Should `run` mark streams as not-writable after using them?

[ericphanson]

julia> io=IOBuffer()
IOBuffer(data=UInt8[...], readable=true, writable=true, seekable=true, append=false, size=0, maxsize=Inf, ptr=1, mark=-1)

julia> iswritable(io)
true

julia> isopen(io)
true

julia> run(pipeline(`echo "hi"`, stdout=io))
Process(`echo hi`, ProcessExited(0))

julia> iswritable(io) # this is the interesting part
false

julia> isopen(io)
true

came up in JuliaLang/Pkg.jl#3430 (comment)

[vtjnash]

IOBuffer is supposed to be disallowed here entirely, but I keep being too busy to implement the error #46768

[ericphanson]

in that case, could you comment in JuliaLang/Pkg.jl#3430 with how Pkg.test(; io) should work? (it internally calls run twice, directing stdout to io). It sounds like it is meant to support IOBuffer at least.