tracing_test/
subscriber.rs

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use std::{
    io,
    sync::{Mutex, MutexGuard},
};

use tracing_core::Dispatch;
use tracing_subscriber::{fmt::MakeWriter, FmtSubscriber};

/// A fake writer that writes into a buffer (behind a mutex).
#[derive(Debug)]
pub struct MockWriter<'a> {
    buf: &'a Mutex<Vec<u8>>,
}

impl<'a> MockWriter<'a> {
    /// Create a new `MockWriter` that writes into the specified buffer (behind a mutex).
    pub fn new(buf: &'a Mutex<Vec<u8>>) -> Self {
        Self { buf }
    }

    /// Give access to the internal buffer (behind a `MutexGuard`).
    fn buf(&self) -> io::Result<MutexGuard<'a, Vec<u8>>> {
        // Note: The `lock` will block. This would be a problem in production code,
        // but is fine in tests.
        self.buf
            .lock()
            .map_err(|_| io::Error::from(io::ErrorKind::Other))
    }
}

impl<'a> io::Write for MockWriter<'a> {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        // Lock target buffer
        let mut target = self.buf()?;

        // Write to stdout in order to show up in tests
        print!("{}", String::from_utf8(buf.to_vec()).unwrap());

        // Write to buffer
        target.write(buf)
    }

    fn flush(&mut self) -> io::Result<()> {
        self.buf()?.flush()
    }
}

impl<'a> MakeWriter<'_> for MockWriter<'a> {
    type Writer = Self;

    fn make_writer(&self) -> Self::Writer {
        MockWriter::new(self.buf)
    }
}

/// Return a new subscriber that writes to the specified [`MockWriter`].
///
/// [`MockWriter`]: struct.MockWriter.html
pub fn get_subscriber(mock_writer: MockWriter<'static>, env_filter: &str) -> Dispatch {
    FmtSubscriber::builder()
        .with_env_filter(env_filter)
        .with_writer(mock_writer)
        .with_level(true)
        .with_ansi(false)
        .into()
}