Also, move a few free-standing functions into StoreAPI and Derivation.

Also, introduce a non-nullable smart pointer, ref<T>, which is just a
wrapper around std::shared_ptr ensuring that the pointer is never
null. (For reference-counted values, this is better than passing a
"T&", because the latter doesn't maintain the refcount. Usually, the
caller will have a shared_ptr keeping the value alive, but that's not
always the case, e.g., when passing a reference to a std::thread via
std::bind.)

For example,

  $ nix-build --hash -A nix-repl.src

will build the fixed-output derivation nix-repl.src (a fetchFromGitHub
call), but instead of *verifying* the hash given in the Nix
expression, it prints out the resulting hash, and then moves the
result to its content-addressed location in the Nix store. E.g

  build produced path ‘/nix/store/504a4k6zi69dq0yjc0bm12pa65bccxam-nix-repl-8a2f5f06-src’ with sha256 hash ‘0cjablz01i0g9smnavhf86imwx1f9mnh5flax75i615ml71gsr88’

The goal of this is to make all nix-prefetch-* scripts unnecessary: we
can just let Nix run the real thing (i.e., the corresponding fetch*
derivation).

Another example:

  $ nix-build --hash -E 'with import <nixpkgs> {}; fetchgit { url = "https://github.com/NixOS/nix.git"; sha256 = "ffffffffffffffffffffffffffffffffffffffffffffffffffff"; }'
  ...
  git revision is 9e7c1a4b
  ...
  build produced path ‘/nix/store/gmsnh9i7x4mb7pyd2ns7n3c9l90jfsi1-nix’ with sha256 hash ‘1188xb621diw89n25rifqg9lxnzpz7nj5bfh4i1y3dnis0dmc0zp’

(Having to specify a fake sha256 hash is a bit annoying...)

Revert "Revert "next try for "don't abort when given unmatched '}' with 'start-condition stack underflow'. This fixes  #751"""

This reverts commit b669d3d2.

Revert "next try for "don't abort when given unmatched '}' with 'start-condition stack underflow'. This fixes  #751""

This reverts commit ed23c856. Let's
merge this *after* the 1.11.1 release.

next try for "don't abort when given unmatched '}' with 'start-condition stack underflow'. This fixes  #751"

This reverts commit 8120b6fb and fixes the regression introduced in
8d22b264.

Revert "don't abort when given unmatched '}' with 'start-condition stack underflow'. This fixes  #751"

This reverts commit 8d22b264. It
breaks Nixpkgs:

$ nix-env -qa
error: syntax error, unexpected IND_STR, expecting '}', at /home/eelco/Dev/nixpkgs-stable/pkgs/top-level/python-packages.nix:7605:8

Similar to 00903fa7. Regardless of -K,
we now also print which output differs.

Previously files in the Nix store were owned by root or by nixbld,
depending on whether they were created by a substituter or by a
builder. This doesn't matter much, but causes spurious diffoscope
differences. So use root everywhere.

This makes it easier to investigate the non-determinism, e.g.

  $ nix-build pkgs/stdenv/linux -A stage1.pkgs.zlib --check -K
  error: derivation ‘/nix/store/l54i8wlw22656i4pk05c52ngv9rpl39q-zlib-1.2.8.drv’ may not be deterministic: output ‘/nix/store/11a27shh6n2ivi4a7s964i65ql80cf27-zlib-1.2.8’ differs from ‘/nix/store/11a27shh6n2ivi4a7s964i65ql80cf27-zlib-1.2.8-check’

  $ diffoscope /nix/store/11a27shh6n2ivi4a7s964i65ql80cf27-zlib-1.2.8 /nix/store/11a27shh6n2ivi4a7s964i65ql80cf27-zlib-1.2.8-check
  ...
  ├── lib/libz.a
  │   ├── metadata
  │   │ @@ -1,15 +1,15 @@
  │   │ -rw-r--r-- 30001/30000   3096 Jan 12 15:20 2016 adler32.o
  ...
  │   │ +rw-r--r-- 30001/30000   3096 Jan 12 15:28 2016 adler32.o
  ...

This occured when sandbox building is disabled, at least one output
exists, and at least one other output does not.

E.g.

  $ nix-build pkgs/stdenv/linux/ -A stage1.pkgs.perl --check
  nix-store: src/libstore/build.cc:1323: void nix::DerivationGoal::tryToBuild(): Assertion `buildMode != bmCheck || validPaths.size() == drv->outputs.size()' failed.

when perl.out exists but perl.man doesn't. The fix is to only check
the outputs that exist. Note that "nix-build -A stage1.pkgs.all
--check" will still give a (proper) error in this case.

This was observed in the deb_debian7x86_64 build:
http://hydra.nixos.org/build/29973215

Calling c_str() on a temporary should be fine because the temporary
shouldn't be destroyed until after the execl() call, but who knows...

If repair found a corrupted/missing path that depended on a
multiple-output derivation, and some of the outputs of the latter were
not present, it failed with a message like

  error: path ‘/nix/store/cnfn9d5fjys1y93cz9shld2xwaibd7nn-bash-4.3-p42-doc’ is not valid

Also show types when nix cannot compare values of different types.
This is also more consistent since types are already shown when comparing values of the same not comparable type.

For example, "${{ foo = "bar"; __toString = x: x.foo; }}" evaluates
to "bar".

With this, we can delay calling functions like mkDerivation,
buildPythonPackage, etc. until we actually need a derivation, enabling
overrides and other modifications to happen by simple attribute set
update.

`BUFSIZ` is defined in header `<cstdio>`.

The text is just a conversion of comment from src/libexpr/primops.cc

This makes Darwin consistent with Linux: Nix expressions can't break
out of the sandbox unless relaxed sandbox mode is enabled.

For the normal sandbox mode this will require fixing #759 however.

Otherwise, since the call to write a "d" character to the lock file
can fail with ENOSPC, we can get an unhandled exception resulting in a
call to terminate().

Caused by 8063fc49. If tmpDir !=
tmpDirInSandbox (typically when there are multiple concurrent builds
with the same name), the *Path attribute would not point to an
existing file. This caused Nixpkgs' writeTextFile to write an empty
file. In particular this showed up as hanging VM builds (because it
would run an empty run-nixos-vm script and then wait for it to finish
booting).

Hopefully fixes Darwin sandbox regression introduced in
8063fc49.

We should probably disallow these, but until then, we shouldn't barf
with an assertion failure.

Fixes #738.

Also, use "#if __APPLE__" instead of "#if SANDBOX_ENABLED" to prevent
ambiguity.

This is arguably nitpicky, but I think this new formulation is even
clearer. My thinking is that it's easier to comprehend when the
calculated hash value is displayed close to the output path. (I think it
is somewhat similar to eliminating double negatives in logic
statements.)

The formulation is inspired / copied from the OpenEmbedded build tool,
bitbake.