Multiple async_executes may not always execute in sequence?

[tanmaykm]

The docstring for async_execute mentions:

LibPQ.jl/src/asyncresults.jl

Lines 203 to 204 in 2c0547d

	Only one `AsyncResult` can be active on a [`Connection`](@ref) at once.
	If multiple `AsyncResult`s use the same `Connection`, they will execute serially.

The actual implementation does this for synchronization in the method:

LibPQ.jl/src/asyncresults.jl

Line 270 in 2c0547d

async_result.result_task = @async lock(jl_conn) do

But it is not clear if that's something that can guarantee the desired behavior. The doubt is based on these questions:

• Are tasks spawned guaranteed to start executing in the order in which they were spawned?
• And are locks guaranteed to be granted in the same order in which they were requested?

In this example below, that is what happens:

julia> using Base.Threads

julia> function test1()
           lck = Threads.Condition()
           val = 10
           @sync begin
               for idx in 1:50
                   @async begin
                       lock(lck) do
                           val = idx
                       end
                   end     
               end
           end
           println("val: $val")
       end
test1 (generic function with 1 method)

julia> test1()
val: 50

julia> test1()
val: 50

But making the task yield randomly makes the result different every time:

julia> function test2()
           lck = Threads.Condition()
           val = 10
           @sync begin
               for idx in 1:50
                   @async begin
                       sleep(rand())
                       lock(lck) do
                           val = idx
                       end
                   end     
               end
           end
           println("val: $val")
       end
test2 (generic function with 1 method)

julia> test2()
val: 45

julia> test2()
val: 8

The question is in absence of any random yield (as in test2), is the execution order (as in test1) something that is guaranteed in Julia?