Fallback to default profile when profiles file is empty

[desertaxle]

This PR adds handling to the profile settings source to load the default ephemeral profile if another profile can't be loaded.

Closes #16101
Closes #16118
supersedes #16112

[codspeed-hq]

CodSpeed Performance Report

Merging #16119 will not alter performance

_{Comparing fix-16101 (bc66125) with main (62b1001)}

Summary

✅ 3 untouched benchmarks

[zzstoatzz]

lgtm!

[dsmfreire]

This doesn't work if, for example, the ephemeral default profile has multiple variables and the user changes a subset of them.

E.g.
default profiles.toml

[profiles.ephemeral]
PREFECT_VAR_1 = "true"
PREFECT_VAR_2 = "true"

~/.prefect/profiles.toml

[profiles.ephemeral]
PREFECT_VAR_1 = "false"

In this case, PREFECT_VAR_2 would evaluate to the settings' class default, not the value from the default profiles.toml file.

Wouldn't it be a better solution to merge the two, overriding defaults with user settings but maintaining the default loaded?

[desertaxle]

Hey @dsmfreire! We want user-defined profiles to take precedence over default profiles and I think that merging user-defined profiles with default profiles has the potential to be more confusing.

We handle users making changes to the default profiles by copying the values for that profile into the user's profiles.toml file when they make their first change via prefect config set.

For example, if I run prefect config set PREFECT_LOGGING_LEVEL="DEBUG" in a brand new install, I end up with a profiles.toml that looks like this:

active = "ephemeral"

[profiles.ephemeral]
PREFECT_SERVER_ALLOW_EPHEMERAL_MODE = "true"
PREFECT_LOGGING_LEVEL = "DEBUG"

where I have ephemeral mode enabled even though I haven't explicitly enabled it via the CLI because that came with the default profile.

This way, the config is visible to the user, which I think is less confusing overall.

I'm happy to discuss this more or other scenarios! I'm going to merge this PR now, so either a new issue or GitHub discussion would be a great place to chat!

[dsmfreire]

Hey @desertaxle! Thank you for the response. I was unaware of the "config set" behavior of copying the defaults to the user's profiles.toml. That's a great solution and way less confusing than what I suggested, so I retract my previous comment. Sorry for the ignorance, and thanks!

[desertaxle]

No worries, thanks for bringing it up! I'm always happy to discuss DX improvements!