Need some way to enable "tokio=trace" filter for the console Layer only #76

sbrocket · 2021-07-25T23:17:29Z

First off...this is super cool, thanks for creating it!

The requirement to enable trace-level events for tokio to use this console means that other layers also receive those events, which makes a fmt Layer pretty unusable without out-of-band filtering.

Per tokio-rs/tracing#302, it looks like per-layer filtering does not yet exist. Before I'd discovered that I'd tried combining separate EnvFilters with a fmt Layer and a console Layer using Layer::and_then, but this clearly doesn't work as expected.

Maybe there's another workaround I haven't thought of?

The text was updated successfully, but these errors were encountered:

hawkw · 2021-08-02T17:50:27Z

For now, the best solution is to use MakeWriterExt to add an output filter that disables tokio=trace to the writer used by your fmt layer. This will prevent it from printing events emitted by Tokio, but still allows the subscriber to record them, so that they can be used for the console.

For example, you could write

use tracing_subscriber::prelude::*;

let fmt = tracing_subscriber::fmt::layer()
    .with_writer(std::io::stdout
        // Don't print logs for events with the `tokio` target.
        .with_filter(|meta| meta.target() != "tokio")
    );
    // ... plus any additional `fmt` configurations you may want...

console_subscriber::build()
    .with(fmt)
    .init();

The one caveat here is that the spans used by the console will not be filtered out when printing events that are enabled by other filter settings, so those spans will be shown as part of the span context for those events. A stopgap solution for until we have proper, generic per-layer filtering could be to add a similar form of filtering to tracing-subscriber::fmt when formatting span contexts, so that the fmt layer could skip over spans that don't match a predicate as it builds a context.

See also #64 (comment)

