history_export: resolve activity dependencies per invocation (retire process-global bind_context)

[andystaples]

Summary

Fixes #182.

The history-export activities previously resolved their runtime dependencies (a durabletask client and a HistoryWriter) from a module-global _context installed once via bind_context(HistoryExportContext(...)). That "activities run in-process within the worker that registered them" assumption holds for a standalone worker but breaks in host-driven, multi-process models such as Azure Functions, where the process that starts an export job is often not the worker process that runs an export activity — so _require_context() raised.

This PR retires the process-global and moves the core extension to a per-invocation resolver / factory pattern, so any hosting model can supply the dependencies lazily at execution time.

Core changes (durabletask.extensions.history_export)

• Added HistoryExportContextResolver (a Callable[[], HistoryExportContext]) plus the pure activity bodies run_list_terminal_instances(context, input) and run_export_instance_history(context, input) that take the resolved context explicitly.
• Added build_activities(resolver) — a factory returning the two worker-registrable activity callables (with the canonical activity names) that invoke resolver() once per activity execution.
• Changed register(worker, resolver) to register resolver-bound activities.
• Removed bind_context(), clear_context(), _require_context(), and the module-global context. ExportHistoryClient.register_worker() now captures a HistoryExportContext in the activity closures — no process-global is installed.

Azure Functions adapter

• Dropped the bind_context shim (_ensure_context_bound). The client-bound wrappers now build a HistoryExportContext from the per-invocation injected durable client (cached once per process) and call the core run_* bodies explicitly — fitting the single core pattern instead of layering Functions-specific plumbing on top of bind_context.

Breaking change

bind_context() / clear_context() (exported in v1.8.0) are removed. Most callers use ExportHistoryClient.register_worker() and are unaffected. Code that called bind_context(HistoryExportContext(client, writer)) directly should register with durabletask.extensions.history_export.activities.register(worker, lambda: HistoryExportContext(client, writer)) (or a lazier resolver). Documented in CHANGELOG.md.

Validation

• flake8: clean on all changed files.
• pyright (strict, repo config): 0 errors on the changed source, the full history_export package, and the example.
• Core history_export tests: 76 passed.
• azure-functions-durable unit suite: 203 passed (incl. the history-export compat tests).
• Smoke-tested DFApp.configure_history_export(...) wiring end to end.

[Copilot]

Pull request overview

This PR refactors the durabletask.extensions.history_export extension to stop relying on a process-global activity context (bind_context) and instead resolve dependencies (durable client + HistoryWriter) per activity invocation via a resolver/factory pattern. This aligns history export with host-driven, multi-process execution models (e.g., Azure Functions) where “the process that created the job” may not be “the process that executes the activity”.

Changes:

• Introduces resolver-based activity registration (HistoryExportContextResolver, pure run_* bodies, and build_activities(resolver)), and removes the module-global context binding model.
• Updates ExportHistoryClient.register_worker() and the Azure Functions adapter to use per-invocation context resolution/caching rather than process-global binding.
• Updates tests to use a per-test mutable context holder + resolver, and documents the breaking change in CHANGELOG.md.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
durabletask/extensions/history_export/activities.py	Replaces process-global context with per-invocation resolver; adds pure run_* activity bodies and activity factory/registration helpers.
durabletask/extensions/history_export/client.py	Wires worker registration through resolver-bound activities (no process-global binding).
durabletask/extensions/history_export/init.py	Updates public exports to include resolver types and pure run_*/factory building blocks (and remove bind/clear exports).
azure-functions-durable/azure/durable_functions/internal/history_export_compat.py	Removes bind_context usage; builds/caches a per-process HistoryExportContext from the injected client + configured writer and calls pure bodies.
CHANGELOG.md	Adds/updates Unreleased notes for the new resolver API and the breaking removal of process-global binding.
tests/durabletask/extensions/history_export/_test_helpers.py	Adds a MutableContextHolder to support resolver-based activity registration in tests.
tests/durabletask/extensions/history_export/test_activities.py	Switches E2E activity tests to resolver+holder pattern; updates assertions/messages accordingly.
tests/durabletask/extensions/history_export/test_client.py	Removes teardown that cleared a now-removed process-global context.
tests/durabletask/extensions/history_export/test_orchestrator.py	Updates orchestrator E2E tests to register activities with a resolver and swap contexts per test.
tests/azure-functions-durable/test_history_export_compat.py	Updates compat tests to pass explicit contexts and validate new caching/closure behavior.
tests/azure-functions-durable/e2e/apps/dtask_style/history_export_routes.py	Updates E2E app documentation to reflect per-invocation resolution (no process-wide context binding).