Fix Issue #2385

[BrandonIrizarry]

What type of PR is this?

bugfix

• bug

What this PR does / why we need it:

Mutual exclusion wasn't being properly enforced when using MutuallyExclusiveFlags.

flag_mutex_test.go

• Introduce a breaking test that illustrates the bug.

flag_mutex.go

• Rewrite function check to pass the breaking test.

Which issue(s) this PR fixes:

Fixes #2385

Testing

I ran make test at the project root.

Release Notes

(REQUIRED)

NONE

[dearchap]

Minor cleanup suggestion: the two scan loops in MutuallyExclusiveFlags.check (the one that finds the group where a flag is set, and the one that scans subsequent groups for a second set flag) are structurally identical. Extracting a small helper, e.g.

func (grp MutuallyExclusiveFlags) findSetFlag(i int) (name string, idx int, ok bool)

that returns the first set flag at or after group index i, would remove the duplication and make the flagGroupLoop labeled-break idiom unnecessary:

name, i, ok := grp.findSetFlag(0)
if ok {
    e.flag1Name = name
    i++
    if name2, _, ok := grp.findSetFlag(i); ok {
        e.flag2Name = name2
        return e
    }
}

Pure refactor, no behavior change. Happy to open a follow-up PR if you prefer.

[BrandonIrizarry]

OK, so this should do the trick. I've added some comments (including a godoc-style comment for 'findSetFlag'); hopefully these are welcome (as I also kind of wanted to vet my understanding of the code I was writing.)