https://hydra.nixos.org/eval/1564374

Add a link to official channels in the docs.

builtins.fromJSON: use nlohmann/json parser instead of custom parser

Add support for unicode escape sequences in fromJSON

As fromTOML supports \u and \U escapes, bring fromJSON on par. As JSON defaults
to UTF-8 encoding (every JSON parser must support UTF-8), this change parses the
`\u hex hex hex hex` sequence (\u followed by 4 hexadecimal digits) into an
UTF-8 representation.

Add a test to verify correct parsing, using all escape sequences from json.org.

Fixes 'building of '/nix/store/00000000000000000000000000000000-': ...'.

build: fix sandboxing on darwin

Starting ba87b08f getEnv now returns an
std::optional which means these getEnv() != "" conditions no longer happen
if the variables are not defined.

build: fix path repairing with the darwin sandbox

This prevents them from being inlined. On gcc 9, this reduces the
stack size needed for

  nix-instantiate '<nixpkgs>' -A texlive.combined.scheme-full --dry-run

from 12.9 MiB to 4.8 MiB.

Handle store path repairing on darwin when sandboxing is enabled. Unlike
on linux sandboxing on darwin still requires hash rewriting.

This shouldn't happen in normal circumstances, but just in case
attempt to move the temporary path back if possible.

passAsFile: leave out the hash prefix

Having a colon in the path may cause issues, and having the hash
function indicated isn't actually necessary. We now verify the path 
format in the tests to prevent regressions.

passAsFile: hash the attribute name instead of numbering sequentially

This makes the paths consistent without relying on ordering.

Co-authored-by: edef <edef@edef.eu>

exportReferencesGraph: support working

Before, we would get:

    [deploy@bastion:~]$ nix-store -r /nix/store/grfnl76cahwls0igd2by2pqv0dimi8h2-nixos-system-eris-19.09.20191213.03f3def.drv
    these derivations will be built:
      /nix/store/3ka4ihvwh6wsyhpd2qa9f59506mnxvx1-initrd-linux-4.19.88.drv
      /nix/store/ssxwmll7v21did1c8j027q0m8w6pg41i-unit-prometheus-alertmanager-irc-notifier.service.drv
      /nix/store/mvyvkj46ay7pp7b1znqbkck2mq98k0qd-unit-script-network-local-commands-start.drv
      /nix/store/vsl1y9mz38qfk6pyirjwnfzfggz5akg6-unit-network-local-commands.service.drv
      /nix/store/wi5ighfwwb83fdmav6z6n2fw6npm9ffl-unit-prometheus-hydra-exporter.service.drv
      /nix/store/x0qkv535n75pbl3xn6nn1w7qkrg9wwyg-unit-prometheus-packet-sd.service.drv
      /nix/store/lv491znsjxdf51xnfxh9ld7r1zg14d52-unit-script-packet-sd-env-key-pre-start.drv
      /nix/store/nw4nzlca49agsajvpibx7zg5b873gk9f-unit-script-packet-sd-env-key-start.drv
      /nix/store/x674wwabdwjrkhnykair4c8mpxa9532w-unit-packet-sd-env-key.service.drv
      /nix/store/ywivz64ilb1ywlv652pkixw3vxzfvgv8-unit-wireguard-wg0.service.drv
      /nix/store/v3b648293g3zl8pnn0m1345nvmyd8dwb-unit-script-acme-selfsigned-status.nixos.org-start.drv
      /nix/store/zci5d3zvr6fgdicz6k7jjka6lmx0v3g4-unit-acme-selfsigned-status.nixos.org.service.drv
      /nix/store/f6pwvnm63d0kw5df0v7sipd1rkhqxk5g-system-units.drv
      /nix/store/iax8071knxk9c7krpm9jqg0lcrawf4lc-etc.drv
      /nix/store/grfnl76cahwls0igd2by2pqv0dimi8h2-nixos-system-eris-19.09.20191213.03f3def.drv
    error: invalid file name 'closure-init-0' in 'exportReferencesGraph'

This was tough to debug, I didn't figure out which one was broken until I did:

    nix-store -r /nix/store/grfnl76cahwls0igd2by2pqv0dimi8h2-nixos-system-eris-19.09.20191213.03f3def.drv 2>&1 | grep  nix/store | xargs -n1 nix-store -r

and then looking at the remaining build graph:

    $ nix-store -r /nix/store/grfnl76cahwls0igd2by2pqv0dimi8h2-nixos-system-eris-19.09.20191213.03f3def.drv
    these derivations will be built:
      /nix/store/3ka4ihvwh6wsyhpd2qa9f59506mnxvx1-initrd-linux-4.19.88.drv
      /nix/store/grfnl76cahwls0igd2by2pqv0dimi8h2-nixos-system-eris-19.09.20191213.03f3def.drv
    error: invalid file name 'closure-init-0' in 'exportReferencesGraph'

and knowing the initrd build is before the system, then:

    $ nix show-derivation /nix/store/3ka4ihvwh6wsyhpd2qa9f59506mnxvx1-initrd-linux-4.19.88.drv
    {
      "/nix/store/3ka4ihvwh6wsyhpd2qa9f59506mnxvx1-initrd-linux-4.19.88.drv": {
        [...]
        "exportReferencesGraph": "closure-init-0 /nix/store/...-stage-1-init.sh closure-mdadm.conf-1 /nix/store/...-mdadm.conf closure-ubuntu.conf-2 ...",
        [...]
      }
    }

I then searched the repo for "in 'exportReferencesGraph'", found this
recently updated regex, and realized it was missing a "-".

Pass -J to tar for xz decompression

Some tar implementations can't auto-detect compression formats, so
they must be specified explicitly.

Disable use-sqlite-wal under WSL

Pass -P to cp to preserve symlinks

This is commonly the default behavior with -R, but POSIX leaves the
default unspecified.

Before:

    $ nix-channel --update
    unpacking channels...
    warning: SQLite database '/nix/var/nix/db/db.sqlite' is busy (SQLITE_PROTOCOL)
    warning: SQLite database '/nix/var/nix/db/db.sqlite' is busy (SQLITE_PROTOCOL)
    warning: SQLite database '/nix/var/nix/db/db.sqlite' is busy (SQLITE_PROTOCOL)
    warning: SQLite database '/nix/var/nix/db/db.sqlite' is busy (SQLITE_PROTOCOL)
    warning: SQLite database '/nix/var/nix/db/db.sqlite' is busy (SQLITE_PROTOCOL)

After:

    $ inst/bin/nix-channel --update
    unpacking channels...
    created 1 symlinks in user environment

I've seen complaints that "sandbox" caused problems under WSL but I'm
having no problems. I think recent changes could have fixed the issue.

This is needed to get the lastModified attribute of GitHub flakes.

Nix is multithreaded so it's not safe to change the cwd.

Fixes #3274.

This allows overriding the priority of substituters, e.g.

  $ nix-store --store ~/my-nix/ -r /nix/store/df3m4da96d84ljzxx4mygfshm1p0r2n3-geeqie-1.4 \
    --substituters 'http://cache.nixos.org?priority=100 daemon?priority=10'

Fixes #3264.

Fixes #3270.