thread safety issue

[norci]

Julia crashed, when Makie function was called in multiple threads.

reproduce code:

using GLMakie
# scatter(rand(1000),rand(1000)) # julia will crash without this
tasks = map(1:30) do i
    Threads.@spawn scatter(rand(1000),rand(1000))
end
scenes = fetch.(tasks)
display(scenes[1])

the font of the images also has problem.

[SimonDanisch]

What backend is this?

[norci]

What backend is this?

it's CairoMakie

[SimonDanisch]

The Cairo fontrendering itself may not be threadsafe

[norci]

I have tested GLMakie, it has the same problem.

The code still has bug, with my PR JuliaPlots/AbstractPlotting.jl#597

julia> scenes = fetch.(tasks)
ERROR: TaskFailedException:
Can't represent character � with any fallback font nor DejaVu Sans!
Stacktrace:
 [1] error(::String) at .\error.jl:33
 [2] find_font_for_char(::Char, ::FreeTypeAbstraction.FTFont) at C:\Users\x\Documents\GitHub\AbstractPlotting\src\utilities\texture_atlas.jl:169       

I have added repo code in above comment. Could you have a look? thanks.

[SimonDanisch]

Sorry, I don't have time to look into this right now. Making Makie threadsafe is a bigger project than just quickly looking into it.

[gdkrmr]

I had no problem creating heatmaps with Cairomakie using a threaded for loop. GLMakie however crashes when used on more than one thread.

[cossio]

I am running into this issue now.

using CairoMakie
tasks = map(1:30) do i
   Threads.@spawn scatter(rand(1000),rand(1000))
end
scenes = fetch.(tasks)
display(scenes[1])

Note that with CairoMakie I also crash Julia with the above example.

[cossio]

Another example that crashes. Start Julia with julia -t 4. Then:

using CairoMakie
function do_a_plot(name)
  fig = scatter(randn(10), randn(10))
  save(name, fig)
end
@sync for i = 1:5
  Threads.@spawn do_a_plot("/home/cossio/temp/$i $(Threads.threadid()).pdf")
end

signal (11): Segmentation fault
in expression starting at REPL[3]:1
in expression starting at REPL[3]:1
FT_List_Find at /home/cossio/.julia/artifacts/54c97eb1b0a6f74bac96297a815ddec2204a7db7/lib/libfreetype.so (unknown line)
FT_Done_Face at /home/cossio/.julia/artifacts/54c97eb1b0a6f74bac96297a815ddec2204a7db7/lib/libfreetype.so (unknown line)
FT_Done_Face at /home/cossio/.julia/packages/FreeType/aNpmE/src/FreeType.jl:769 [inlined]
safe_free at /home/cossio/.julia/packages/FreeTypeAbstraction/Gphwd/src/types.jl:112
unknown function (ip: 0x7f6440a7bbf7)
FT_List_Find at /workspace/srcdir/freetype-2.10.4/src/base/ftutil.c:255
Segmentation fault (core dumped)

[cossio]

A workaround is to use locks to make sure only one thread plots at a time. Useful if plotting is not the bottleneck.

The following runs fine.

using CairoMakie

const plotting_lock = ReentrantLock()

function do_a_plot(name)
	lock(plotting_lock) do
		fig = scatter(randn(10), randn(10))
		save(name, fig)
	end
end

@sync for i = 1:5
	Threads.@spawn do_a_plot("/home/cossio/temp/$i $(Threads.threadid()).pdf")
end

Thanks @jpsamaroo for clarifying the use of locks here.

(I guess there is only bits of CairoMakie that are not thread-safe. It would be much more efficient to know those and put the lock only there)

[ffreyer]

Seems fixed to me