hawkw · 2021-08-27T18:09:44Z

upstream change tokio-rs/tracing#1523 should fix this in a somewhat nicer way

## Motivation Currently, filtering with `Layer`s is always _global_. If a `Layer` performs filtering, it will disable a span or event for _all_ layers that compose the current subscriber. In some cases, however, it is often desirable for individual layers to see different spans or events than the rest of the `Layer` stack. Issues in other projects, such as tokio-rs/console#64 and tokio-rs/console#76, linkerd/linkerd2-proxy#601, influxdata/influxdb_iox#1012 and influxdata/influxdb_iox#1681, jackwaudby/spaghetti#86, etc; as well as `tracing` feature requests like #302, #597, and #1348, all indicate that there is significant demand for the ability to add filters to individual `Layer`s. Unfortunately, doing this nicely is somewhat complicated. Although a naive implementation that simply skips events/spans in `Layer::on_event` and `Layer::new_span` based on some filter is relatively simple, this wouldn't really be an ideal solution, for a number of reasons. A proper per-layer filtering implementation would satisfy the following _desiderata_: * If a per-layer filter disables a span, it shouldn't be present _for the layer that filter is attached to_ when iterating over span contexts (such as `Context::event_scope`, `SpanRef::scope`, etc), or when looking up a span's parents. * When _all_ per-layer filters disable a span or event, it should be completely disabled, rather than simply skipped by those particular layers. This means that per-layer filters should participate in `enabled`, as well as being able to skip spans and events in `new_span` and `on_event`. * Per-layer filters shouldn't interfere with non-filtered `Layer`s. If a subscriber contains layers without any filters, as well as layers with per-layer filters, the non-filtered `Layer`s should behave exactly as they would without any per-layer filters present. * Similarly, per-layer filters shouldn't interfere with global filtering. If a `Layer` in a stack is performing global filtering (e.g. the current filtering behavior), per-layer filters should also be effected by the global filter. * Per-layer filters _should_ be able to participate in `Interest` caching, _but only when doing so doesn't interfere with non-per-layer-filtered layers_. * Per-layer filters should be _tree-shaped_. If a `Subscriber` consists of multiple layers that have been `Layered` together to form new `Layer`s, and some of the `Layered` layers have per-layer filters, those per-layer filters should effect all layers in that subtree. Similarly, if `Layer`s in a per-layer filtered subtree have their _own_ per-layer filters, those layers should be effected by the union of their own filters and any per-layer filters that wrap them at higher levels in the tree. Meeting all these requirements means that implementing per-layer filtering correctly is somewhat more complex than simply skipping events and spans in a `Layer`'s `on_event` and `new_span` callbacks. ## Solution This branch has a basic working implementation of per-layer filtering for `tracing-subscriber` v0.2. It should be possible to add this in a point release without a breaking change --- in particular, the new per-layer filtering feature _doesn't_ interfere with the global filtering behavior that layers currently have when not using the new per-layer filtering APIs, so existing configurations should all behave exactly as they do now. ### Implementation The new per-layer filtering API consists of a `Filter` trait that defines a filtering strategy and a `Filtered` type that combines a `Filter` with a `Layer`. When `enabled` is called on a `Filtered` layer, it checks the metadata against the `Filter` and stores whether that filter enabled the span/event in a thread-local cell. If every per-layer filter disables a span/event, and there are no non-per-layer-filtered layers in use, the `Registry`'s `enabled` will return `false`. Otherwise, when the span/event is recorded, each `Filtered` layer's `on_event` and `new_span` method checks with the `Registry` to see whether _it_ enabled or disabled that span/event, and skips passing it to the wrapped layer if it was disabled. When a span is recorded, the `Registry` which filters disabled it, so that they can skip it when looking up spans in other callbacks. A new `on_layer` method was added to the `Layer` trait, which allows running a callback when adding a `Layer` to a `Subscriber`. This allows mutating both the `Layer` and the `Subscriber`, since they are both passed by value when layering a `Layer` onto a `Subscriber`, and a new `register_filter` method was added to `LookupSpan`, which returns a `FilterId` type. When a `Filtered` layer is added to a subscriber that implements `LookupSpan`, it calls `register_filter` in its `on_layer` hook to generate a unique ID for its filter. `FilterId` can be used to look up whether a span or event was enabled by the `Filtered` layer. Internally, the filter results are stored in a simple bitset to reduce the performance overhead of adding per-layer filtering as much as possible. The `FilterId` type is actually a bitmask that's used to set the bit corresponding to a `Filtered` layer in that bitmap when it disables a span or event, and check whether the bit is set when querying if a span or event was disabled. Setting a bit in the bitset and testing whether its set is extremely efficient --- it's just a couple of bitwise ops. Additionally, the "filter map" that tracks which filters disabled a span or event is just a single `u64` --- we don't need to allocate a costly `HashSet` or similar every time we want to filter an event. This *does* mean that we are limited to 64 unique filters per `Registry`...however, I would be _very_ surprised if anyone _ever_ builds a subscriber with 64 layers, let alone 64 separate per-layer filters. Additionally, the `Context` and `SpanRef` types now also potentially carry `FilterId`s, so that they can filter span lookups and span traversals for `Filtered` layers. When `Filtered` layers are nested using the `Layered`, the `Context` _combines_ the filter ID of the inner and outer layers so that spans can be skipped if they were disabled by either. Finally, an implementation of the `Filter` trait was added for `LevelFilter`, and new `FilterFn` and `DynFilterFn` structs were added to produce a `Filter` implementation from a function pointer or closure. ### Notes Some other miscellaneous factors to consider when reviewing this change include: * I did a bit of unrelated refactoring in the `layer` module. Since we were adding more code to the `Layer` module (the `Filter` trait), the `layer.rs` file got significantly longer and was becoming somewhat hard to navigate. Therefore, I split out the `Context` and `Layered` types into their own files. Most of the code in those files is the same as it was before, although some of it was changed in order to support per-layer filtering. Apologies in advance to reviewers for this... * In order to test this change, I added some new test-support code in `tracing-subscriber`. The `tracing` test support code provides a `Subscriber` implementation that can be configured to expect a particular set of spans and events, and assert that they occurred as expected. In order to test per-layer filtering, I added a new `Layer` implementation that does the same thing, but implements `Layer` rather than `Subscriber`. This allows testing per-layer filter behavior in the same way we test global filter behavior. Reviewers should feel free to just skim the test the test support changes, or skip reviewing them entirely. ## Future Work There is a bunch of additional stuff we can do once this change lands. In order to limit the size of this PR, I didn't make these changes yet, but once this merges, we will want to consider some important follow up changes: * [ ] Currently, _only_ `tracing-subscriber`'s `Registry` is capable of serving as a root subscriber for per-layer filters. This is in order to avoid stabilizing a lot of the per-layer filtering design as public API without taking the time to ensure there are no serious issues. In the future, we will want to add new APIs to allow users to implement their own root `Subscriber`s that can host layers with per-layer filtering. * [ ] The `EnvFilter` type doesn't implement the `Filter` trait and thus cannot currently be used as a per-layer filter. We should add an implementation of `Filter` for `EnvFilter`. * [ ] The new `FilterFn` and `DynFilterFn` types could conceivably _also_ be used as global filters. We could consider adding `Layer` implementations to allow them to be used for global filtering. * [ ] We could consider adding new `Filter` implementations for performing common filtering predicates. For example, "only enable spans/events with a particular target or a set of targets", "only enable spans", "only enable events", etc. Then, we could add a combinator API to combine these predicates to build new filters. * [ ] It would be nice if the `Interest` system could be used to cache the result of multiple individual per-layer filter evaluations, so that we don't need to always use `enabled` when there are several per-layer filters that disagree on whether or not they want a given span/event. There are a lot of unused bits in `Interest` that could potentially be used for this purpose. However, this would require new API changes in `tracing-core`, and is therefore out of scope for this PR. Closes #302 Closes #597 Closes #1348 Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Currently, the `console_subscriber::build()` and `console_subscriber::init()` functions configure an `EnvFilter` that enables the `tokio` and `runtime` targets, plus any targets configured by `RUST_LOG`. This means that the `tokio` spans used by the console will always be enabled at the `TRACE` level for all layers. This includes the `fmt` layer we previously added in `init`. Since this resulted in the `fmt` layer logs being cluttered with a bunch of `tokio=trace` spans and events even when only high-level application logs are enabled by `RUST_LOG`, we removed the `fmt` layer in PR #64. However, tokio-rs/tracing#1523 was recently merged and [released][1], adding support for [per-layer filtering][2] to `tracing-subscriber`. We can now use the per-layer filtering API to selectively enable the `tokio`/`runtime` spans and events at the `TRACE` level _only_ for the `TasksLayer`, and add a filter to the `fmt` layer based on `RUST_LOG`. This allows us to put back the `fmt` layer in `console_subscriber::init`. Closes #76

