On Wed, Jun 02, 2021 at 10:21:43AM +0200, Sasi Olin wrote:

On Wed, Jun 2 2021 at 09:40:40 +0200, Adam Majer wrote:

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The third issue is packaging this mess. How many times have any of you made an error with update-alternatives that then broke an update and became almost impossible to fix without further scriptlet hacks? It's fragile, to say the least. The system is also changed during installation and not at runtime -- this makes it potentially inconsistent between snapshots.

It was suggested to handle this as an abstraction in rpm specs, with update-alternatives being handled with rpmdb, which is probably the method I would like the most https://github.com/rpm-software-management/rpm/issues/993

Scriplets are an awful way to deal with this, I agree, but I think managing this through rpm is a better way to go

Yes, this is a way of avoiding the scriptlet issue but it doesn't address other issues. The issue with inconsistent snapshots remains especially for transactional updates. It's a little funny how update-alternatives became entrenched and how many packages are using it now. - Adam

Am 02.06.21 um 10:26 schrieb Jan Engelhardt:

python3 would hardly exec /usr/bin/python (v2). They're still sane.

Many ignorant python scripts not strictly adhering to the PEP 394 recommendations [1] unfortunately do. /usr/bin/python pointing to python v2 instead of python3 is a distro choice, not a Python convention. There is a reason why we have thousands of "replace the script interpreter" sed calls in our python package specfiles. [1] https://www.python.org/dev/peps/pep-0394/#for-python-script-publishers

On Wed, Jun 02, 2021 at 10:50:11AM +0200, Jan Engelhardt wrote:

On Wednesday 2021-06-02 10:45, Ben Greiner wrote:

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Am 02.06.21 um 10:26 schrieb Jan Engelhardt:

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python3 would hardly exec /usr/bin/python (v2). They're still sane.

Many ignorant python scripts

Ignorant scripts are one thing. Ignorant node runtimes is another level.

Well, I kind of detect a bias? Here's a naive python example, f133:~ # cat worker.py #!/usr/bin/python3 import platform print(platform.python_version()) f133:~ # cat daemon.py #!/usr/bin/python3 import subprocess subprocess.Popen(['./worker.py']).wait() f133:~ # python3. python3.6 python3.6m python3.8 python3.9 f133:~ # python3.6 ./daemon.py 3.8.10 f133:~ # python3.6 ./worker.py 3.6.13 So, with node, if you did this experiment, and you started it with node10, you would get node10 in the worker irrespective what /usr/bin/node points to. Basically, once we allow multiple interpreters to be co-installed, they actually should run as expected. No? Saying the problem is "out of scope" tells me that something is abused for purposes that were not intended (looking at update-alternatives here). Anyway, this is an example of why pure symlinks are bad and actually result in confusion at runtime. - Adam

On Wed, Jun 2, 2021 at 2:22 PM Ben Greiner wrote:

Am 02.06.21 um 14:52 schrieb Adam Majer:

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On Wed, Jun 02, 2021 at 10:50:11AM +0200, Jan Engelhardt wrote:

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On Wednesday 2021-06-02 10:45, Ben Greiner wrote:

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Am 02.06.21 um 10:26 schrieb Jan Engelhardt:

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python3 would hardly exec /usr/bin/python (v2). They're still sane.

Many ignorant python scripts

Ignorant scripts are one thing. Ignorant node runtimes is another level.

Well, I kind of detect a bias? Here's a naive python example,

f133:~ # cat worker.py #!/usr/bin/python3

import platform print(platform.python_version())

f133:~ # cat daemon.py #!/usr/bin/python3

import subprocess

subprocess.Popen(['./worker.py']).wait()

f133:~ # python3. python3.6 python3.6m python3.8 python3.9 f133:~ # python3.6 ./daemon.py 3.8.10 f133:~ # python3.6 ./worker.py 3.6.13

So, with node, if you did this experiment, and you started it with node10, you would get node10 in the worker irrespective what /usr/bin/node points to.

Basically, once we allow multiple interpreters to be co-installed, they actually should run as expected.

