Deprecate zero-copy keyword argument in @ez.subscriber

[griffinmilsap]

Closes #209

Summary

• Deprecate the zero_copy argument on @subscriber (ignored for years) and emit a warning when it’s explicitly passed.
• Update internal usages to remove redundant zero_copy=True.

Changes

• src/ezmsg/core/unit.py: Treat zero_copy as deprecated, warn when provided, and always set ZERO_COPY_ATTR=True.
• src/ezmsg/util/perf/impl.py: Remove zero_copy=True from @ez.subscriber.
• src/ezmsg/util/debuglog.py: Remove zero_copy=True from @ez.subscriber.
• src/ezmsg/util/messages/modify.py: Remove zero_copy=True from @ez.subscriber.
• src/ezmsg/util/messages/key.py: Remove zero_copy=True from @ez.subscriber.

Deprecation

• @subscriber(..., zero_copy=...) is now deprecated and ignored. A DeprecationWarning is emitted when used.

Testing

• Not run (not requested).

[KonradPilch]

nice

[Copilot]

Pull request overview

This pull request deprecates the zero_copy keyword argument in the @ez.subscriber decorator, addressing issue #209 which identified that the parameter was being ignored. The PR takes the approach of deprecating the parameter entirely and always setting the internal ZERO_COPY_ATTR to True, while actual zero-copy behavior is now controlled by the InputStream's leaky property (non-leaky streams receive zero-copy messages, leaky streams receive deepcopied messages).

Changes:

• Deprecated the zero_copy parameter on @subscriber decorator with a DeprecationWarning
• Removed all internal usages of zero_copy=True from utility modules
• Removed the ZERO_COPY_ATTR import and associated deepcopy check from backendprocess.py

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/ezmsg/core/unit.py	Adds deprecation warning for zero_copy parameter, always sets ZERO_COPY_ATTR=True, updates documentation
src/ezmsg/core/backendprocess.py	Removes ZERO_COPY_ATTR import and removes deepcopy check for pass-through messages
src/ezmsg/util/perf/impl.py	Removes zero_copy=True from three @ez.subscriber decorators
src/ezmsg/util/debuglog.py	Removes zero_copy=True from @ez.subscriber decorator
src/ezmsg/util/messages/modify.py	Removes zero_copy=True from @ez.subscriber decorator
src/ezmsg/util/messages/key.py	Removes zero_copy=True from two @ez.subscriber decorators

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[Copilot]

The docstring states "Subscribers always receive zero-copy messages" which is misleading. The actual behavior depends on whether the InputStream is leaky: leaky subscribers receive deepcopied messages via recv(), while non-leaky subscribers receive zero-copy messages via recv_zero_copy() (see backendprocess.py lines 413-443). Consider clarifying that the parameter is deprecated because zero-copy behavior is now controlled by the InputStream's leaky property rather than an explicit parameter.

[griffinmilsap]

This is accurate, though asserts tighter coupling between leaky behavior and deep-copy than was intended. The current implementation of leaky streams depends on copying the messages as more of a workaround. In the future, leaky streams may not deepcopy the messages while still providing the same leaky functionality.

[Copilot]

This removes the safety check that prevented zero-copy message pass-through when obj is msg. Previously, if a subscriber yielded the exact same object it received, a deepcopy would be created to prevent unintended mutations. Now, the same object reference can be passed through to downstream subscribers. This is a significant behavior change that could lead to mutation bugs if multiple subscribers modify shared message objects. If this is intentional as part of the new "zero-copy by default" design, it should be clearly documented. Otherwise, consider restoring this safety check without the ZERO_COPY_ATTR condition.

Suggested change

	async for stream, obj in result:
	async for stream, obj in result:
	# Prevent unintended zero-copy pass-through when a subscriber
	# yields the exact same object it received.
	if obj is msg:
	obj = deepcopy(obj)

[griffinmilsap]

This is an astute observation - The change is documented appropriately in the ezmsg docs repo. We will not be restoring that deepcopy.