arc_summary: prefer python3 version and install when there is no python

[eli-schwartz]

Motivation and Context

Fixes installation in a minimal build chroot without python available.

Description

This matches the behavior of other python scripts, such as arcstat and dbufstat, which are always installed but whose install-exec-hook actions will simply touch up the shebang if a python interpreter was configured and that interpreter is a python2 interpreter.

How Has This Been Tested?

When configuring zfs with --enable-pyzfs=no, and with --with-python="$PWD/python3-fake" in order to fool the configure script into thinking python is available in order to work around #8731, the arc_summary script was not installed, while arcstat was installed (and used a python3 shebang). With this change, they are both installed, with a python3 shebang.

Tested in a minimal Arch Linux chroot without python available.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commit messages are properly formatted and contain Signed-off-by.

[codecov]

Codecov Report

Merging #8851 into master will increase coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #8851      +/-   ##
==========================================
+ Coverage   78.81%   78.96%   +0.14%     
==========================================
  Files         382      382              
  Lines      117771   117771              
==========================================
+ Hits        92821    92997     +176     
+ Misses      24950    24774     -176

Flag	Coverage Δ
#kernel	79.45% <ø> (+0.07%)	⬆️
#user	67.95% <ø> (+0.37%)	⬆️

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[behlendorf]

@freqlabs would you mind reviewing this change related to #8731.

[ghost]

With #8731 merged, none of the Python scripts will be installed when ZFS is configured without Python, and you won't be able to fool the configure script anymore. I think the most straightforward solution is to have the desired version of Python installed when building ZFS if you intend to install the Python scripts.

[eli-schwartz]

Hmm, I missed that that makes SUBDIRS conditional.

[eli-schwartz]

Actually, this works fine:

./configure --with-python=$PWD/python3-fake PYTHON=$PWD/python3-fake --enable-pyzfs=no

[behlendorf]

As I understand it the intent here is to default to installing the python3 version when a non-standard python version has be specified. That seems entirely reasonable to me since it's most likely to be correct version and won't cause any harm.

[ghost]

@eli-schwartz I get this error with that command:
configure: error: Cannot find /home/ryan/ZoF/python3-fake in your system path

[eli-schwartz]

That comes from the pyzfs code, though. (At least the error message is from AX_PYTHON_DEVEL which is part of config/always-pyzfs.m4)

[ghost]

I ran exactly the command you gave. This comes from on line 72 in config/always-python.m4. Make sure you are testing with the commit that was merged for #8731

[behlendorf]

Without this change applied, and assuming the $PWD/python3-fake exists and is executable; then I can confirm that arc_summary does not get installed (but arcstat and dbufstat do).

So while I think it would be best to build with Python installed in the environment, if a fake version is specified we do still want what's installed to be consistent. So this change makes sense to me. For example, with this change applied:

$ touch $PWD/python3-fake
$ chmod 775 $PWD/python3-fake
$ ./configure --with-python=$PWD/python3-fake PYTHON=$PWD/python3-fake --enable-pyzfs=no
$ make
$ DESTDIR=/var/tmp/zfs-install make install
$ ls /var/tmp/zfs-install/usr/local/bin/
arcstat  arc_summary  dbufstat  raidz_test  zgenhostid

[ghost]

I am working on a solution that removes the need to have Python at configure time when using --disable-pyzfs. The goal is to allow for example --with-python=python2 or --with-python=python3.4 and only check the version number in the string, instead of trying to find that actual executable to get a version number. That removes the need to create a dummy python executable just to trick the configure script. In doing so, I make sure that PYTHON_VERSION is set in such a way that the existing tests against that variable will work. I am planning to do the PR for this in the next few days.

I have no objection to this PR, if the idea was to have created a dummy python executable beforehand, which I had not done when I tested it.