Add mutex to ensure struct gets initialized once

[Bryan1041]

Running depguard via golangci-lint run on a large number of packages (e.g. over 600) sometimes results in "is not in the allowlist" errors for packages that are in the allowlist.

It looks like Run/initialize is being called by multiple goroutines on a shared struct in golangci-lint, so prefixPackages and possibly other struct members are ending up with duplicate, missing and/or incomplete package names.

This change ensures that the struct gets initialized once on the first call.

[dixonwille]

I'll take a look at this. Curious what performance hit this may incur. Was this a change golangci lint made recently that causes it to call a linter's initializer more than once? I don't recall this being the case when this was initially developed.

[dixonwille]

Thanks for the PR. I am concerned about performance, but as I am working on a V2 for depguard, this shouldn't be an issue moving forward (using go/analysis which is structured to be concurrent access I believe). Also poor planning on my part for initializing during run instead of inside a NewFunc() that handles it before the execution...

[Bryan1041]

@dixonwille Thanks for the review (and the linter). It's been a while since I looked at it, but I don't remember seeing any changes to how plugins are initialized in golangci-lint. I will say that when I tested this, the likelihood of a false positive flag increased with the number of packages being linted and seems to be pretty rare unless you are linting a lot of packages.

Can we create a tag for this? If so, I will open a PR to upgrade the dependency in golangci-lint as a temporary fix until V2.

[dixonwille]

@Bryan1041 just tagged v1.1.1