[tracing-subscriber]: add chrono crate implementations of FormatTime (#…

…2690)

Issue #2080 explains that it's not
possible to soundly use
[`tracing_subscriber::fmt::time::LocalTime`](https://docs.rs/tracing-subscriber/latest/tracing_subscriber/fmt/time/struct.LocalTime.html)
in a multithreaded context. It proposes adding alternative time formatters that
use the [chrono crate](https://docs.rs/chrono/latest/chrono/) to workaround
which is what this PR offers.

A new source file 'chrono_crate.rs' is added to the 'tracing-subscriber'
package implementing `mod chrono_crate` providing two new tag types `LocalTime`
and `Utc` with associated `time::FormatTime` trait implementations that call
`chrono::Local::now().to_rfc3339()` and `chrono::Utc::now().to_rfc3339()`
respectively. Simple unit-tests of the new functionality accompany the
additions.
---------

Co-authored-by: David Barsky <me@davidbarsky.com>
Co-authored-by: Shayne Fletcher <shaynefletcher@meta.com>

tracing-subscriber/Cargo.toml

@@ -59,6 +59,7 @@ tracing-serde = { path = "../tracing-serde", version = "0.1.3", optional = true
 
 # opt-in deps
 parking_lot = { version = "0.12.1", optional = true }
+chrono = { version = "0.4.26", default-features = false, features = ["clock", "std"], optional = true }
 
