feat(embed): support baking PythonCall into a juliacall system image

[ncudlenco]

Summary

Lets PythonCall itself be compiled into a juliacall system image (opt-in). With it baked in, the using PythonCall that import juliacall performs becomes a memory-map instead of a multi-second load+compile. In a containerised juliacall workload we measured, fresh-process startup drops from ~18 s to ~1.9 s. Off by default — no behaviour change unless enabled. It's meant as a low-risk interim until the automatic fix discussed in JuliaPy#436 (or related work) lands; it doesn't conflict with or preclude that.

Motivation

In short-lived Python processes that embed Julia via juliacall — serverless or autoscaled containers (AWS Lambda, queue workers, CI jobs) that start, handle one request, and exit — there's no long-lived process to amortise Julia start-up. Every cold start pays it again, and the dominant part is the using PythonCall that import juliacall runs to bring the bridge up.

Measured, fresh container to first call:

	container init
stock juliacall (only app packages in the sysimage)	~18 s
PythonCall also baked into the sysimage (this PR)	~1.9 s

(~0.68 s of the ~1.9 s is the Julia stack; the rest is container/runtime overhead common to any container.) The ~16 s saved is essentially the repeated using PythonCall load+compile.

PYTHON_JULIACALL_SYSIMAGE already exists, but a sysimage with only the application packages still runs using PythonCall every cold start. Baking PythonCall in too is the obvious fix, but it currently fails: when PythonCall is in the sysimage its __init__ runs during jl_init_with_image, before juliacall's bootstrap has set Main.__PythonCall_libptr. init_context() decides "embedded" only from that global, so it sees "not embedded"; since Python is mid-import juliacall, init_juliacall() then errors with 'juliacall' module already exists.

Change

A second, opt-in way to detect the embedded case, using the same getpref mechanism added in 0.9.33 (preference embedded / env JULIA_PYTHONCALL_EMBEDDED, default no):

• src/Utils/Utils.jl — add getpref_embedded(), alongside the existing getpref_exe / getpref_lib / getpref_pickle.
• src/C/context.jl — if the Main global isn't set but embedded is, take the embedded path and get libpython from the existing lib preference / JULIA_PYTHONCALL_LIB (it's already loaded in the host process, so dlopen just hands back a handle).
• docs/src/juliacall.md — a short "Baking PythonCall into a system image" section.
• CHANGELOG.md — Unreleased entry.

In practice it's used together with PYTHON_JULIACALL_SYSIMAGE (PythonCall baked in) and PYTHON_JULIACALL_EXE / PROJECT.

Backward compatibility

Off by default. The new path is only reached when embedded is explicitly set and Main.__PythonCall_libptr is absent. The normal juliacall path and the PythonCall-as-host path are unchanged.

Testing

The new code path only runs when explicitly enabled, so default behaviour is unchanged and the existing test suites are unaffected.

It was verified manually: with PythonCall baked into a system image and embedded enabled, import juliacall and calling Julia both work. No automated test is included because exercising it requires building a system image in CI, which is slow — happy to add one, or document the steps, if you'd prefer.

Related issues

Relates to JuliaPy#436 and JuliaPy#600 — this unblocks their use case (baking PythonCall into a custom system image) as an interim. The root-cause fix the maintainer described in JuliaPy#436 — resetting PythonCall's sysimage-persisted state in __init__ so it works with no opt-in — is the longer-term path; this PR doesn't attempt or preclude it.

Also relevant to JuliaPy#762 ("Improve juliacall startup time?", which explicitly asks about compiling a system image) and a building block for JuliaPy#76 ("Compile and use custom sysimages automatically").

Does not address JuliaPy#129 (a different failure: no environment in the LOAD_PATH depends on CondaPkg).

[ncudlenco]

Merged into main. Do not delete the feat/embed-sysimage branch — it is the head of upstream JuliaPy#773 and must remain until that PR is resolved.