Currently, the `console_subscriber::build()` and `console_subscriber::init()` functions configure an `EnvFilter` that enables the `tokio` and `runtime` targets, plus any targets configured by `RUST_LOG`. This means that the `tokio` spans used by the console will always be enabled at the `TRACE` level for all layers. This includes the `fmt` layer we previously added in `init`. Since this resulted in the `fmt` layer logs being cluttered with a bunch of `tokio=trace` spans and events even when only high-level application logs are enabled by `RUST_LOG`, we removed the `fmt` layer in PR #64. However, tokio-rs/tracing#1523 was recently merged and [released][1], adding support for [per-layer filtering][2] to `tracing-subscriber`. We can now use the per-layer filtering API to selectively enable the `tokio`/`runtime` spans and events at the `TRACE` level _only_ for the `TasksLayer`, and add a filter to the `fmt` layer based on `RUST_LOG`. This allows us to put back the `fmt` layer in `console_subscriber::init`. Now, if we run the example app (which uses `init`) with a `RUST_LOG` value that enables only the app's logs, we get nice output: ![image](https://user-images.githubusercontent.com/2796466/133124582-608c2b72-db2f-4588-bb75-585312ac8d66.png) However, we can also enable the `console-subscriber` crate's internal logging: ![image](https://user-images.githubusercontent.com/2796466/133124746-7df10f14-cf8c-40ef-96f3-5046039a8577.png) And, we can still enable `tokio=trace` ourselves, if we actually _want_ to see all the task spans and waker events: ![image](https://user-images.githubusercontent.com/2796466/133124851-3e4af25a-06a4-4912-9950-bfbab6dba4c7.png) I also added some `trace!` and `debug!` macros in `examples/app.rs` to demo that `console_subscriber::init()` also enables logging. Closes #76

## Motivation Currently, filtering with `Layer`s is always _global_. If a `Layer` performs filtering, it will disable a span or event for _all_ layers that compose the current subscriber. In some cases, however, it is often desirable for individual layers to see different spans or events than the rest of the `Layer` stack. Issues in other projects, such as tokio-rs/console#64 and tokio-rs/console#76, linkerd/linkerd2-proxy#601, influxdata/influxdb_iox#1012 and influxdata/influxdb_iox#1681, jackwaudby/spaghetti#86, etc; as well as `tracing` feature requests like #302, #597, and #1348, all indicate that there is significant demand for the ability to add filters to individual `Layer`s. Unfortunately, doing this nicely is somewhat complicated. Although a naive implementation that simply skips events/spans in `Layer::on_event` and `Layer::new_span` based on some filter is relatively simple, this wouldn't really be an ideal solution, for a number of reasons. A proper per-layer filtering implementation would satisfy the following _desiderata_: * If a per-layer filter disables a span, it shouldn't be present _for the layer that filter is attached to_ when iterating over span contexts (such as `Context::event_scope`, `SpanRef::scope`, etc), or when looking up a span's parents. * When _all_ per-layer filters disable a span or event, it should be completely disabled, rather than simply skipped by those particular layers. This means that per-layer filters should participate in `enabled`, as well as being able to skip spans and events in `new_span` and `on_event`. * Per-layer filters shouldn't interfere with non-filtered `Layer`s. If a subscriber contains layers without any filters, as well as layers with per-layer filters, the non-filtered `Layer`s should behave exactly as they would without any per-layer filters present. * Similarly, per-layer filters shouldn't interfere with global filtering. If a `Layer` in a stack is performing global filtering (e.g. the current filtering behavior), per-layer filters should also be effected by the global filter. * Per-layer filters _should_ be able to participate in `Interest` caching, _but only when doing so doesn't interfere with non-per-layer-filtered layers_. * Per-layer filters should be _tree-shaped_. If a `Subscriber` consists of multiple layers that have been `Layered` together to form new `Layer`s, and some of the `Layered` layers have per-layer filters, those per-layer filters should effect all layers in that subtree. Similarly, if `Layer`s in a per-layer filtered subtree have their _own_ per-layer filters, those layers should be effected by the union of their own filters and any per-layer filters that wrap them at higher levels in the tree. Meeting all these requirements means that implementing per-layer filtering correctly is somewhat more complex than simply skipping events and spans in a `Layer`'s `on_event` and `new_span` callbacks. ## Solution This branch has a basic working implementation of per-layer filtering for `tracing-subscriber` v0.2. It should be possible to add this in a point release without a breaking change --- in particular, the new per-layer filtering feature _doesn't_ interfere with the global filtering behavior that layers currently have when not using the new per-layer filtering APIs, so existing configurations should all behave exactly as they do now. ### Implementation The new per-layer filtering API consists of a `Filter` trait that defines a filtering strategy and a `Filtered` type that combines a `Filter` with a `Layer`. When `enabled` is called on a `Filtered` layer, it checks the metadata against the `Filter` and stores whether that filter enabled the span/event in a thread-local cell. If every per-layer filter disables a span/event, and there are no non-per-layer-filtered layers in use, the `Registry`'s `enabled` will return `false`. Otherwise, when the span/event is recorded, each `Filtered` layer's `on_event` and `new_span` method checks with the `Registry` to see whether _it_ enabled or disabled that span/event, and skips passing it to the wrapped layer if it was disabled. When a span is recorded, the `Registry` which filters disabled it, so that they can skip it when looking up spans in other callbacks. A new `on_layer` method was added to the `Layer` trait, which allows running a callback when adding a `Layer` to a `Subscriber`. This allows mutating both the `Layer` and the `Subscriber`, since they are both passed by value when layering a `Layer` onto a `Subscriber`, and a new `register_filter` method was added to `LookupSpan`, which returns a `FilterId` type. When a `Filtered` layer is added to a subscriber that implements `LookupSpan`, it calls `register_filter` in its `on_layer` hook to generate a unique ID for its filter. `FilterId` can be used to look up whether a span or event was enabled by the `Filtered` layer. Internally, the filter results are stored in a simple bitset to reduce the performance overhead of adding per-layer filtering as much as possible. The `FilterId` type is actually a bitmask that's used to set the bit corresponding to a `Filtered` layer in that bitmap when it disables a span or event, and check whether the bit is set when querying if a span or event was disabled. Setting a bit in the bitset and testing whether its set is extremely efficient --- it's just a couple of bitwise ops. Additionally, the "filter map" that tracks which filters disabled a span or event is just a single `u64` --- we don't need to allocate a costly `HashSet` or similar every time we want to filter an event. This *does* mean that we are limited to 64 unique filters per `Registry`...however, I would be _very_ surprised if anyone _ever_ builds a subscriber with 64 layers, let alone 64 separate per-layer filters. Additionally, the `Context` and `SpanRef` types now also potentially carry `FilterId`s, so that they can filter span lookups and span traversals for `Filtered` layers. When `Filtered` layers are nested using the `Layered`, the `Context` _combines_ the filter ID of the inner and outer layers so that spans can be skipped if they were disabled by either. Finally, an implementation of the `Filter` trait was added for `LevelFilter`, and new `FilterFn` and `DynFilterFn` structs were added to produce a `Filter` implementation from a function pointer or closure. ### Notes Some other miscellaneous factors to consider when reviewing this change include: * I did a bit of unrelated refactoring in the `layer` module. Since we were adding more code to the `Layer` module (the `Filter` trait), the `layer.rs` file got significantly longer and was becoming somewhat hard to navigate. Therefore, I split out the `Context` and `Layered` types into their own files. Most of the code in those files is the same as it was before, although some of it was changed in order to support per-layer filtering. Apologies in advance to reviewers for this... * In order to test this change, I added some new test-support code in `tracing-subscriber`. The `tracing` test support code provides a `Subscriber` implementation that can be configured to expect a particular set of spans and events, and assert that they occurred as expected. In order to test per-layer filtering, I added a new `Layer` implementation that does the same thing, but implements `Layer` rather than `Subscriber`. This allows testing per-layer filter behavior in the same way we test global filter behavior. Reviewers should feel free to just skim the test the test support changes, or skip reviewing them entirely. ## Future Work There is a bunch of additional stuff we can do once this change lands. In order to limit the size of this PR, I didn't make these changes yet, but once this merges, we will want to consider some important follow up changes: * [ ] Currently, _only_ `tracing-subscriber`'s `Registry` is capable of serving as a root subscriber for per-layer filters. This is in order to avoid stabilizing a lot of the per-layer filtering design as public API without taking the time to ensure there are no serious issues. In the future, we will want to add new APIs to allow users to implement their own root `Subscriber`s that can host layers with per-layer filtering. * [ ] The `EnvFilter` type doesn't implement the `Filter` trait and thus cannot currently be used as a per-layer filter. We should add an implementation of `Filter` for `EnvFilter`. * [ ] The new `FilterFn` and `DynFilterFn` types could conceivably _also_ be used as global filters. We could consider adding `Layer` implementations to allow them to be used for global filtering. * [ ] We could consider adding new `Filter` implementations for performing common filtering predicates. For example, "only enable spans/events with a particular target or a set of targets", "only enable spans", "only enable events", etc. Then, we could add a combinator API to combine these predicates to build new filters. * [ ] It would be nice if the `Interest` system could be used to cache the result of multiple individual per-layer filter evaluations, so that we don't need to always use `enabled` when there are several per-layer filters that disagree on whether or not they want a given span/event. There are a lot of unused bits in `Interest` that could potentially be used for this purpose. However, this would require new API changes in `tracing-core`, and is therefore out of scope for this PR. Closes #302 Closes #597 Closes #1348 Signed-off-by: Eliza Weisman <eliza@buoyant.io>

