Add persistent_term:put_new

[Benjamin-Philip]

This PR adds the persistent_term:put_new to conditionally add a key-value only if the key is new.

Closes #9681.

/cc @jhogberg.

[github-actions]

CT Test Results

    3 files    141 suites   50m 9s ⏱️
1 615 tests 1 560 ✅ 54 💤 1 ❌
2 335 runs  2 259 ✅ 74 💤 2 ❌

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Results for commit 5ec1438.

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// Erlang/OTP Github Action Bot

[josevalim]

Instead of adding put_new, would it be more useful to add a has_key function that could be used in more cases? I recognize put_new has the benefit of being atomic but I assume that the value you need to store is likely expensive to compute anyway, so you want to check if the key exists before you do any work.

[Benjamin-Philip]

I think a has_key would be a good addition, but not as a replacement to put_new. Like you said put_new has the benefit of being atomic which enables one to "correctly" (in a formal sense) update parallely out of the box. See the linked issue for my use case.

You can define a rudimentary has_key already by setting Default in get/2 to undefined or some other more specific atom. Nevertheless, has_key would be faster and less error prone.

TLDR; I think we should add both functions.

[Benjamin-Philip]

@jhogberg, I think this PR is review ready now. Sorry about the delay - I was busy the past 2 weeks...