tracing-subscriber: implement Clone for EnvFilter #2398
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Benjamin-L
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This is useful when using `EnvFilter` for multiple identical per-layer filters, as well as with clap and similar libraries that have `Clone` bounds. We generally expect users to be cloning an `EnvFilter` before attaching it to a subscriber, rather than cloning `EnvFilters` that are already attached. Because of this, we reset all the accumulated dynamic state when cloning. This means that some spans and callsites might be missed when an already-attached `EnvFilter` is cloned, but the presence of the dynamic state mean that detaching and attaching `EnvFilter`s to existing subscribers (e.g. with `reload`) already doesn't work very well. This isn't a new class of problem. There was a previous implementation of this in tokio-rs#2398, that shared the dynamic state between all cloned filters behind an `Arc`. I chose not do go for that approach because it causes inconsistencies if the cloned filters are attached to different subscribers. Fixes: tokio-rs#2360
Benjamin-L
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Apr 26, 2024
This is useful when using `EnvFilter` for multiple identical per-layer filters, as well as with clap and similar libraries that have `Clone` bounds. We generally expect users to be cloning an `EnvFilter` before attaching it to a subscriber, rather than cloning `EnvFilters` that are already attached. Because of this, we reset all the accumulated dynamic state when cloning. This means that some spans and callsites might be missed when an already-attached `EnvFilter` is cloned, but the presence of the dynamic state mean that detaching and attaching `EnvFilter`s to existing subscribers (e.g. with `reload`) already doesn't work very well. This isn't a new class of problem. There was a previous implementation of this in tokio-rs#2398, that shared the dynamic state between all cloned filters behind an `Arc`. I chose not do go for that approach because it causes inconsistencies if the cloned filters are attached to different subscribers. Fixes: tokio-rs#2360
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Motivation
As of
clapv4, there'sClonebound, thus making it impossible forEnvFilterto be aclapcommand line argument.Solution
Implement
CloneforEnvFilter.