subscriber: default timestamp formatter without `chrono` is `SystemTime`'s `Debug` impl

[hawkw]

Bug Report

Version

tracing-subscriber 0.2.7

Crates

tracing-subscriber

Description

By default, tracing-subscriber's fmt module uses the chrono crate for formatting timestamps. However, the chrono dependency is optional and can be disabled independently of fmt. This is correct --- users who are using the Uptime timestamp formatter, providing their own custom timestamp formatter, or not using timestamps don't need the chrono dependency and may want to disable it.

However, when the chrono feature flag is disabled, the default formatter still enables SystemTime timestamps by default:

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

Lines 184 to 194 in 7dc37dd

	impl Default for Format<Full, SystemTime> {
	fn default() -> Self {
	Format {
	format: Full,
	timer: SystemTime,
	ansi: true,
	display_target: true,
	display_level: true,
	}
	}
	}

So, the formatter will still emit timestamps unless without_time(true) is called.

What do we do when timestamps are enabled, but we can't use chrono to format them because the dependency is not enabled?

It turns out that we fall back to SystemTime's fmt::Debug impl:

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

Lines 119 to 124 in 7dc37dd

	#[cfg(not(feature = "chrono"))]
	impl FormatTime for SystemTime {
	fn format_time(&self, w: &mut dyn fmt::Write) -> fmt::Result {
	write!(w, "{:?}", std::time::SystemTime::now())
	}
	}

The formatted output from this is...not great:

I do think it's correct to rely on chrono (or arbitrary user-defined timestamp formatters) for complex human-readable formatting. But the fmt::Debug implementation is not ideal. It includes the struct's name, field names, and curly braces, which add a lot of noise and make the log lines much longer. It leaks the SystemTime type's internal layout, which may be different depending on OS. And, the width of the numeric fields are not padded to a fixed width, so the log lines are not aligned.

We should try to fall back to something a little nicer when chrono isn't present. It doesn't need to be fancy --- we can (and should!) rely on chrono for human readable formats. Maybe just the number of seconds since the Unix epoch or something?

[danburkert]

In case you want something a little fancier, feel free to copy this: https://github.com/danburkert/kudu-rs/blob/master/src/timestamp.rs#L5-L18 (original attribution should go to musl I believe).