`Base.unsafe_convert(Ptr, ::Vector}` started throwing on master

[Drvi]

Hey folks,

This used to work prior to 1.11:

julia> Base.unsafe_convert(Ptr{Float64}, Int[0]) 
Ptr{Float64} @0x00000002992d5d00

julia> Base.unsafe_convert(Ptr{Int}, Int[0])
Ptr{Int64} @0x00000002998d5b40

But now it throws:

julia> Base.unsafe_convert(Ptr{Float64}, Int[0])
ERROR: conversion to pointer not defined for Vector{Int64}

julia> Base.unsafe_convert(Ptr{Int}, Int[0])
ERROR: conversion to pointer not defined for Vector{Int64}

To give more context, we do something like:

struct W{Int}
    x::Int
end

SRC_T = Tuple{String,W{Int}}
DST_T = Tuple{String,Int}

x = SRC_T[("a", W(1)), ("b", W(2))]
ptr = Base.unsafe_convert(Ptr{DST_T}, x)
Base.unsafe_wrap(Vector{DST_T}, ptr, length(x))

Which would on 1.9 / 1.10 yield:

julia> ptr = Base.unsafe_convert(Ptr{DST_T}, x)
Ptr{Tuple{String, Int64}} @0x0000000160d55060

julia> Base.unsafe_wrap(Vector{DST_T}, ptr, length(x))
2-element Vector{Tuple{String, Int64}}:
 ("a", 1)
 ("b", 2)

But throws in the unsafe_convert step on master.

Is this expected? This broke our master-tracking CI.

Would a reasonable fix for this be rewriting theunsafe_convert step to

ptr = Base.unsafe_convert(Ptr{DST_T}, pointer(x))

(adding the pointer(...) call)?

Is this fundamentally impossible to make safe and there is a better idiom for this kind of unsafe conversion?

Thanks!

[KristofferC]

Dup of #51962 I think

[vtjnash]

It seems like this is some sort of reinterpret call? It is not well-defined what happens if you call unsafe_wrap on an pointer derived from any existing Julia object. In particular for the example here, any gc_wb would hit the wrong object and lead to memory corruption.

In the particular unsafe_convert example, the docs for ccall state that you must call cconvert first and GC.@preserve the result of that.

[Drvi]

@vtjnash Thanks for explaining! To answer your question, yes, what we try to do here is to have a reinterpret that would work on non-isbits types (tuples very similar to the one I used as an example). The actual code does try to ensure the source object is not outlived by the reinterpreted one -- is the approach fundamentally flawed even then? Is there a better way to achieve that (really just getting from SRC_T[("a", W(1)), ("b", W(2))] to DST_T[("a", 1), ("b", 2)])?

[vtjnash]

The operation is illegal in most programming languages (for example, in C it is undefined behavior), so Julia is no different in that respect. That is why reinterpret only works with cases where it can legally do so, and errors for cases that are invalid. Even just "try to ensure the source object is not outlived" is very complicated, and would require you to implement your own garbage collector (a simple RC one is fine) such that each unsafe call incremented the counter and registered a finalizer which decremented the counter.

[vtjnash]

Note that the GC property you must ensure is not "the source object is not outlived by the reinterpreted one" but "the lifetime of the source object is exactly identical to the lifetime of the reinterpreted one"

[oscardssmith]

specifically because if the reinterpreted one dies first, gc might try to free the reinterpreted one.

[vtjnash]

There is no free on the reinterpreted one. But if the reinterpreted one dies first, the gc may forget to traverse the source one, resulting in part of it getting freed earlier than you expected

[nsajko]

Can't repro anymore. Though I guess the idea is still to deprecate at some point?

[giordano]

I think this was fixed by #53589