; This is the 1st commit message: subscriber: implement per-subscriber filtering (#1523) ## Motivation Currently, filtering with `Subscribe`s is always _global_. If a `Subscribe` performs filtering, it will disable a span or event for _all_ subscribers that compose the current subscriber. In some cases, however, it is often desirable for individual subscribers to see different spans or events than the rest of the `Subscribe` stack. Issues in other projects, such as tokio-rs/console#64 and tokio-rs/console#76, linkerd/linkerd2-proxy#601, influxdata/influxdb_iox#1012 and influxdata/influxdb_iox#1681, jackwaudby/spaghetti#86, etc; as well as `tracing` feature requests like #302, #597, and #1348, all indicate that there is significant demand for the ability to add filters to individual `Subscribe`s. Unfortunately, doing this nicely is somewhat complicated. Although a naive implementation that simply skips events/spans in `Subscribe::on_event` and `Subscribe::new_span` based on some filter is relatively simple, this wouldn't really be an ideal solution, for a number of reasons. A proper per-subscriber filtering implementation would satisfy the following _desiderata_: * If a per-subscriber filter disables a span, it shouldn't be present _for the subscriber that filter is attached to_ when iterating over span contexts (such as `Context::event_scope`, `SpanRef::scope`, etc), or when looking up a span's parents. * When _all_ per-subscriber filters disable a span or event, it should be completely disabled, rather than simply skipped by those particular subscribers. This means that per-subscriber filters should participate in `enabled`, as well as being able to skip spans and events in `new_span` and `on_event`. * Per-subscriber filters shouldn't interfere with non-filtered `Subscribe`s. If a subscriber contains subscribers without any filters, as well as subscribers with per-subscriber filters, the non-filtered `Subscribe`s should behave exactly as they would without any per-subscriber filters present. * Similarly, per-subscriber filters shouldn't interfere with global filtering. If a `Subscribe` in a stack is performing global filtering (e.g. the current filtering behavior), per-subscriber filters should also be effected by the global filter. * Per-subscriber filters _should_ be able to participate in `Interest` caching, _but only when doing so doesn't interfere with non-per-subscriber-filtered subscribers_. * Per-subscriber filters should be _tree-shaped_. If a `Subscriber` consists of multiple subscribers that have been `Subscribeed` together to form new `Subscribe`s, and some of the `Subscribeed` subscribers have per-subscriber filters, those per-subscriber filters should effect all subscribers in that subtree. Similarly, if `Subscribe`s in a per-subscriber filtered subtree have their _own_ per-subscriber filters, those subscribers should be effected by the union of their own filters and any per-subscriber filters that wrap them at higher levels in the tree. Meeting all these requirements means that implementing per-subscriber filtering correctly is somewhat more complex than simply skipping events and spans in a `Subscribe`'s `on_event` and `new_span` callbacks. ## Solution This branch has a basic working implementation of per-subscriber filtering for `tracing-subscriber` v0.2. It should be possible to add this in a point release without a breaking change --- in particular, the new per-subscriber filtering feature _doesn't_ interfere with the global filtering behavior that subscribers currently have when not using the new per-subscriber filtering APIs, so existing configurations should all behave exactly as they do now. ### Implementation The new per-subscriber filtering API consists of a `Filter` trait that defines a filtering strategy and a `Filtered` type that combines a `Filter` with a `Subscribe`. When `enabled` is called on a `Filtered` subscriber, it checks the metadata against the `Filter` and stores whether that filter enabled the span/event in a thread-local cell. If every per-subscriber filter disables a span/event, and there are no non-per-subscriber-filtered subscribers in use, the `Registry`'s `enabled` will return `false`. Otherwise, when the span/event is recorded, each `Filtered` subscriber's `on_event` and `new_span` method checks with the `Registry` to see whether _it_ enabled or disabled that span/event, and skips passing it to the wrapped subscriber if it was disabled. When a span is recorded, the `Registry` which filters disabled it, so that they can skip it when looking up spans in other callbacks. A new `on_subscriber` method was added to the `Subscribe` trait, which allows running a callback when adding a `Subscribe` to a `Subscriber`. This allows mutating both the `Subscribe` and the `Subscriber`, since they are both passed by value when subscribering a `Subscribe` onto a `Subscriber`, and a new `register_filter` method was added to `LookupSpan`, which returns a `FilterId` type. When a `Filtered` subscriber is added to a subscriber that implements `LookupSpan`, it calls `register_filter` in its `on_subscriber` hook to generate a unique ID for its filter. `FilterId` can be used to look up whether a span or event was enabled by the `Filtered` subscriber. Internally, the filter results are stored in a simple bitset to reduce the performance overhead of adding per-subscriber filtering as much as possible. The `FilterId` type is actually a bitmask that's used to set the bit corresponding to a `Filtered` subscriber in that bitmap when it disables a span or event, and check whether the bit is set when querying if a span or event was disabled. Setting a bit in the bitset and testing whether its set is extremely efficient --- it's just a couple of bitwise ops. Additionally, the "filter map" that tracks which filters disabled a span or event is just a single `u64` --- we don't need to allocate a costly `HashSet` or similar every time we want to filter an event. This *does* mean that we are limited to 64 unique filters per `Registry`...however, I would be _very_ surprised if anyone _ever_ builds a subscriber with 64 subscribers, let alone 64 separate per-subscriber filters. Additionally, the `Context` and `SpanRef` types now also potentially carry `FilterId`s, so that they can filter span lookups and span traversals for `Filtered` subscribers. When `Filtered` subscribers are nested using the `Subscribeed`, the `Context` _combines_ the filter ID of the inner and outer subscribers so that spans can be skipped if they were disabled by either. Finally, an implementation of the `Filter` trait was added for `LevelFilter`, and new `FilterFn` and `DynFilterFn` structs were added to produce a `Filter` implementation from a function pointer or closure. ### Notes Some other miscellaneous factors to consider when reviewing this change include: * I did a bit of unrelated refactoring in the `subscriber` module. Since we were adding more code to the `Subscribe` module (the `Filter` trait), the `subscriber.rs` file got significantly longer and was becoming somewhat hard to navigate. Therefore, I split out the `Context` and `Subscribeed` types into their own files. Most of the code in those files is the same as it was before, although some of it was changed in order to support per-subscriber filtering. Apologies in advance to reviewers for this... * In order to test this change, I added some new test-support code in `tracing-subscriber`. The `tracing` test support code provides a `Subscriber` implementation that can be configured to expect a particular set of spans and events, and assert that they occurred as expected. In order to test per-subscriber filtering, I added a new `Subscribe` implementation that does the same thing, but implements `Subscribe` rather than `Subscriber`. This allows testing per-subscriber filter behavior in the same way we test global filter behavior. Reviewers should feel free to just skim the test the test support changes, or skip reviewing them entirely. ## Future Work There is a bunch of additional stuff we can do once this change lands. In order to limit the size of this PR, I didn't make these changes yet, but once this merges, we will want to consider some important follow up changes: * [ ] Currently, _only_ `tracing-subscriber`'s `Registry` is capable of serving as a root subscriber for per-subscriber filters. This is in order to avoid stabilizing a lot of the per-subscriber filtering design as public API without taking the time to ensure there are no serious issues. In the future, we will want to add new APIs to allow users to implement their own root `Subscriber`s that can host subscribers with per-subscriber filtering. * [ ] The `EnvFilter` type doesn't implement the `Filter` trait and thus cannot currently be used as a per-subscriber filter. We should add an implementation of `Filter` for `EnvFilter`. * [ ] The new `FilterFn` and `DynFilterFn` types could conceivably _also_ be used as global filters. We could consider adding `Subscribe` implementations to allow them to be used for global filtering. * [ ] We could consider adding new `Filter` implementations for performing common filtering predicates. For example, "only enable spans/events with a particular target or a set of targets", "only enable spans", "only enable events", etc. Then, we could add a combinator API to combine these predicates to build new filters. * [ ] It would be nice if the `Interest` system could be used to cache the result of multiple individual per-subscriber filter evaluations, so that we don't need to always use `enabled` when there are several per-subscriber filters that disagree on whether or not they want a given span/event. There are a lot of unused bits in `Interest` that could potentially be used for this purpose. However, this would require new API changes in `tracing-core`, and is therefore out of scope for this PR. Closes #302 Closes #597 Closes #1348 Signed-off-by: Eliza Weisman <eliza@buoyant.io> ; The commit message #2 will be skipped: ; fixup: start making changes for filter ; The commit message #3 will be skipped: ; fixup: move stuff to `subscribe` folder from `layer ; The commit message #4 will be skipped: ; fixup: additional compilation errors

