Default to `spawn` multiprocessing context across platforms

[FernandoCelmer]

Labels: enhancement

Context

dotflow/core/workflow.py:20-31:

if sys.platform == "win32":
    _mp = get_context("spawn")
else:
    if sys.platform == "darwin":
        os.environ.setdefault("OBJC_DISABLE_INITIALIZE_FORK_SAFETY", "YES")
    _mp = get_context("fork")

Problems:

• macOS fork with the Objective-C runtime is a known deadlock
  source. OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES only suppresses the warning; native libraries (numpy, cryptography, grpcio) can still crash silently.
• Python 3.14 changes the macOS default to spawn for this reason.
  The library will inherit that behavior whether we like it or not; shipping ahead of the change avoids surprise breakage.
• fork inherits all open file descriptors from the parent, which
  causes subtle bugs with database connection pools (the same socket is shared between processes).

Concept

Default mp_context to spawn on every platform. Allow override via env var or Config.

Proposed change

import os

DEFAULT_MP_CONTEXT = os.getenv("DOTFLOW_MP_CONTEXT", "spawn")
_mp = get_context(DEFAULT_MP_CONTEXT)

Optionally exposed on Config:

config = Config(mp_context="fork")  # opt back in if you accept the risk

Migration notes

spawn re-imports the worker module on each process start. Workflows that rely on importable steps will work; workflows that pass non-picklable closures will fail. Document the requirement.

Acceptance criteria

• Default context is spawn on darwin and linux
• DOTFLOW_MP_CONTEXT env var accepted (spawn | fork |
  forkserver)
• CHANGELOG entry highlights the behavioral change
• Documentation includes a "step picklability" note

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