[ruby-core:111029] [Ruby master Feature#19090] Do not duplicate an unescaped string in CGI.escapeHTML

Issue #19090 has been updated by Eregon (Benoit Daloze).


Right, I forgot CGI is a default gem too.
I think it seems cleaner for other template engines (e.g. haml, slim, etc) to depend (as in require "cgi") on CGI vs depending on ERB, i.e. CGI feels smaller/like a subset of ERB. In other words, it doesn't seem ideal for other template engines to depend on the ERB template engine just for escaping (although practically speaking there is no issue, just from a design perspective).

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Feature #19090: Do not duplicate an unescaped string in CGI.escapeHTML
https://bugs.ruby-lang.org/issues/19090#change-100282

* Author: k0kubun (Takashi Kokubun)
* Status: Closed
* Priority: Normal
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## Proposal
Stop guaranteeing that `GGI.escapeHTML` returns a new string even if there's nothing to be escaped.

More specifically, stop calling this `rb_str_dup` https://github.com/ruby/cgi/blob/v0.3.3/ext/cgi/escape/escape.c#L72 for the case that nothing needs to be escaped.

## Background
My original implementation https://github.com/ruby/ruby/pull/1164 was not calling it. The reason why `rb_str_dup` was added was that [Bug #11858] claimed returning the argument object for non-escaped cases is a backward incompatibility because the original `gsub`-based implementation always returns a new object. As a result, even while many people use `CGI.escapeHTML` as an optimized implementation for escaping HTML today, it ended up having a compromised performance.

## Motivation
The motivation is to improve performance. By just doing so, escaping a pre-allocated `"string"` becomes 1.34x faster on my machine https://gist.github.com/k0kubun/f66d6fe1e6ba821e4263257e504ba28f.

The most major use case of `CGP.escapeHTML` is to safely embed a user input. When the result is just embedded in another string, the allocated new object will be just wasted. It's pretty common that an embedded string fragment doesn't contain any of `'"&<>` characters. So we should stop wasting that to optimize that case.

[Bug #11858] wasn't really a use case but just "I think this is backward incompatibility" based on frozen Hello World. Unlike user input, you usually don't need to escape your own string literal. It feels like the ticket addressed a problem that doesn't exist in actual applications. It should have cited existing code that could be broken by that, and I can't find such code with `gem-codesearch` today.

The only reason to maintain the current behavior would be to allow using a return value of `CGI.escapeHTML` as a buffer for creating another longer string starting with the escaped value, but using `CGI.escapeHTML` to initialize a string buffer feels like an abuse. Relying on the behavior never makes sense as an "optimization" either because it makes all other cases (the result is not used as a string buffer) suboptimal.

## Why not an optional flag like `CGI.escapeHTML(str, dup: false)`?
Two reasons:

* The non-dup behavior should be used 99.999..9% of the time. We shouldn't make code using `CGI.escapeHTML` less readable just for maintaining a use case that doesn't exist.
* Passing keyword arguments to a C extension is unfortunately slow, and it defeats the optimization purpose. In core classes, we could use `Primitive` to address that, but this is a default gem and we can't use that.
  * We could workaround that if we choose `CGI.escapeHTML(str, false)`, but again it'd spoil the readability for maintaining an invalid use case.

## Why not a new method?

It's a good idea actually, but with `escapeHTML`, `escape_html`, and `h` aliased to it already, I can't think of a good name for it. And again, not calling it `escapeHTML` or `escape_html` would spoil the readability for no valid reason.



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