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Eliza Weisman b8f63308d7 trace-core: Pass dispatcher by ref to dispatcher::with_default (#971 )

* trace-core: Pass dispatcher by ref to `dispatcher::with_default`

As requested by @carllerche in https://github.com/tokio-rs/tokio/pull/966#discussion_r264380005, this branch changes the
`dispatcher::with_default` function in `tokio-trace-core` to take the
dispatcher by ref and perform the clone internally. This makes this
function more consistant with other `with_default` functions in other
crates.

Signed-off-by: Eliza Weisman <[email protected]>

* trace: Don't set the default dispatcher on entering a span

Setting the default dispatcher on span entry is a relic of when spans
tracked their parent's ID. At that time, it was necessary to ensure that
any spans created inside a span were observed by the same subscriber
that originally provided the entered span with an ID, as otherwise, new
spans would be created with parent IDs that did not originate from that
subscriber.

Now that spans don't track their parent ID, this is no longer necessary.
However, removing this behavior does mean that if a span is entered
outside of the subscriber context it was created in, any subsequent
spans will be observed by the current default subscriber and thus will
not be part of the original span's trace tree. Since subscribers are not
expected to change frequently, and spans are not expected to move
between them, this is likely acceptable.

I've removed the tests for the old behavior.

Note that this change improves the performance of span entry/exit fairly
significantly. Here are the results of running a benchmark that enters
a span, does nothing, and immediately exits it, before this change:

```
test enter_span              ... bench:          93 ns/iter (+/- 14)
```

...and after:

```
test enter_span              ... bench:          51 ns/iter (+/- 9)
```

Signed-off-by: Eliza Weisman <[email protected]>

2019-03-11 15:29:00 -07:00

src

trace-core: Pass dispatcher by ref to dispatcher::with_default (#971 )

2019-03-11 15:29:00 -07:00

Cargo.toml

Introduce tokio-trace (#827 )

2019-02-19 12:15:01 -08:00

LICENSE

Introduce tokio-trace (#827 )

2019-02-19 12:15:01 -08:00

README.md

trace: Fix tokio-trace documentation url in the README (#939 )

2019-03-01 15:31:59 -08:00

README.md

tokio-trace-core

Core primitives for tokio-trace.

Documentation

Overview

tokio-trace is a framework for instrumenting Rust programs to collect structured, event-based diagnostic information. This crate defines the core primitives of tokio-trace.

The crate provides:

Span identifies a span within the execution of a program.
Event represents a single event within a trace.
Subscriber, the trait implemented to collect trace data.
Metadata and Callsite provide information describing Spans.
Field, FieldSet, Value, and ValueSet represent the structured data attached to a Span.
Dispatch allows span events to be dispatched to Subscribers.

In addition, it defines the global callsite registry and per-thread current dispatcher which other components of the tracing system rely on.

Application authors will typically not use this crate directly. Instead, they will use the tokio-trace crate, which provides a much more fully-featured API. However, this crate's API will change very infrequently, so it may be used when dependencies must be very stable.

License

This project is licensed under the MIT license.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in Tokio by you, shall be licensed as MIT, without any additional terms or conditions.