v0.4.30

In this release, we have decided to swap out the chrono::Duration type (which has been a re-export of time 0.1 Duration type) with our own definition, which exposes a strict superset of the time::Duration API. This helps avoid warnings about the CVE-2020-26235 and RUSTSEC-2020-0071 advisories for downstream users and allows us to improve the Duration API going forward.

While this is technically a SemVer-breaking change, we expect the risk of downstream users experiencing actual incompatibility to be exceedingly limited (see our analysis of public code using a crater-like experiment), and not enough justification for the large ecosystem churn of a 0.5 release. If you have any feedback on these changes, please let us know in #1268.

Additions

• Add NaiveDate::leap_year (#1261)

Documentation

• Update main documentation from README (#1260, thanks @Stygmates)
• Add history of relation between chrono and time 0.1 to documentation (#1264, #1266)
• Clarify Timelike::num_seconds_from_midnight is a simple mapping (#1255)

Relation between chrono and time 0.1

Rust first had a time module added to std in its 0.7 release. It later moved to libextra, and then to a libtime library shipped alongside the standard library. In 2014 work on chrono started in order to provide a full-featured date and time library in Rust. Some improvements from chrono made it into the standard library; notably, chrono::Duration was included as std::time::Duration (rust#15934) in 2014.

In preparation of Rust 1.0 at the end of 2014 libtime was moved out of the Rust distro and into the time crate to eventually be redesigned (rust#18832, rust#18858), like the num and rand crates. Of course chrono kept its dependency on this time crate. time started re-exporting std::time::Duration during this period. Later, the standard library was changed to have a more limited unsigned Duration type (rust#24920, RFC 1040), while the time crate kept the full functionality with time::Duration. time::Duration had been a part of chrono's public API.

By 2016 time 0.1 lived under the rust-lang-deprecated organisation and was not actively maintained (time#136). chrono absorbed the platform functionality and Duration type of the time crate in chrono#478 (the work started in chrono#286). In order to preserve compatibility with downstream crates depending on time and chrono sharing a Duration type, chrono kept depending on time 0.1. chrono offered the option to opt out of the time dependency by disabling the oldtime feature (swapping it out for an effectively similar chrono type). In 2019, @jhpratt took over maintenance on the time crate and released what amounts to a new crate as time 0.2.

Security advisories

In November of 2020 CVE-2020-26235 and RUSTSEC-2020-0071 were opened against the time crate. @quininer had found that calls to localtime_r may be unsound (chrono#499). Eventually, almost a year later, this was also made into a security advisory against chrono as RUSTSEC-2020-0159, which had platform code similar to time.

On Unix-like systems a process is given a timezone id or description via the TZ environment variable. We need this timezone data to calculate the current local time from a value that is in UTC, such as the time from the system clock. time 0.1 and chrono used the POSIX function localtime_r to do the conversion to local time, which reads the TZ variable.

Rust assumes the environment to be writable and uses locks to access it from multiple threads. Some other programming languages and libraries use similar locking strategies, but these are typically not shared across languages. More importantly, POSIX declares modifying the environment in a multi-threaded process as unsafe, and getenv in libc can't be changed to take a lock because it returns a pointer to the data (see rust#27970 for more discussion).

Since version 4.20 chrono no longer uses localtime_r, instead using Rust code to query the timezone (from the TZ variable or via iana-time-zone as a fallback) and work with data from the system timezone database directly. The code for this was forked from the tz-rs crate by @x-hgg-x. As such, chrono now respects the Rust lock when reading the TZ environment variable. In general, code should avoid modifying the environment.

Removing time 0.1

Because time 0.1 has been unmaintained for years, however, the security advisory mentioned above has not been addressed. While chrono maintainers were careful not to break backwards compatibility with the time::Duration type, there has been a long stream of issues from users inquiring about the time 0.1 dependency with the vulnerability. We investigated the potential breakage of removing the time 0.1 dependency in chrono#1095 using a crater-like experiment and determined that the potential for breaking (public) dependencies is very low. We reached out to those few crates that did still depend on compatibility with time 0.1.

As such, for chrono 0.4.30 we have decided to swap out the time 0.1 Duration implementation for a local one that will offer a strict superset of the existing API going forward. This will prevent most downstream users from being affected by the security vulnerability in time 0.1 while minimizing the ecosystem impact of semver-incompatible version churn.

Thanks to all contributors on behalf of the chrono team, @djc and @pitdicker!