But how do you know what "is expected"? In the Python case we can only assume that PEP 394 is the de-facto standard:

---PEP 394--- - When reinvoking the interpreter from a Python script, querying sys.executable to avoid hardcoded assumptions regarding the interpreter location remains the preferred approach. ---PEP 394---

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No? Saying the problem is "out of scope" tells me that something is abused for purposes that were not intended (looking at update-alternatives here).> Anyway, this is an example of why pure symlinks are bad and actually result in confusion at runtime.

Your example does not involve u-a at all. We don't put the python3 command under u-a control. We even go out of our way and fix the "ignorant scripts", who install wrong script interpreter lines and do something to the extent of replacing `subprocess.call("./worker.py")` or `subprocess.call("python", "./worker.py")` with `subprocess.call(sys.executable, "./worker.py")`.

Am 02.06.21 um 10:46 schrieb Adam Majer:

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python3 exec python3, of course. But what happens if you have python3.8 and python3.10 installed and want your app to use python3.8? That app then starts a child process and you are in python3.10 then?

If you know you want python3.8, then call python3.8. If the app depends on the same python version as it is called on the toplevel call, it MUST use `sys.executable`. `"/usr/bin/python3"` is always the current TW primary python3 flavor and all dependencies for the script calling it must be provided in that flavor.

When Python setuptools installs "entrypoints" it takes care of the correct script interpreter automatically. In the case of multiple flavors, this is overwritten by `%python_clone -a %{buildroot}%{_bindir}/myentrypoint` creating the u-a symlinks and adjusting the script interpreter lines.

Any python package, which installs commands into %{_bindir} must either be for the primary python3 flavor only or the handling of different flavors must be taken care of by u-a and adjusted script interpreter lines as described above.

Apparently Node.js is more lenient on different flavors. But as you describe, this calls for problems with u-a.

Why are we using alternatives with Python anyway? Why not just use the swappable package strategy for `/usr/bin/python` and `/usr/bin/python3`? -- 真実はいつも一つ！/ Always, there's only one truth!

On Wed, Jun 02, 2021 at 10:26:24AM +0200, Jan Engelhardt wrote:

On Wednesday 2021-06-02 09:40, Adam Majer wrote:

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symlinks -- they only really allow a SINGLE choice to be preferred.

It so happens that any ordering of elements in a set implies that one is the first.

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But, what happens when you run `node14` and then that program re-execs #!/usr/bin/node in a subprocess? The subprocess runs node16 and you are out of luck.

exec is a willfull boundary. (Just like /usr/libexec and /usr/bin.) Programs using these mechanisms _must not_ care about changes in architecture, language, memory layout, etc. nodejs's behavior sounds incredibly stupid.

Well, it needs to exec g++ and python3 - that's the GYP, build system. This in turns needs to find parameters, which is from node process and you end up with wrong one.

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I expect the same issue exists with ruby and python3 and other interpreters

python3 would hardly exec /usr/bin/python (v2). They're still sane.

python3 exec python3, of course. But what happens if you have python3.8 and python3.10 installed and want your app to use python3.8? That app then starts a child process and you are in python3.10 then?

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The second issue that comes up is inability to actually manage preferences on a user level. Imagine you have all editors installed on a multi-user system. The user cannot specify their preferences bypass update-alternatives and resort to manual symlink hackery in ~/.bin or similar. [The plan is] moving the logic into a library. The /usr/bin/node would link to it and exec the preferred node version based on user preferences

Making /usr/bin/editor a program that painstakingly loads a ton of ELF libraries, parses config files is such a bad idea. Both

export EDITOR=/usr/bin/joe alias editor='$EDITOR' export PAGER=/usr/bin/less

or

PATH="$HOME/bin:$PATH" ln -s /usr/bin/joe ~/bin/editor

exec the "right" program, "right away" (after perhaps a search through PATH - it's still a _lot_ less syscalls).

Not sure editor cares about syscall overhead. But yes, there is about 3 or 4 syscalls + the exec(). Starting actual interpreters like bash is a lot more overhead. - Adam