Nix Build Enhancements: Closure Exports, Build Termination, Sandbox Paths, User Namespaces, Internal Changes, and Contributors

Nix writes a script named `.attrs.sh` to the builder’s directory that initialises shell variables corresponding to all attributes that are representable in Bash. This includes non-nested (associative) arrays. For example, the attribute `hardening.format = true` ends up as the Bash associative array element `${hardening[format]}`. - Builders can [now](https://github.com/NixOS/nix/commit/88e6bb76de5564b3217be9688677d1c89101b2a3) communicate what build phase they are in by writing messages to the file descriptor specified in `NIX_LOG_FD`. The current phase is shown by the `nix` progress indicator. - In Linux sandbox builds, we [now](https://github.com/NixOS/nix/commit/a2d92bb20e82a0957067ede60e91fab256948b41) provide a default `/bin/sh` (namely `ash` from BusyBox). - In structured attribute mode, `exportReferencesGraph` [exports](https://github.com/NixOS/nix/commit/c2b0d8749f7e77afc1c4b3e8dd36b7ee9720af4a) extended information about closures in JSON format. In particular, it includes the sizes and hashes of paths. This is primarily useful for NixOS image builders. - Builds are [now](https://github.com/NixOS/nix/commit/21948deed99a3295e4d5666e027a6ca42dc00b40) killed as soon as Nix receives EOF on the builder’s stdout or stderr. This fixes a bug that allowed builds to hang Nix indefinitely, regardless of timeouts. - The `sandbox-paths` configuration option can now specify optional paths by appending a `?`, e.g. `/dev/nvidiactl?` will bind-mount `/dev/nvidiactl` only if it exists. - On Linux, builds are now executed in a user namespace with UID 1000 and GID 100. A number of significant internal changes were made: - Nix no longer depends on Perl and all Perl components have been rewritten in C++ or removed. The Perl bindings that used to be part of Nix have been moved to a separate package, `nix-perl`. - All `Store` classes are now thread-safe. `RemoteStore` supports multiple concurrent connections to the daemon. This is primarily useful in multi-threaded programs such as `hydra-queue-runner`. This release has contributions from Adrien Devresse, Alexander Ried, Alex Cruice, Alexey Shmalko, AmineChikhaoui, Andy Wingo, Aneesh Agrawal, Anthony Cowley, Armijn Hemel, aszlig, Ben Gamari, Benjamin Hipple, Benjamin Staffin, Benno Fünfstück, Bjørn Forsman, Brian McKenna, Charles Strahan, Chase Adams, Chris Martin, Christian Theune, Chris Warburton, Daiderd Jordan, Dan Connolly, Daniel Peebles, Dan Peebles, davidak, David McFarland, Dmitry Kalinkin, Domen Kožar, Eelco Dolstra, Emery Hemingway, Eric Litak, Eric Wolf, Fabian Schmitthenner, Frederik Rietdijk, Gabriel Gonzalez, Giorgio Gallo, Graham Christensen, Guillaume Maudoux, Harmen, Iavael, James Broadhead, James Earl Douglas, Janus Troelsen, Jeremy Shaw, Joachim Schiele, Joe Hermaszewski, Joel Moberg, Johannes 'fish' Ziemke, Jörg Thalheim, Jude Taylor, kballou, Keshav Kini, Kjetil Orbekk, Langston Barrett, Linus Heckemann, Ludovic Courtès, Manav Rathi, Marc Scholten, Markus Hauck, Matt Audesse, Matthew Bauer, Matthias Beyer, Matthieu Coudron, N1X, Nathan Zadoks, Neil Mayhew, Nicolas B. Pierron, Niklas Hambüchen, Nikolay Amiantov, Ole Jørgen Brønner, Orivej Desh, Peter Simons, Peter Stuart, Pyry Jahkola, regnat, Renzo Carbonara, Rhys, Robert Vollmert, Scott Olson, Scott R. Parish, Sergei Trofimovich, Shea Levy, Sheena Artrip, Spencer Baugh, Stefan Junker, Susan Potter, Thomas Tuegel, Timothy Allen, Tristan Hume, Tuomas Tynkkynen, tv, Tyson Whitehead, Vladimír Čunát, Will Dietz, wmertens, Wout Mertens, zimbatm and Zoran Plesivčak.

The Nix build environment has been updated to include the sizes and hashes of paths in closure exports in JSON format, terminate builds immediately upon receiving EOF on stdout or stderr, allow optional sandbox paths, and execute builds in a user namespace with UID 1000 and GID 100 on Linux. Internal changes include the removal of Perl dependencies, rewriting components in C++, and making Store classes thread-safe. This release includes contributions from numerous individuals.