This is consistent with the behaviour of the old
download-from-binary-cache substituter.

This corresponds to the NixOS default.

Fixes #1283.

NixOps needs this.

nix-shell/pure: keep environment variable SHLVL

Previously, we tried to compute the closure in the local store, which
obviously doesn't work.

For example, this cuts "nix-copy-closure --from" on a NixOS system
closure from 15.9s to 0.5s.

This avoids the latency of the standard implementation, which can make
a huge difference (e.g. 16.5s -> 0.5s on a NixOS system closure).

They convey no useful information.

  $ ./inst/bin/nix-copy-closure --to bla $(type -p firefox)
  nix-copy-closure: src/libstore/store-api.cc:80: std::__cxx11::string nix::storePathToHash(const Path&): Assertion `base.size() >= storePathHashLen' failed.

For example, if we call brotli with an empty input, it shouldn't read
from the caller's stdin.

So if "text-compression=br", the .ls file in S3 will get a
Content-Encoding of "br". Brotli appears to compress better than xz
for this kind of file and is natively supported by browsers.

This is necessary for serving log files to browsers.

You can now set the store parameter "text-compression=br" to compress
textual files in the binary cache (i.e. narinfo and logs) using
Brotli. This sets the Content-Encoding header; the extension of
compressed files is unchanged.

You can separately specify the compression of log files using
"log-compression=br". This is useful when you don't want to compress
narinfo files for backward compatibility.

  nix: src/libutil/compression.cc:142: virtual nix::XzSink::~XzSink(): Assertion `finished' failed.

Build logs on cache.nixos.org are compressed using Brotli (since this
allows them to be decompressed automatically by Chrome and Firefox),
so it's handy if "nix log" can decompress them.

We assume that build logs are stored under log/<drv>, e.g.

  /nix/store/q7ab198v13p0f8x8wgnd75dva7d5mip6-friday-devil-0.1.1.1.drv

maps to

  https://cache.nixos.org/log/q7ab198v13p0f8x8wgnd75dva7d5mip6-friday-devil-0.1.1.1.drv

This replaces "nix-store --read-log". It checks the local store and
any configured substituters for the requested logs.

This allows various Store implementations to provide different ways to
get build logs. For example, BinaryCacheStore can get the build logs
from the binary cache.

Also, remove the log-servers option since we can use substituters for
this.

Add missing header <sstream>

Properly set the caFile for aws-sdk-cpp s3