First I'll show you the symptoms, then I'll explain what I did to shootmyself in the foot:

# zpool create tank01 c0t38d0# zpool create tank02 c0t39d0# zpool replace tank01 c0t38d0 c0t42d0

(Time passes, the resilver finishes, and zpool status tank01 shows nouse of c0t38d0.)

# zpool attach

There's a lot of times where a function wants to return more than aplain boolean; for example, validation routines often want to returnsome sort of explanation of the validation error (if there is one).At the same time, the most natural way to use the function is oftenin the flow of a conditional:

if blah(o):    ....elif not validates(o):    # whoops, lost the error reason

There's a variety of traditional answers to this,