This is a huge surprise but a very welcome one. I've not seen any official announcement, and they seemingly did it very quietly but Gaijin Entertainment has open sourced their Dagor Engine used by War Thunder.
Seems it appeared a few days ago on their official GitHub account and it includes tools and samples to go along with it, not only that but this is proper open source too under the BSD 3-Clause License.
Documentation as a whole is pretty much non-existing for it so far, but now they've done the initial drop hopefully that will be expanded upon because all it does right now is include build instructions. Currently they only include instructions to build it on Windows, but no doubt now it's open someone will sort it for Linux - it's always only a matter of time when this sort of thing happens.
Really great to see more gaming companies embrace open source! Especially so when it's used by one of the most popular games around with War Thunder constantly in the top list on Steam (at time of writing it's #8 for most players online).
Gaijin have a bunch of other projects on their main GitHub Organisation too like daScript a "high-performance statically strong typed scripting language" and Quirrel the "Gaijin Entertainment dynamic script language" along with other projects.