parts.rb: Use a mutable string

[olleolleolle]

Description

Use an explicitly mutable string, to support RUBYOPT=--enable-frozen-string-literal

I ran into it when trying to get support for that in octokit.rb, and into github-changelog-generator.

https://github.com/octokit/octokit.rb/pull/1354/checks?check_run_id=3298433252

Types of Changes

• Bug fix, perhaps maintenance

Testing

Would it be OK to add the RUBYOPT setting mentioned above to CI? Oh, we don't have a running CI, I logged #71.

[Lewiscowles1986]

I got curious https://ruby-doc.org/core-2.3.0/String.html#method-i-2B-40
Then I made a CI in PR.
Then I manually cherry-pick this commit (using copy paste 😁 )

I Think Ruby 2.0 should be gone, but maybe this is the feedback this needs?

https://github.com/Lewiscowles1986/multipart-post/runs/3305811062?check_suite_focus=true

This library in Travis, was supporting (5 months ago build); Some quite old versions of Ruby.

I saw someone make an interesting commit elsewhere which was to have Ruby 2.5 do something with frozen string literals. I Do not understand this in a robust way. Does this help?

[ioquatix]

I think I slightly prefer String.new to make a mutable buffer. What do you think?

[ioquatix]

Anything older than Ruby 2.3 we definitely don't care about.

Anything older than Ruby 2.6 I'm okay with dropping if it's inconvenient.

[Lewiscowles1986]

I got 2.5-3.0 working with some playing around on a less tired brain
https://github.com/Lewiscowles1986/multipart-post/tree/frozen-stringd

[Lewiscowles1986]

@olleolleolle will very likely have improvements to my takes on their code.

• 2.5 patch is one that mimics their work on octokit.
• I learned about frozen strings and workarounds from them.
• Reading their commits I saw the need for CI.

Now I have green and I understand this better, the only thing I'm a little upset about is that the library requires downstream consumers to "".dup (through whatever means); which is large surface-area for coordination.

Might there be patterns where we can receive the string, and only mutate within this library; so that read is safe. Otherwise it feels like read should be read!. Is the buf name convention enough to say "this is a mutable outvar buffer".
Perhaps I have totally misunderstood.

[olleolleolle]

@Lewiscowles1986 👋 Hi! The --enable-frozen-string-literal is a sort of "nod to the future" where Ruby strings may become immutable. https://freelancing-gods.com/2017/07/27/an-introduction-to-frozen-string-literals.html is a nice blog post where Pat, an activist for "frozen string literals" to happen, describes the feature a little.

Nice to see you again, and I hope you had some fun!