Genie.Renderer.Json causes world age issues and instability in code it should not affect

[mrarat]

Describe the bug
Genie causes repeated recompilation problems, increases allocation count and makes it non-deterministic in code it should not affect e.g. OrdinaryDiffEq solvers. Furthermore, recompilations caused by Genie have extremely high values e.g. 5000%.

It seems that version 5.35.7 made it worse, but the issue still persists on 5.35.6.

To reproduce
Below is MWE code that reproduces the issue:

# Minimal Working Example: Genie 5.35.7 causes ODE solver recompilation
#
# After warming up an ODE solve (QNDF + KLUFactorization), subsequent calls
# show 0% compilation time as expected.  After `import Genie.Renderer.Json: json`,
# the very next ODE solves shows recompilation and increased allocations.
#
# Root-cause hypothesis
# ---------------------
# Genie 5.35.7 added in src/JSONParser.jl (commit 7add658):
#
#   const UNDEFINED_REPLACEMENT = Ref{Any}(nothing)
#   const UNDEFINED_TYPE = Ref{DataType}(typeof(UNDEFINED_REPLACEMENT[]))
#   typify(x::String) = x == UNDEFINED_PLACEHOLDER[] ? UNDEFINED_REPLACEMENT[] : x
#
# The module-level evaluation of `typeof(UNDEFINED_REPLACEMENT[])` dereferences
# a `Ref{Any}` holding `nothing`, triggering new @generated dispatch
# specialisations that intersect with OrdinaryDiffEq / LinearSolve / SparseArrays
# dispatch paths.  This invalidates previously cached method specialisations in
# the ODE solver.

ENV["GENIE_ENV"] = "prod"  # disable Revise — issue persists regardless

using OrdinaryDiffEq
using LinearSolve
using SparseArrays

# ── Simple stiff ODE (Robertson chemical kinetics) ───────────────────────────
function rober!(du, u, p, t)
    y₁, y₂, y₃ = u
    k₁, k₂, k₃ = p
    du[1] = -k₁ * y₁ + k₃ * y₂ * y₃
    du[2] =  k₁ * y₁ - k₂ * y₂^2 - k₃ * y₂ * y₃
    du[3] =  k₂ * y₂^2
end

const JAC_SPARSITY = float(sparse(Bool[
    1 1 1
    1 1 1
    0 1 0
]))

function run_ode()
    u0    = [1.0, 0.0, 0.0]
    p     = (0.04, 3e7, 1e4)
    tspan = (0.0, 1e5)
    f     = ODEFunction(rober!; jac_prototype = JAC_SPARSITY)
    prob  = ODEProblem(f, u0, tspan, p)
    return solve(prob, QNDF(linsolve = KLUFactorization());
                 abstol = 1e-8, reltol = 1e-6)
end

# ── Phase 1: warm up ODE solver ───────────────────────────────────────────────
println("=== Run 1  (first compilation — expected slow) ===")
@time run_ode()

println("\n=== Run 2  (expected: 0% compilation) ===")
@time run_ode()

println("\n=== Run 3  (expected: 0% compilation) ===")
@time run_ode()

# ── Load Genie.Renderer.Json ──────────────────────────────────────────────────
println("\nLoading Genie.Renderer.Json (Genie 5.35.7)...")
import Genie.Renderer.Json: json
println("Loaded.\n")

# ── Phase 2: same ODE, same solver, same types ────────────────────────────────
println("=== Run 4 (bug: Genie 5.35.7 shows compilation here, expected 0% compilation) ===")
@time run_ode()

println("\n=== Run 5 (bug: Genie 5.35.7 shows compilation here, expected 0% compilation) ===")
@time run_ode()

println("\n=== Run 6 (bug: Genie 5.35.7 shows compilation here, expected 0% compilation) ===")
@time run_ode()

println("\n=== Version Info ===")
import Pkg
Pkg.status()
using InteractiveUtils   
versioninfo()

output:

=== Run 1  (first compilation — expected slow) ===
  4.144455 seconds (10.20 M allocations: 509.281 MiB, 12.61% gc time, 99.91% compilation time)

=== Run 2  (expected: 0% compilation) ===
  0.000528 seconds (4.44 k allocations: 210.344 KiB)

=== Run 3  (expected: 0% compilation) ===
  0.000577 seconds (4.44 k allocations: 210.344 KiB)

Loading Genie.Renderer.Json (Genie 5.35.7)...
Loaded.

=== Run 4 (bug: Genie 5.35.7 shows compilation here, expected 0% compilation) ===
  0.099837 seconds (684.82 k allocations: 32.385 MiB, 198.45% compilation time: 50% of which was recompilation)

=== Run 5 (bug: Genie 5.35.7 shows compilation here, expected 0% compilation) ===
  0.000664 seconds (5.47 k allocations: 261.617 KiB, 5137.18% compilation time)

=== Run 6 (bug: Genie 5.35.7 shows compilation here, expected 0% compilation) ===
  0.000644 seconds (6.02 k allocations: 287.109 KiB, 5013.85% compilation time)

=== Version Info ===
Status `~/Code/MWE/GenieWorldAgeProblem/Project.toml`
  [c43c736e] Genie v5.35.7
  [7ed4a6bd] LinearSolve v3.65.0
  [1dea7af3] OrdinaryDiffEq v6.108.0
  [2f01184e] SparseArrays v1.12.0
Julia Version 1.12.5
Commit 5fe89b8ddc1 (2026-02-09 16:05 UTC)
Build Info:
  Official https://julialang.org release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 14 × Intel(R) Core(TM) Ultra 7 165U
  WORD_SIZE: 64
  LLVM: libLLVM-18.1.7 (ORCJIT, alderlake)
  GC: Built with stock GC
Threads: 1 default, 1 interactive, 1 GC (on 14 virtual cores)

Expected behavior
No compilation or increased allocation count on function calls that happen after import Genie.Renderer.Json: json

Additional context
Version info provided in the output of the MWE script

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Please answer these optional questions to help us understand, prioritise, and assign the issue

2/ Can you give us, in a few words, some details about the app you're building with Genie?

I am trying to host some ODE solvers via REST API and I cannot afford to lose their performance or stability when calling them from Genie.