Recompilation in an editable install hides all compiler output

[Cadair]

This line:

meson-python/mesonpy/_editable.py

Line 346 in 8e98598

subprocess.run(self._build_cmd, cwd=self._build_path, env=env, stdout=subprocess.DEVNULL, check=True)

hides all build output, so if the build fails due to missing build requirements or some other reason, you don't get any indication as to why.

I would expect the compilation output to be shown at import time.

I found this while trying to do an editable install of numpy, and because the initial install of numpy used build isolation the import time recompile did not have the build requirements (cython).

[rgommers]

Does https://mesonbuild.com/meson-python/how-to-guides/editable-installs.html#verbose-mode help?

There might be an improvement to make there, showing the build output by default only if the build fails.

[Cadair]

That documentation page is helpful. It would be helpful for any compilation errors to show up. Apart from build environment issues I encountered, if you are developing the C code and you make a mistake you are going to want to see why!

[lesteve]

Discussing with @Cadair IRL there are at least two things:

• he was not getting an error when using meson editable install in non-verbose mode. If that's the case this seems like a bug. I was involved in Editable install: raise ImportError when rebuilding fails #648 a while ago and I think it was supposed to raise an ImportError when compilation fails? Maybe we are missing some context or this was a bit of an edge case.
• should the default editable be verbose by default? I guess this is a bit more controversial but I lean towards "verbose by default" as well. If you are doing editable install you are likely in "developer mode" and you are probably interested to see some output when things are recompiled (be it only to be reassured something is happening).

[dnicolodi]

• he was not getting an error when using meson editable install in non-verbose mode

This is not what should happen. There is a test that verify that an ImportError is raised

meson-python/tests/test_editable.py

Lines 317 to 346 in 8e98598

	@pytest.mark.parametrize('verbose', [False, True], ids=('', 'verbose'))
	def test_editable_rebuild_error(package_purelib_and_platlib, tmp_path, verbose):
	with mesonpy._project({'builddir': os.fspath(tmp_path)}) as project:
	finder = _editable.MesonpyMetaFinder(
	project._metadata.name, {'plat', 'pure'},
	os.fspath(tmp_path), project._build_command,
	verbose=verbose,
	)
	path = package_purelib_and_platlib / 'plat.c'
	code = path.read_text()
	try:
	# Install editable hooks
	sys.meta_path.insert(0, finder)
	# Insert invalid code in the extension module source code
	path.write_text('return')
	# Import module and trigger rebuild: the build fails and ImportErrror is raised
	stdout = io.StringIO()
	with redirect_stdout(stdout):
	with pytest.raises(ImportError, match='re-building the purelib-and-platlib '):
	import plat # noqa: F401
	assert not verbose or stdout.getvalue().startswith('meson-python: building ')
	finally:
	del sys.meta_path[0]
	sys.modules.pop('pure', None)
	path.write_text(code)

If this is not the case, we would need a way to reproduce the issue to investigate further.

• should the default editable be verbose by default?

The reason for making the rebuild silent by default is that editable installs may be used for packages that install scripts. The output of those scripts becomes unusable when it is intermixed with compilation logging. This problem is more severe because ninja output the compilation log on the standard output and not on the standard error. Someone went to great lengths to suppress any output when there is nothing to be built even when verbose output is enabled, thus there is definitely a group of users that does not want output emitted. What could be done is to redirect the compilation output to a file and display it (or the last N lines of it) when the compilation fails.

[Cadair]

I got an error (the import failed) but there was no indication as to why (in my case I was missing Cython). No ninja output was shown by the import error so I didn't know how to fix it.

[lesteve]

Someone went to great lengths to suppress any output when there is nothing to be built even when verbose output is enabled, thus there is definitely a group of users that does not want output emitted

(I think?) I am one of the user who did not want any output when there is no work to do. I tried to open a PR to do it in #579, which @dnicolodi finished in #579 🙏.

Yet at the same time, where there is an error, I would have liked a quick tip about meson-editable verbose, mostly because I am neither a meson nor meson-python expert 😅 and it took me a while to debug the situation and understand that things were not recompiling as I naively thought. As a scikit-learn maintainer, I am guessing other scientific Python package maintainers may be in a similar boat.

I tried to mention the MESON_EDITABLE_ERROR in the ImportError message in #648 (comment) but @dnicolodi was not a great fan of the approach which I guess is completely understandable.

To be explicit, I am not trying to sway anyone, but mostly summarize the situation at this point 😉.

[rgommers]

It is a complicated UX question given the various usage scenarios plus the issues with different tools emitting content to stdout vs. sterr inconsistently. However, I would say that the main use case - using an editable build for developing a package that uses meson-python in a regular environment - does have the requirement of showing the traceback on a build error.

What could be done is to redirect the compilation output to a file and display it (or the last N lines of it) when the compilation fails.

I like this idea. Perhaps the last N lines + the path to the file (which will be preserved in the build dir) if the user needs the full output?