## Motivation Currently, filtering with `Subscribe`s is always _global_. If a `Subscribe` performs filtering, it will disable a span or event for _all_ subscribers that compose the current subscriber. In some cases, however, it is often desirable for individual subscribers to see different spans or events than the rest of the `Subscribe` stack. Issues in other projects, such as tokio-rs/console#64 and tokio-rs/console#76, linkerd/linkerd2-proxy#601, influxdata/influxdb_iox#1012 and influxdata/influxdb_iox#1681, jackwaudby/spaghetti#86, etc; as well as `tracing` feature requests like #302, #597, and #1348, all indicate that there is significant demand for the ability to add filters to individual `Subscribe`s. Unfortunately, doing this nicely is somewhat complicated. Although a naive implementation that simply skips events/spans in `Subscribe::on_event` and `Subscribe::new_span` based on some filter is relatively simple, this wouldn't really be an ideal solution, for a number of reasons. A proper per-subscriber filtering implementation would satisfy the following _desiderata_: * If a per-subscriber filter disables a span, it shouldn't be present _for the subscriber that filter is attached to_ when iterating over span contexts (such as `Context::event_scope`, `SpanRef::scope`, etc), or when looking up a span's parents. * When _all_ per-subscriber filters disable a span or event, it should be completely disabled, rather than simply skipped by those particular subscribers. This means that per-subscriber filters should participate in `enabled`, as well as being able to skip spans and events in `new_span` and `on_event`. * Per-subscriber filters shouldn't interfere with non-filtered `Subscribe`s. If a subscriber contains subscribers without any filters, as well as subscribers with per-subscriber filters, the non-filtered `Subscribe`s should behave exactly as they would without any per-subscriber filters present. * Similarly, per-subscriber filters shouldn't interfere with global filtering. If a `Subscribe` in a stack is performing global filtering (e.g. the current filtering behavior), per-subscriber filters should also be effected by the global filter. * Per-subscriber filters _should_ be able to participate in `Interest` caching, _but only when doing so doesn't interfere with non-per-subscriber-filtered subscribers_. * Per-subscriber filters should be _tree-shaped_. If a `Subscriber` consists of multiple subscribers that have been `Subscribeed` together to form new `Subscribe`s, and some of the `Subscribeed` subscribers have per-subscriber filters, those per-subscriber filters should effect all subscribers in that subtree. Similarly, if `Subscribe`s in a per-subscriber filtered subtree have their _own_ per-subscriber filters, those subscribers should be effected by the union of their own filters and any per-subscriber filters that wrap them at higher levels in the tree. Meeting all these requirements means that implementing per-subscriber filtering correctly is somewhat more complex than simply skipping events and spans in a `Subscribe`'s `on_event` and `new_span` callbacks. ## Solution This branch has a basic working implementation of per-subscriber filtering for `tracing-subscriber` v0.2. It should be possible to add this in a point release without a breaking change --- in particular, the new per-subscriber filtering feature _doesn't_ interfere with the global filtering behavior that subscribers currently have when not using the new per-subscriber filtering APIs, so existing configurations should all behave exactly as they do now. ### Implementation The new per-subscriber filtering API consists of a `Filter` trait that defines a filtering strategy and a `Filtered` type that combines a `Filter` with a `Subscribe`. When `enabled` is called on a `Filtered` subscriber, it checks the metadata against the `Filter` and stores whether that filter enabled the span/event in a thread-local cell. If every per-subscriber filter disables a span/event, and there are no non-per-subscriber-filtered subscribers in use, the `Registry`'s `enabled` will return `false`. Otherwise, when the span/event is recorded, each `Filtered` subscriber's `on_event` and `new_span` method checks with the `Registry` to see whether _it_ enabled or disabled that span/event, and skips passing it to the wrapped subscriber if it was disabled. When a span is recorded, the `Registry` which filters disabled it, so that they can skip it when looking up spans in other callbacks. A new `on_subscriber` method was added to the `Subscribe` trait, which allows running a callback when adding a `Subscribe` to a `Subscriber`. This allows mutating both the `Subscribe` and the `Subscriber`, since they are both passed by value when subscribering a `Subscribe` onto a `Subscriber`, and a new `register_filter` method was added to `LookupSpan`, which returns a `FilterId` type. When a `Filtered` subscriber is added to a subscriber that implements `LookupSpan`, it calls `register_filter` in its `on_subscriber` hook to generate a unique ID for its filter. `FilterId` can be used to look up whether a span or event was enabled by the `Filtered` subscriber. Internally, the filter results are stored in a simple bitset to reduce the performance overhead of adding per-subscriber filtering as much as possible. The `FilterId` type is actually a bitmask that's used to set the bit corresponding to a `Filtered` subscriber in that bitmap when it disables a span or event, and check whether the bit is set when querying if a span or event was disabled. Setting a bit in the bitset and testing whether its set is extremely efficient --- it's just a couple of bitwise ops. Additionally, the "filter map" that tracks which filters disabled a span or event is just a single `u64` --- we don't need to allocate a costly `HashSet` or similar every time we want to filter an event. This *does* mean that we are limited to 64 unique filters per `Registry`...however, I would be _very_ surprised if anyone _ever_ builds a subscriber with 64 subscribers, let alone 64 separate per-subscriber filters. Additionally, the `Context` and `SpanRef` types now also potentially carry `FilterId`s, so that they can filter span lookups and span traversals for `Filtered` subscribers. When `Filtered` subscribers are nested using the `Subscribeed`, the `Context` _combines_ the filter ID of the inner and outer subscribers so that spans can be skipped if they were disabled by either. Finally, an implementation of the `Filter` trait was added for `LevelFilter`, and new `FilterFn` and `DynFilterFn` structs were added to produce a `Filter` implementation from a function pointer or closure. ### Notes Some other