Comment 2 for bug 1719045

https://golang.org/src/runtime/mgcwork.go around line 306

  // workbuf factory routines. These funcs are used to manage the
  // workbufs.
  // If the GC asks for some work these are the only routines that
  // make wbufs available to the GC.

  func (b *workbuf) checknonempty() {
   if b.nobj == 0 {
    throw("workbuf is empty")
   }
  }

  func (b *workbuf) checkempty() {
   if b.nobj != 0 {
    throw("workbuf is not empty")
   }
  }