CORS: the middleware needlessly compiles the same regexps during each of its invocations

[jub0bs]

Issue Description

The CORS middleware currently recompiles the same regexps over and over during each of its invocations. However, for performance, compiling regexps on the hot path should be avoided when possible.

In this case, the regexps could be compiled once and for all during middleware initialisation. The following line

allowOriginPatterns := []string{}

could advantageously be replaced by

var allowOriginRegexp []*regexp.Regexp

and the rest of the necessary changes would flow from this simple change.

Checklist

• Dependencies installed
• No typos
• Searched existing issues and docs

Expected behaviour

No regexp compilation on the hot path.

Actual behaviour

Regexp compilation on the hot path.

Version/commit

v4.12.0

[aldas]

Thanks for noticing. #2709 reworks that part