 # registry
 sharded-slab = { version = "0.1.4", optional = true }

tracing-subscriber/src/fmt/time/chrono_crate.rs

@@ -0,0 +1,177 @@
+use crate::fmt::format::Writer;
+use crate::fmt::time::FormatTime;
+
+use std::sync::Arc;
+
+/// Formats [local time]s and [UTC time]s with `FormatTime` implementations
+/// that use the [`chrono` crate].
+///
+/// [local time]: [`chrono::offset::Local`]
+/// [UTC time]: [`chrono::offset::Utc`]
+/// [`chrono` crate]: [`chrono`]
+
+/// Formats the current [local time] using a [formatter] from the [`chrono`] crate.
+///
+/// [local time]: chrono::Local::now()
+/// [formatter]: chrono::format
+#[cfg_attr(docsrs, doc(cfg(feature = "chrono")))]
+#[derive(Debug, Clone, Eq, PartialEq, Default)]
+pub struct ChronoLocal {
+    format: Arc<ChronoFmtType>,
+}
+
+impl ChronoLocal {
+    /// Format the time using the [`RFC 3339`] format
+    /// (a subset of [`ISO 8601`]).
+    ///
+    /// [`RFC 3339`]: https://tools.ietf.org/html/rfc3339
+    /// [`ISO 8601`]: https://en.wikipedia.org/wiki/ISO_8601
+    pub fn rfc_3339() -> Self {
+        Self {
+            format: Arc::new(ChronoFmtType::Rfc3339),
+        }
+    }
+
+    /// Format the time using the given format string.
+    ///
+    /// See [`chrono::format::strftime`] for details on the supported syntax.
+    pub fn new(format_string: String) -> Self {
+        Self {
+            format: Arc::new(ChronoFmtType::Custom(format_string)),
+        }
+    }
+}
+
+impl FormatTime for ChronoLocal {
+    fn format_time(&self, w: &mut Writer<'_>) -> alloc::fmt::Result {
+        let t = chrono::Local::now();
+        match self.format.as_ref() {
+            ChronoFmtType::Rfc3339 => {
+                use chrono::format::{Fixed, Item};
+                write!(
+                    w,
+                    "{}",
+                    t.format_with_items(core::iter::once(Item::Fixed(Fixed::RFC3339)))
+                )
+            }
+            ChronoFmtType::Custom(fmt) => {
+                write!(w, "{}", t.format(fmt))
+            }
+        }
+    }
+}
+
+/// Formats the current [UTC time] using a [formatter] from the [`chrono`] crate.
+///
+/// [UTC time]: chrono::Utc::now()
+/// [formatter]: chrono::format
+#[cfg_attr(docsrs, doc(cfg(feature = "chrono")))]
+#[derive(Debug, Clone, Eq, PartialEq, Default)]
+pub struct ChronoUtc {
+    format: Arc<ChronoFmtType>,
+}
+
+impl ChronoUtc {
+    /// Format the time using the [`RFC 3339`] format
+    /// (a subset of [`ISO 8601`]).
+    ///
+    /// [`RFC 3339`]: https://tools.ietf.org/html/rfc3339
+    /// [`ISO 8601`]: https://en.wikipedia.org/wiki/ISO_8601
+    pub fn rfc_3339() -> Self {
+        Self {
+            format: Arc::new(ChronoFmtType::Rfc3339),
+        }
+    }
+
+    /// Format the time using the given format string.
+    ///
+    /// See [`chrono::format::strftime`] for details on the supported syntax.
+    pub fn new(format_string: String) -> Self {
+        Self {
+            format: Arc::new(ChronoFmtType::Custom(format_string)),
+        }
+    }
+}
+
+impl FormatTime for ChronoUtc {
+    fn format_time(&self, w: &mut Writer<'_>) -> alloc::fmt::Result {
+        let t = chrono::Utc::now();
+        match self.format.as_ref() {
+            ChronoFmtType::Rfc3339 => w.write_str(&t.to_rfc3339()),
+            ChronoFmtType::Custom(fmt) => w.write_str(&format!("{}", t.format(fmt))),
+        }
+    }
+}
+
+/// The RFC 3339 format is used by default but a custom format string
+/// can be used. See [`chrono::format::strftime`]for details on
+/// the supported syntax.
+///
+/// [`chrono::format::strftime`]: https://docs.rs/chrono/0.4.9/chrono/format/strftime/index.html
+#[derive(Debug, Clone, Eq, PartialEq)]
+enum ChronoFmtType {
+    /// Format according to the RFC 3339 convention.
+    Rfc3339,
+    /// Format according to a custom format string.
+    Custom(String),
+}
+
+impl Default for ChronoFmtType {
+    fn default() -> Self {
+        ChronoFmtType::Rfc3339
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::fmt::format::Writer;
+    use crate::fmt::time::FormatTime;
+
+    use std::sync::Arc;
+
+    use super::ChronoFmtType;
+    use super::ChronoLocal;
+    use super::ChronoUtc;
+
+    #[test]
+    fn test_chrono_format_time_utc_default() {
+        let mut buf = String::new();
+        let mut dst: Writer<'_> = Writer::new(&mut buf);
+        assert!(FormatTime::format_time(&ChronoUtc::default(), &mut dst).is_ok());
+        // e.g. `buf` contains "2023-08-18T19:05:08.662499+00:00"
+        assert!(chrono::DateTime::parse_from_str(&buf, "%FT%H:%M:%S%.6f%z").is_ok());
+    }
+
+    #[test]
+    fn test_chrono_format_time_utc_custom() {
+        let fmt = ChronoUtc {
+            format: Arc::new(ChronoFmtType::Custom("%a %b %e %T %Y".to_owned())),
+        };
+        let mut buf = String::new();
+        let mut dst: Writer<'_> = Writer::new(&mut buf);
+        assert!(FormatTime::format_time(&fmt, &mut dst).is_ok());
+        // e.g. `buf` contains "Wed Aug 23 15:53:23 2023"
+        assert!(chrono::NaiveDateTime::parse_from_str(&buf, "%a %b %e %T %Y").is_ok());
+    }
+
+    #[test]
+    fn test_chrono_format_time_local_default() {
+        let mut buf = String::new();
+        let mut dst: Writer<'_> = Writer::new(&mut buf);
+        assert!(FormatTime::format_time(&ChronoLocal::default(), &mut dst).is_ok());
+        // e.g. `buf` contains "2023-08-18T14:59:08.662499-04:00".
+        assert!(chrono::DateTime::parse_from_str(&buf, "%FT%H:%M:%S%.6f%z").is_ok());
+    }
+
+    #[test]
+    fn test_chrono_format_time_local_custom() {
+        let fmt = ChronoLocal {
+            format: Arc::new(ChronoFmtType::Custom("%a %b %e %T %Y".to_owned())),
+        };
+        let mut buf = String::new();
+        let mut dst: Writer<'_> = Writer::new(&mut buf);
+        assert!(FormatTime::format_time(&fmt, &mut dst).is_ok());
+        // e.g. `buf` contains "Wed Aug 23 15:55:46 2023".
+        assert!(chrono::NaiveDateTime::parse_from_str(&buf, "%a %b %e %T %Y").is_ok());
+    }
+}

tracing-subscriber/src/fmt/time/mod.rs

@@ -7,6 +7,7 @@ mod datetime;
 
 #[cfg(feature = "time")]
 mod time_crate;
+
 #[cfg(feature = "time")]
 #[cfg_attr(docsrs, doc(cfg(feature = "time")))]
 pub use time_crate::UtcTime;
@@ -19,6 +20,18 @@ pub use time_crate::LocalTime;
 #[cfg_attr(docsrs, doc(cfg(feature = "time")))]
 pub use time_crate::OffsetTime;
 
+/// [`chrono`]-based implementation for [`FormatTime`].
+#[cfg(feature = "chrono")]
+mod chrono_crate;
+
+#[cfg(feature = "chrono")]
+#[cfg_attr(docsrs, doc(cfg(feature = "chrono")))]
+pub use chrono_crate::ChronoLocal;
+
+#[cfg(feature = "chrono")]
+#[cfg_attr(docsrs, doc(cfg(feature = "chrono")))]
+pub use chrono_crate::ChronoUtc;
+
 /// A type that can measure and format the current time.
 ///
 /// This trait is used by `Format` to include a timestamp with each `Event` when it is logged.