stackoverflow in type inference caused by handling a StackOverflow exception

[NHDaly]

We saw this rather comical exception pop up in our production cloud deployment:

Internal error: stack overflow in type inference of close(Base.Channel{Bool}, StackOverflowError).

As described above, this comes from a place in our code that performs a generic operation like this:

function http_handler()
    result = Channel()
    do_stuff()
    return_result_or_report_error(result)
end

# ... much later, after much processing on the server  ...
function do_stuff_inner_function(result_channel)
    try
        put!(result_channel, user_func())
    catch e
        close(result_channel, e)
    end
end

In this case, the try block happened to execute when the stack was very nearly full, so the user_func() threw a StackOverflowException. This is perfectly fine -- the code is set up to handle exceptions, and so the result should now contain the StackOverflowException, which, once everything unwinds, could be reported at the top level http handler.

However, what actually happens is the close(::Channel, ::StackOverflowExeption) triggers type inference, which causes another stack overflow! 😅

---

Looking at that function, i'm pretty sure that it the exception argument should be marked @nospecialize, since the generated code is going to be identical no matter what: c.excp is type Union{Exception, Nothing}, and all the other functions it calls with excp are already @nospecialize:

julia/base/channels.jl

Lines 186 to 198 in 2d14390

	function close(c::Channel, excp::Exception=closed_exception())
	lock(c)
	try
	c.excp = excp
	@atomic :release c.state = :closed
	notify_error(c.cond_take, excp)
	notify_error(c.cond_wait, excp)
	notify_error(c.cond_put, excp)
	finally
	unlock(c)
	end
	nothing
	end

[NHDaly]

To add some more detail: looking at our stacktrace, we can see that the dynamic dispatch was directly on the call to close().

Internal error: stack overflow in type inference of close(Base.Channel{Bool}, StackOverflowError).
This might be caused by recursion over very long tuples or argument lists.
Internal error: encountered unexpected error during compilation of close:
StackOverflowError()
emit_expr at /build/source/src/codegen.cpp:4630
emit_call at /build/source/src/codegen.cpp:3812
...
ijl_apply_generic at /build/source/src/gf.c:2549
_set_exception at src/<internal-file>.jl:290

[NHDaly]

BTW here's a simple reproducer:

julia> ch = let result = Channel()

       foo() = try
           foo()
       catch e;
           close(result, e)
       end

       foo()

       result
       end
Internal error: stack overflow in type inference of close(Base.Channel{Any}, StackOverflowError).
This might be caused by recursion over very long tuples or argument lists.
Internal error: encountered unexpected error during compilation of close:
StackOverflowError()
Internal error: stack overflow in type inference of close(Base.Channel{Any}, StackOverflowError).
This might be caused by recursion over very long tuples or argument lists.
Internal error: encountered unexpected error during compilation of close:
StackOverflowError()
_ZL9emit_exprR12jl_codectx_tP11_jl_value_tl at /Users/nathandaly/builds/julia-1.8/usr/lib/libjulia-codegen.1.8.dylib (unknown line)
_ZL9emit_exprR12jl_codectx_tP11_jl_value_tl at /Users/nathandaly/builds/julia-1.8/usr/lib/libjulia-codegen.1.8.dylib (unknown line)