This is primarily useful for version string generation, where we need
a monotonically increasing number. The revcount is the preferred thing
to use, but isn't available for GitHub flakes (since it requires
fetching the entire history). The last commit timestamp OTOH can be
extracted from GitHub tarballs.
This ensures that flakes don't get garbage-collected, which is
important to get nix-channel-like behaviour.
For example, running
$ nix build hydra:
will create a GC root
~/.cache/nix/flake-closures/hydra -> /nix/store/xarfiqcwa4w8r4qpz1a769xxs8c3phgn-flake-closure
where the contents/references of the linked file in the store are the
flake source trees used by the 'hydra' flake:
/nix/store/n6d5f5lkpfjbmkyby0nlg8y1wbkmbc7i-source
/nix/store/vbkg4zy1qd29fnhflsv9k2j9jnbqd5m2-source
/nix/store/z46xni7d47s5wk694359mq9ay353ar94-source
Note that this in itself is not enough to allow offline use; the
fetcher for the flakeref (e.g. fetchGit or downloadCached) must not
fail if it cannot fetch the latest version of the file, so long as it
knows a cached version.
Issue #2868.
This causes 'nix' to print build log output to stderr rather than
showing the last log line in the progress bar. Log lines are prefixed
by the name of the derivation (minus the version string), e.g.
binutils> make[1]: Leaving directory '/build/binutils-2.31.1'
binutils-wrapper> unpacking sources
binutils-wrapper> patching sources
...
binutils-wrapper> Using dynamic linker: '/nix/store/kr51dlsj9v5cr4n8700jliyz8v5b2q7q-bootstrap-stage0-glibc/lib/ld-linux-x86-64.so.2'
bootstrap-stage2-gcc-wrapper> unpacking sources
...
linux-headers> unpacking sources
linux-headers> unpacking source archive /nix/store/8javli69jhj3bkql2c35gsj5vl91p382-linux-4.19.16.tar.xz
It was getting confused between logical and real store paths.
Also, make fetchGit and fetchMercurial update allowedPaths properly.
(Maybe the evaluator, rather than the caller of the evaluator, should
apply toRealPath(), but that's a bigger change.)