miscellaneous factors to consider when reviewing this change include: * I did a bit of unrelated refactoring in the `subscriber` module. Since we were adding more code to the `Subscribe` module (the `Filter` trait), the `subscriber.rs` file got significantly longer and was becoming somewhat hard to navigate. Therefore, I split out the `Context` and `Subscribeed` types into their own files. Most of the code in those files is the same as it was before, although some of it was changed in order to support per-subscriber filtering. Apologies in advance to reviewers for this... * In order to test this change, I added some new test-support code in `tracing-subscriber`. The `tracing` test support code provides a `Subscriber` implementation that can be configured to expect a particular set of spans and events, and assert that they occurred as expected. In order to test per-subscriber filtering, I added a new `Subscribe` implementation that does the same thing, but implements `Subscribe` rather than `Subscriber`. This allows testing per-subscriber filter behavior in the same way we test global filter behavior. Reviewers should feel free to just skim the test the test support changes, or skip reviewing them entirely. ## Future Work There is a bunch of additional stuff we can do once this change lands. In order to limit the size of this PR, I didn't make these changes yet, but once this merges, we will want to consider some important follow up changes: * [ ] Currently, _only_ `tracing-subscriber`'s `Registry` is capable of serving as a root subscriber for per-subscriber filters. This is in order to avoid stabilizing a lot of the per-subscriber filtering design as public API without taking the time to ensure there are no serious issues. In the future, we will want to add new APIs to allow users to implement their own root `Subscriber`s that can host subscribers with per-subscriber filtering. * [ ] The `EnvFilter` type doesn't implement the `Filter` trait and thus cannot currently be used as a per-subscriber filter. We should add an implementation of `Filter` for `EnvFilter`. * [ ] The new `FilterFn` and `DynFilterFn` types could conceivably _also_ be used as global filters. We could consider adding `Subscribe` implementations to allow them to be used for global filtering. * [ ] We could consider adding new `Filter` implementations for performing common filtering predicates. For example, "only enable spans/events with a particular target or a set of targets", "only enable spans", "only enable events", etc. Then, we could add a combinator API to combine these predicates to build new filters. * [ ] It would be nice if the `Interest` system could be used to cache the result of multiple individual per-subscriber filter evaluations, so that we don't need to always use `enabled` when there are several per-subscriber filters that disagree on whether or not they want a given span/event. There are a lot of unused bits in `Interest` that could potentially be used for this purpose. However, this would require new API changes in `tracing-core`, and is therefore out of scope for this PR. Closes #302 Closes #597 Closes #1348 Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Currently, filtering with `Layer`s is always _global_. If a `Layer` performs filtering, it will disable a span or event for _all_ layers that compose the current subscriber. In some cases, however, it is often desirable for individual layers to see different spans or events than the rest of the `Layer` stack. Issues in other projects, such as tokio-rs/console#64 and tokio-rs/console#76, linkerd/linkerd2-proxy#601, influxdata/influxdb_iox#1012 and influxdata/influxdb_iox#1681, jackwaudby/spaghetti#86, etc; as well as `tracing` feature requests like the ability to add filters to individual `Layer`s. Unfortunately, doing this nicely is somewhat complicated. Although a naive implementation that simply skips events/spans in `Layer::on_event` and `Layer::new_span` based on some filter is relatively simple, this wouldn't really be an ideal solution, for a number of reasons. A proper per-layer filtering implementation would satisfy the following _desiderata_: * If a per-layer filter disables a span, it shouldn't be present _for the layer that filter is attached to_ when iterating over span contexts (such as `Context::event_scope`, `SpanRef::scope`, etc), or when looking up a span's parents. * When _all_ per-layer filters disable a span or event, it should be completely disabled, rather than simply skipped by those particular layers. This means that per-layer filters should participate in `enabled`, as well as being able to skip spans and events in `new_span` and `on_event`. * Per-layer filters shouldn't interfere with non-filtered `Layer`s. If a subscriber contains layers without any filters, as well as layers with per-layer filters, the non-filtered `Layer`s should behave exactly as they would without any per-layer filters present. * Similarly, per-layer filters shouldn't interfere with global filtering. If a `Layer` in a stack is performing global filtering (e.g. the current filtering behavior), per-layer filters should also be effected by the global filter. * Per-layer filters _should_ be able to participate in `Interest` caching, _but only when doing so doesn't interfere with non-per-layer-filtered layers_. * Per-layer filters should be _tree-shaped_. If a `Subscriber` consists of multiple layers that have been `Layered` together to form new `Layer`s, and some of the `Layered` layers have per-layer filters, those per-layer filters should effect all layers in that subtree. Similarly, if `Layer`s in a per-layer filtered subtree have their _own_ per-layer filters, those layers should be effected by the union of their own filters and any per-layer filters that wrap them at higher levels in the tree. Meeting all these requirements means that implementing per-layer filtering correctly is somewhat more complex than simply skipping events and spans in a `Layer`'s `on_event` and `new_span` callbacks. This branch has a basic working implementation of per-layer filtering for `tracing-subscriber` v0.2. It should be possible to add this in a point release without a breaking change --- in particular, the new per-layer filtering feature _doesn't_ interfere with the global filtering behavior that layers currently have when not using the new per-layer filtering APIs, so existing configurations should all behave exactly as they do now. The new per-layer filtering API consists of a `Filter` trait that defines a filtering strategy and a `Filtered` type that combines a `Filter` with a `Layer`. When `enabled` is called on a `Filtered` layer, it checks the metadata