Is `@async` memory safe?

General Usage
8

Consider the following program:

using StructArrays

function f(a, b)
    v = StructVector((a, b))
    @sync begin
        @async push!(v, (1, 2))

        x = @inbounds v[end]
        @show x
    end
end

Is this program well-defined for f(::Vector, ::Vector)? Likely yes. (But there is no public API guaranteeing this is definitely the case.)

Is this program well-defined for f(::AbstractVector, ::AbstractVector)? No. This is because push!(::AbstractVector, 1) may contain an I/O; e.g., reading a file, accessing internet, or even just a @debug statement. In such case, the scheduler may decide to switch to the parent task while executing push!(a, 1) or push!(b, 2) in the child task. Now, in the parent task, the invariance of StructVector (e.g., the underlying arrays have consistent size) can completely be violated and no API can be expected to work.

The semantics of @async and @spawn tasks allow arbitrary interleaving of “pieces of code.” The only difference is that the granularity of the “pieces of code”. In @async, it’s the parts of the code delimited by the yield points (“I/O” operations). In @spawn, it’s individual machine instructions. So, this is why people tend to worry about the correctness of concurrent programs only when @spawn is used. More interleavings are possible and so it increases the possibility that bad things happen. However, since there is nothing in Julia language that makes sure certain function/method call does not contain a yield point [1], it is not correct to assume that manipulating a data structure from multiple @async tasks is safe, unless you read all the code reachable from the functions you are calling. (I guess it sounds rather like a FUD but I feel it is necessary for clarifying the current situation of concurrent programming in Julia.)

[1] In contrast, many other languages such as Python, JavaScript, C++, Rust, … that added coroutines after the language is designed tend to have async/awit keywords which clarify when the coroutine may yield to the scheduler. They have another issue with it though.

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