Many Drawable implementations have their own Invalidate and OnInvalidate
functions, with an unexported onInvalidate field. However OnInvalidate and
Invalidate are usually not called in the same goroutine. This results in a race
on this field, e.g.:
Read at 0x00c000094748 by goroutine 7:
git.sr.ht/~sircmpwn/aerc2/widgets.NewDirectoryList.func1()
/home/simon/src/aerc2/widgets/dirlist.go:85 +0x56
git.sr.ht/~sircmpwn/aerc2/widgets.(*Spinner).Start.func1()
/home/simon/src/aerc2/widgets/spinner.go:93 +0x1bb
Previous write at 0x00c000094748 by main goroutine:
[failed to restore the stack]
Goroutine 7 (running) created at:
git.sr.ht/~sircmpwn/aerc2/widgets.(*Spinner).Start()
/home/simon/src/aerc2/widgets/spinner.go:46 +0x8f
git.sr.ht/~sircmpwn/aerc2/widgets.NewDirectoryList()
/home/simon/src/aerc2/widgets/dirlist.go:37 +0x286
git.sr.ht/~sircmpwn/aerc2/widgets.NewAccountView()
/home/simon/src/aerc2/widgets/account.go:50 +0x5ca
git.sr.ht/~sircmpwn/aerc2/widgets.NewAerc()
/home/simon/src/aerc2/widgets/aerc.go:60 +0x800
main.main()
/home/simon/src/aerc2/aerc.go:65 +0x33e
To fix this, introduce a new type, Invalidatable, which protects the field.
Unfortunately the Drawable must be passed to the callback function in
Invalidate, so we still need to re-implement this in each Invalidatable user.