against the `Filter` and stores whether that filter enabled the span/event in a thread-local cell. If every per-layer filter disables a span/event, and there are no non-per-layer-filtered layers in use, the `Registry`'s `enabled` will return `false`. Otherwise, when the span/event is recorded, each `Filtered` layer's `on_event` and `new_span` method checks with the `Registry` to see whether _it_ enabled or disabled that span/event, and skips passing it to the wrapped layer if it was disabled. When a span is recorded, the `Registry` which filters disabled it, so that they can skip it when looking up spans in other callbacks. A new `on_layer` method was added to the `Layer` trait, which allows running a callback when adding a `Layer` to a `Subscriber`. This allows mutating both the `Layer` and the `Subscriber`, since they are both passed by value when layering a `Layer` onto a `Subscriber`, and a new `register_filter` method was added to `LookupSpan`, which returns a `FilterId` type. When a `Filtered` layer is added to a subscriber that implements `LookupSpan`, it calls `register_filter` in its `on_layer` hook to generate a unique ID for its filter. `FilterId` can be used to look up whether a span or event was enabled by the `Filtered` layer. Internally, the filter results are stored in a simple bitset to reduce the performance overhead of adding per-layer filtering as much as possible. The `FilterId` type is actually a bitmask that's used to set the bit corresponding to a `Filtered` layer in that bitmap when it disables a span or event, and check whether the bit is set when querying if a span or event was disabled. Setting a bit in the bitset and testing whether its set is extremely efficient --- it's just a couple of bitwise ops. Additionally, the "filter map" that tracks which filters disabled a span or event is just a single `u64` --- we don't need to allocate a costly `HashSet` or similar every time we want to filter an event. This *does* mean that we are limited to 64 unique filters per `Registry`...however, I would be _very_ surprised if anyone _ever_ builds a subscriber with 64 layers, let alone 64 separate per-layer filters. Additionally, the `Context` and `SpanRef` types now also potentially carry `FilterId`s, so that they can filter span lookups and span traversals for `Filtered` layers. When `Filtered` layers are nested using the `Layered`, the `Context` _combines_ the filter ID of the inner and outer layers so that spans can be skipped if they were disabled by either. Finally, an implementation of the `Filter` trait was added for `LevelFilter`, and new `FilterFn` and `DynFilterFn` structs were added to produce a `Filter` implementation from a function pointer or closure. Some other miscellaneous factors to consider when reviewing this change include: * I did a bit of unrelated refactoring in the `layer` module. Since we were adding more code to the `Layer` module (the `Filter` trait), the `layer.rs` file got significantly longer and was becoming somewhat hard to navigate. Therefore, I split out the `Context` and `Layered` types into their own files. Most of the code in those files is the same as it was before, although some of it was changed in order to support per-layer filtering. Apologies in advance to reviewers for this... * In order to test this change, I added some new test-support code in `tracing-subscriber`. The `tracing` test support code provides a `Subscriber` implementation that can be configured to expect a particular set of spans and events, and assert that they occurred as expected. In order to test per-layer filtering, I added a new `Layer` implementation that does the same thing, but implements `Layer` rather than `Subscriber`. This allows testing per-layer filter behavior in the same way we test global filter behavior. Reviewers should feel free to just skim the test the test support changes, or skip reviewing them entirely. There is a bunch of additional stuff we can do once this change lands. In order to limit the size of this PR, I didn't make these changes yet, but once this merges, we will want to consider some important follow up changes: * [ ] Currently, _only_ `tracing-subscriber`'s `Registry` is capable of serving as a root subscriber for per-layer filters. This is in order to avoid stabilizing a lot of the per-layer filtering design as public API without taking the time to ensure there are no serious issues. In the future, we will want to add new APIs to allow users to implement their own root `Subscriber`s that can host layers with per-layer filtering. * [ ] The `EnvFilter` type doesn't implement the `Filter` trait and thus cannot currently be used as a per-layer filter. We should add an implementation of `Filter` for `EnvFilter`. * [ ] The new `FilterFn` and `DynFilterFn` types could conceivably _also_ be used as global filters. We could consider adding `Layer` implementations to allow them to be used for global filtering. * [ ] We could consider adding new `Filter` implementations for performing common filtering predicates. For example, "only enable spans/events with a particular target or a set of targets", "only enable spans", "only enable events", etc. Then, we could add a combinator API to combine these predicates to build new filters. * [ ] It would be nice if the `Interest` system could be used to cache the result of multiple individual per-layer filter evaluations, so that we don't need to always use `enabled` when there are several per-layer filters that disagree on whether or not they want a given span/event. There are a lot of unused bits in `Interest` that could potentially be used for this purpose. However, this would require new API changes in `tracing-core`, and is therefore out of scope for this PR. Closes tokio-rs#302 Closes tokio-rs#597 Closes tokio-rs#1348 Signed-off-by: Eliza Weisman <eliza@buoyant.io>

hawkw mentioned this issue Sep 3, 2021

subscriber: implement per-layer filtering tokio-rs/tracing#1523

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hawkw mentioned this issue Sep 13, 2021

feat(subscriber): use per-layer filtering #140

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hawkw closed this as completed in #140 Sep 13, 2021

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Need some way to enable "tokio=trace" filter for the console Layer only #76

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Need some way to enable "tokio=trace" filter for the console Layer only #76

Need some way to enable "tokio=trace" filter for the console Layer only #76

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sbrocket commented Jul 25, 2021

hawkw commented Aug 2, 2021

hawkw commented Aug 27, 2021