BuildError denotes a permanent build failure, which is not the case
here.

This is UB and causes buffer overflow and crash on linux.

In particular, don't use base-64, which we don't support. (We do have
base-32 redirects for hysterical reasons.)

Also, add a test for the hashed mirror feature.

In particular, this allows it to be disabled in our tests.

E.g.

  $ nix path-info --json --store https://cache.nixos.org nixpkgs.thunderbird -S
  ...
      "downloadHash": "sha256:1jlixpzi225wwa0f4xdrwrqgi47ip1qpj9p06fyxxg07sfmyi4q0",
      "downloadSize": 43047620,
      "closureDownloadSize": 84745960
    }
  ]

This doesn't work in read-only mode, ensuring that operations like

  nix path-info --store https://cache.nixos.org -S nixpkgs.hello

(asking for the closure size of nixpkgs.hello in cache.nixos.org) work
when nixpkgs.hello doesn't exist in the local store.

Now you get

  [
    {
      "path": "/nix/store/fzvliz4j5xzvnd0w5zgw2l0ksqh578yk-bla",
      "valid": false
    }
  ]

On second though this was annoying. E.g. "nix log nixpkgs.hello" would
build/download Hello first, even though the log can be fetched
directly from the binary cache.

May need to revisit this.

Fixes #1464.

Not really necessary anymore for #849, but still nice to have.

Fixes #849.

Fixes #1443

Fixes #1432.

Fixes #1438.

As shlevy pointed out, static variables in C++11 have thread-safe
initialisation.

This allows builds to call setuid binaries. This was previously
possible until we started using seccomp. Turns out that seccomp by
default disallows processes from acquiring new privileges. Generally,
any use of setuid binaries (except those created by the builder
itself) is by definition impure, but some people were relying on this
ability for certain tests.

Example:

  $ nix build '(with import <nixpkgs> {}; runCommand "foo" {} "/run/wrappers/bin/ping -c 1 8.8.8.8; exit 1")' --no-allow-new-privileges
  builder for ‘/nix/store/j0nd8kv85hd6r4kxgnwzvr0k65ykf6fv-foo.drv’ failed with exit code 1; last 2 log lines:
    cannot raise the capability into the Ambient set
    : Operation not permitted

  $ nix build '(with import <nixpkgs> {}; runCommand "foo" {} "/run/wrappers/bin/ping -c 1 8.8.8.8; exit 1")' --allow-new-privileges
  builder for ‘/nix/store/j0nd8kv85hd6r4kxgnwzvr0k65ykf6fv-foo.drv’ failed with exit code 1; last 6 log lines:
    PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
    64 bytes from 8.8.8.8: icmp_seq=1 ttl=46 time=15.2 ms

Fixes #1429.

Also simplify the Hash API.

Fixes #1437.

Fixes #824.

Functions like copyClosure() had 3 bool arguments, which creates a
severe risk of mixing up arguments.

Also, implement copyClosure() using copyPaths().

Cygwin sqlite3 is patched to call SetDllDirectory("/usr/bin") on init, which
affects the current process and is inherited by child processes.  It causes
DLLs to be loaded from /usr/bin/ before $PATH, which breaks all sorts of
things.  A typical failures would be header/lib version mismatches (e.g.
openssl when running checkPhase on openssh).  We'll just set it back to the
default value.

Note that this is a problem with the cygwin version of sqlite3 (currently
3.18.0).  nixpkgs doesn't have the problematic patch.

There's no reason to restrict this to Error exceptions. This shouldn't
matter to #1407 since the repl doesn't catch non-Error exceptions
anyway, but you never know...

Recently aws-sdk-cpp quietly switched to using S3 virtual host URIs
(https://github.com/aws/aws-sdk-cpp/commit/69d9c53882), i.e. it sends
requests to http://<bucket>.<region>.s3.amazonaws.com rather than
http://<region>.s3.amazonaws.com/<bucket>. However this interacts
badly with curl connection reuse. For example, if we do the following:

1) Check whether a bucket exists using GetBucketLocation.
2) If it doesn't, create it using CreateBucket.
3) Do operations on the bucket.

then 3) will fail for a minute or so with a NoSuchBucket exception,
presumably because the server being hit is a fallback for cases when
buckets don't exist.

Disabling the use of virtual hosts ensures that 3) succeeds
immediately. (I don't know what S3's consistency guarantees are for
bucket creation, but in practice buckets appear to be available
immediately.)

This is returned by recent versions. Also handle NO_SUCH_KEY even
though the library doesn't actually return that at the moment.

Newer versions of aws-sdk-cpp call CalculateDelayBeforeNextRetry()
even for non-retriable errors (like NoSuchKey) whih causes log spam in
hydra-queue-runner.

Sandboxes cannot be nested, so if Nix's build runs inside a sandbox,
it cannot use a sandbox itself. I don't see a clean way to detect
whether we're in a sandbox, so use a test-specific hack.

https://github.com/NixOS/nix/issues/1413

In particular, UF_IMMUTABLE (uchg) needs to be cleared to allow the
path to be garbage-collected or optimised.

See https://github.com/NixOS/nixpkgs/issues/25819.
+       the file from being garbage-collected.

Untested, hopefully fixes #1406.

This broke "nix-store --serve".