qj1479hhmzr90n5s4m0000gn/T/pip-pip-egg-info-u8mn3a2m/psycopg2.egg-info/PKG-INFO
      writing dependency_links to /private/var/folders/s0/b8snf7qj1479hhmzr90n5s4m0000gn/T/pip-pip-egg-info-u8mn3a2m/psycopg2.egg-info/dependency_links.txt
      writing top-level names to /private/var/folders/s0/b8snf7qj1479hhmzr90n5s4m0000gn/T/pip-pip-egg-info-u8mn3a2m/psycopg2.egg-info/top_level.txt
      writing manifest file '/private/var/folders/s0/b8snf7qj1479hhmzr90n5s4m0000gn/T/pip-pip-egg-info-u8mn3a2m/psycopg2.egg-info/SOURCES.txt'
      Error: b''
      [end of output]
  
  note: This error originates from a subprocess, and is likely not a problem with pip.

That Error: b'' line is coming from

psycopg2/setup.py

Lines 122 to 123 in 333b3b7

	if not result:
	raise Warning(pg_config_process.stderr.readline())

which is treating an empty output from pg_config as an error.

But in this case it's not an error -- pg_config --cppflags is legitimately empty on this platform and build configuration -- so there's no stderr message. It looks like maybe the query() helper should be looking at the process returncode instead, or maybe differentiating queries where an empty response is acceptable from queries where it's not?

[dvarrazzo]

Can you propose a patch? Thank you!