使用tracing_subscriber的info_span!结合Axum时触发panic问题求助

尝试在Axum中结合tracing与tracing_subscriber实现JSON格式日志输出，访问接口时触发panic。

代码片段 #

主函数 #

fn main() -> Result<(), io::Error> {
    let rt = runtime::Builder::new_current_thread()
        .enable_io()
        .enable_time()
        .build()?;
    tracing_subscriber::fmt().event_format(format().json()).init();
     if let Err(e) = rt.block_on(axum_main()) {
        e.source();
    }
    Ok(())
}

axum_main函数 #

let app: Router<()> = Router::new().route(
        "/",
        routing::get(|| {
            async {
                tracing::info!("wowo here");
                "HELLO,world"
            }.instrument(tracing::info_span!("hello_span"))
        }),
    );

    let addr: SocketAddr = "192.168.42.200:9090".parse().unwrap();
    let socket = TcpSocket::new_v4()?;

    #[cfg(all(unix, not(target_os = "solaris"), not(target_os = "illumos")))]
    {
        socket.set_reuseport(true)?;
        socket.set_reuseaddr(true)?;
    }
    socket.bind(addr)?;
    let listener = socket.listen(1024)?;

    let r = axum::serve(listener, app.into_make_service()).await?;

    Ok(r)

报错信息 #

thread 'main' panicked at /root/.cargo/registry/src/rsproxy.cn-e3de039b2554c837/tracing-subscriber-0.3.19/src/fmt/format/json.rs:197:49:
span 'hello_span' had malformed fields! this is a bug.
  error: EOF while parsing a value at line 1 column 0
  fields: FormattedFields { fields: "", formatter: tracing_subscriber::fmt::format::DefaultFields, was_ansi: true }
stack backtrace:
   0: rust_begin_unwind
             at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/panicking.rs:692:5
   1: core::panicking::panic_fmt
             at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/core/src/panicking.rs:75:14
   2: <tracing_subscriber::fmt::format::json::SerializableSpan<Span,N> as serde::ser::Serialize>::serialize
             at /root/.cargo/registry/src/rsproxy.cn-e3de039b2554c837/tracing-subscriber-0.3.19/src/fmt/format/json.rs:197:49
   3: <serde_json::ser::Compound<W,F> as serde::ser::SerializeMap>::serialize_value

补充说明 #

将tracing_subscriber::fmt().event_format(format().json()).init()替换为tracing_subscriber::fmt().event_format(format().compact()).init()时，运行正常。

Cargo.toml依赖 #

[dependencies]
axum = "0.8.1"
tokio = { version = "1.44.1", features = ["macros", "rt"] }
tracing = "0.1.41"
tracing-subscriber = {version = "0.3.19",features = ["env-filter","chrono","json"]}

这个panic是因为tracing-subscriber的JSON事件格式化器和默认的DefaultFields字段格式化器不兼容导致的——默认字段格式化器输出的是文本格式的字段，无法被JSON序列化器解析，空字段场景下直接触发EOF错误。以下两种方法可以解决：

方法一：显式指定JSON专用字段格式化器 #

在初始化订阅器时，搭配JsonFields代替默认的字段格式化器：

use tracing_subscriber::fmt::format::JsonFields;

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

方法二：使用JSON格式快捷构造 #

直接调用fmt().json()方法，它会自动配置好兼容的事件和字段格式化器：

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

内容的提问来源于stack exchange，提问作者Stop