Add a switch command to pgenv

[thanodnl]

The switch command acts mostly like use, however, it does not manage any database for you, it simply changes the version linked and thus in the path of the user.

When developing the citus extension for postgres we often need to manage many different postgres databases running on the same machine (different ports) at once. We have our own tooling to create these databases from the postgres version installed on the path.

By omitting the starting, stopping and initializing of the database we can still use pgenv to manage different versions of postgres on the same machine.

Last years we used a fork of pgenv with some more functionality hacked in. Over time most of that functionality have been better implemented by the pgenv community. The last missing piece for now is a lightweight way of switching which postgres version is on the path without managing a database.

[fluca1978]

This is interesting, but seems to me it should do the very same thing as use without starting the PostgreSQL version (but stopping the one running if any).
Should we implemented with a subcase of use then?

[thanodnl]

I think stopping a database if running makes a lot of sense.

with a subcase of use, do you think about adding a flag (I don't see many flags used within pgenv) or reuse most of the logics from use, simply skipping the starting of the database.

I will push a change with the latter implemented.

EDIT: after implementing the switch as a subcase of use the code became less repetitive, however, use had the behaviour of an error when changing versions while the old version was not running. This error might make sense for use, but less so for switch.

error:

$ pgenv switch 14.2
PostgreSQL 14.1 not running

I don't see an easy way to omit that error message though :(.
@fluca1978 do you have an idea on how to omit the error message while shutting down a non-running version?

[fluca1978]

Uhm, the error is coming from pgenv_stop_or_restart_instance. However such message seems only informative, to warn the user.
What if we change such message from https://github.com/theory/pgenv/blob/master/bin/pgenv#L927:

echo "PostgreSQL $v not running"

to something like:

echo "WARN: PostgreSQL $v not running" >&2

so that it is clear that the message is not an error for pgenv. After all, you do a switch and the program reminds you the system is not running. What do you think?

Then the switch + use branch should look fine.

[thanodnl]

To start with, I am ok with the output as is. Although a bit strange in our workflow, there is nothing problematic with the message.

I would refrain from routing the message to stderr, as it is not really an error. Better to keep it on stdout for now.

That said, it is a bit strange in UX that it would only say something about the old version when switching. Not to say I would want to add more output telling what the new version is. In general I would expect tools to be quiet when things simply work. And only provide information if it needed to course correct to make it work.

eg. instead of logging that postgres wasn't running on the old version I would only tell the user we have detected the old version was running and we are shutting it down. This provides good insights in what the tool was doing, not wat it was not doing. However this would change the normal workflow for people being used to the use command.

Lets merge this as is, and see what others think about its chattyness.

[fluca1978]

Well, I'm fine with removing messages that told "something did not happen" in favor of what "did happen".

I think this can be merged, and imrpove the messaging later. The only doubt I've is if we need to work on messaging before the new release is done. @theory are you okay with this?

[theory]

Maybe emit the message only when in a verbose or debugging mode?

[fluca1978]

@thanodnl would you mind fixing the documentation as suggested and resolving the pending comments, then we can squash and merge.

[thanodnl]

This is interesting, but seems to me it should do the very same thing as use without starting the PostgreSQL version (but stopping the one running if any).
Should we implemented with a subcase of use then?

I don't see any suggestions in this thread, am I simply not seeing them? Or could it be you have unsubmitted comments on the PR?

Once I know what to fix I will do so happily!

[fluca1978]

Uhm, I see a couple of pending comments regarding documentation. It could be that the approval has removed these comments on your side?
By the way, if you don't see any pending comment please advice, I will merge and fix a few things in the documentation.

[thanodnl]

If you see the word pending on the comment like below:

No one, but you, can see the comments. To make them visible you should submit your review from the files tab via the big green button saying Finish Review

[fluca1978]

I would "group" switch with clear and start with use. The former two are related to symlink management, the latters are related to runtime.

[thanodnl]

makes sense to me, I have re-arranged use/start/stop/restart to be together for that reason, and moved, switch/clear below that section.

[fluca1978]

Sorry @thanodnl , my fault! Now you should be able to proceed to changes.
Also please rebase, since I've removed the stop message from the output.

[fluca1978]

Thnaks @thanodnl