There are several places in the gopls codebase that assume a source.Options may be cloned by passing it by value. Perhaps this was once correct, but isn't correct anymore, as for example UserOptions.Env is a slice.

One place where this shows up is in addView, which calls session.Options() and then passes that into session.NewView. By passing around the session.options.UserOptions.Env slice, each view can end up mutating the same underlying array when appending to the slice during fetchConfig.

Another place this manifests is in didChangeConfiguration. This fetchConfig call, which is supposed to just fetch the new view options, actually mutates the existing view options:
https://cs.opensource.google/go/tools/+/master:internal/lsp/workspace.go;l=49;drc=9778d966fa8150a8a0027e8169749bae53a2ee86
Later on, we check if there were any major changes, and fail to detect a UserOptions.Env change:
https://cs.opensource.google/go/tools/+/master:internal/lsp/cache/view.go;l=191;drc=9001f16f58f90e85dd6b3c2b727e3ba0d4f85838

We should explictly clone source.Options before mutating it. Note this would also make it safe to pass around the (quite large) options type by reference, and/or make it opaque.

cc @stamblerre @heschik