Bugs caused by lack of `GC.@preserve`ability of pointers to `InineString`

[jakobnissen]

Our saga begin with this bug:

julia> readline(IOBuffer(codeunits(String7("wq"))))
"0;"

Digging down, we hit this bug:

julia> function foo(s)
           GC.@preserve s findfirst(==(0x0a), codeunits(s))
       end
foo (generic function with 1 method)

julia> s = String7("abc\nss\n")
"abc\nss\n"

julia> foo(s) === nothing
true

Which is caused by:

julia> s = String7("abc\nss\n")
"abc\nss\n"

julia> function foo(s)
           u = reinterpret(UInt, s)
           GC.@preserve s begin
               p = pointer(codeunits(s))
               p2 = @ccall memchr(p::Ptr{UInt8}, 0x0a::Cchar, 7::Csize_t)::Ptr{Nothing}
               p2 == C_NULL ? nothing : p2 - p + 1
           end
       end
foo (generic function with 1 method)

julia> foo(s) === nothing
true

Which I believe ultimately is caused by the lack of GC.@preserve in the pointer function.

To check it, let's compare:

julia> function foo(s)
           GC.@preserve s begin
               p = pointer(codeunits(s))
               p2 = @ccall memchr(p::Ptr{UInt8}, 0x0a::Cchar, 7::Csize_t)::Ptr{Nothing}
               p2 == C_NULL ? nothing : p2 - p + 1
           end
       end
foo (generic function with 1 method)

julia> function bar(s)
           r = Ref(bswap(reinterpret(UInt, s)))
           GC.@preserve r begin
               p = Ptr{UInt8}(pointer_from_objref(r))
               p2 = @ccall memchr(p::Ptr{UInt8}, 0x0a::Cchar, 7::Csize_t)::Ptr{Nothing}
               p2 == C_NULL ? nothing : p2 - p + 1
           end
       end
bar (generic function with 1 method)

julia> foo(s)

julia> bar(s)
0x0000000000000004

Note that foo and bar do the same thing - the definition of pointer(codeunits(::InlineString)) is just inlined into bar. The only difference is that in bar, the Ref is GC.@preserved, which it isn't in foo