Panic when using JSON format and #[instrument]

[gahag-cw]

Bug Report

Version

├── tracing v0.1.29
│   ├── tracing-attributes v0.1.18 (proc-macro)
│   └── tracing-core v0.1.21
└── tracing-subscriber v0.3.3
    ├── tracing-core v0.1.21 (*)
    ├── tracing-log v0.1.2
    │   └── tracing-core v0.1.21 (*)
    └── tracing-serde v0.1.2
        └── tracing-core v0.1.21 (*)

Platform

Linux 5.10.84-1-lts #1 SMP Wed, 08 Dec 2021 10:17:01 +0000 x86_64 GNU/Linux

Crates

• tracing-subscriber

Description

When using the JSON format in conjunction with the #[instrument] macro, we get a panic.

Minimal code:

fn main() {
    tracing_subscriber::fmt()
        .event_format(tracing_subscriber::fmt::format().json())
        .init();

    test("hai")
}

#[tracing::instrument]
fn test(input: &str) {
    tracing::info!("got: {}", input);
}

Which results in:

thread 'main' panicked at 'span 'test' had malformed fields! this is a bug.
  error: expected value at line 1 column 1
  fields: FormattedFields { fields: "input=\"hai\"", formatter: tracing_subscriber::fmt::format::DefaultFields }', ~/.local/share/cargo/registry/src/github.com-1ecc6299db9ec823/tracing-subscriber-0.3.0/src/fmt/format/json.rs:166:49
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

If we comment the #[tracing::instrument] line, it works as expected.

[davidbarsky]

Sorry for the delay and the fact that you encountered this issue. The issue is the same as #1365; you'll also need add a JSON-specific field formatter, which is needed to record span fields (in reproduction, that would be input).

tracing_subscriber::fmt()
    .event_format(tracing_subscriber::fmt::format().json())
    .fmt_fields(tracing_subscriber::fmt::format::JsonFields::new())
    .init();

An alternative would be to use the .json() builder method on fmt::SubscriberBuilder:

tracing_subscriber::fmt()
    .json()
    .init();

[gahag-cw]

No worries, thanks for taking your time into this. Eventually, I stumbled on the json example in the repository and found out that I should use tracing_subscriber::fmt().json() directly. Not sure why I was using it the other way previously, perhaps there is some tutorial somewhere that's misleading.