Merge branch 'nix'.

This is a squashed commit of the following:

commit 0dccab9f417b406f5d4aedc81900fc7b2f16c9f6
Author: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date:   Thu Jul 2 00:30:16 2015 +0200

    Typo

commit 2cd28517b13524c242c7758783b0b2d8250fdded
Author: Ludovic Courtès <ludo@gnu.org>
Date:   Wed Jul 1 14:56:34 2015 +0200

    Preserve supplementary groups of build users

    The following patch is an attempt to address this bug (see
    <http://bugs.gnu.org/18994>) by preserving the supplementary groups of
    build users in the build environment.

    In practice, I would expect that supplementary groups would contain only
    one or two groups: the build users group, and possibly the “kvm” group.

    [Changed &at(0) to data() and removed tabs - Eelco]

commit 6e38685ef65284093df79ebe7378bac33b0e7e5d
Author: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date:   Tue Jun 30 21:41:26 2015 +0200

    GC: Handle ENOSPC creating/moving to the trash directory

    Issue #564.

commit 5e0a9ae2e25a1016389f4893a6ed6682aadcf51d
Author: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date:   Mon Jun 22 15:54:55 2015 +0200

    Use posix_fallocate to create /nix/var/nix/db/reserved

commit 4e5ab98d6d14f8b0e3bd1d77b2f4f2354e7a49a8
Author: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date:   Mon Jun 22 15:47:40 2015 +0200

    Make /nix/var/nix/db/reserved bigger

    Issue #564.

commit 60bda60fc06135aa97a93301b1a9e2270768f5b3
Author: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date:   Wed Jun 10 16:17:06 2015 +0200

    Export outputPaths function

    This is useful for the new hydra-queue-runner.

commit 5dfea34048aa8541f20aeb2fbcd163561b609a49
Author: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date:   Thu Jul 2 22:51:33 2015 +0200

    Use std::vector::data()

commit 2459458bc8257734ca78cb7a2db3df20bd730ec0
Author: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date:   Thu Jun 4 16:04:41 2015 +0200

    Allow substitutes for builds that have preferLocalBuild set

    Not substituting builds with "preferLocalBuild = true" was a bad idea,
    because it didn't take the cost of dependencies into account. For
    instance, if we can't substitute a fetchgit call, then we have to
    download/build git and all its dependencies.

    Partially reverts 5558652709f27e8a887580b77b93c705659d7a4b and adds a
    new derivation attribute "allowSubstitutes" to specify whether a
    derivation may be substituted.

2 files changed, 6 insertions, 7 deletions

diff --git a/nix/libutil/util.cc b/nix/libutil/util.cc
index dab4235b04..14026ab829 100644
--- a/nix/libutil/util.cc
+++ b/nix/libutil/util.cc
@@ -897,10 +897,10 @@ pid_t startProcess(std::function<void()> fun,
 }
 
 
-std::vector<const char *> stringsToCharPtrs(const Strings & ss)
+std::vector<char *> stringsToCharPtrs(const Strings & ss)
 {
-    std::vector<const char *> res;
-    for (auto & s : ss) res.push_back(s.c_str());
+    std::vector<char *> res;
+    for (auto & s : ss) res.push_back((char *) s.c_str());
     res.push_back(0);
     return res;
 }
@@ -921,12 +921,11 @@ string runProgram(Path program, bool searchPath, const Strings & args)
 
         Strings args_(args);
         args_.push_front(program);
-        auto cargs = stringsToCharPtrs(args_);
 
         if (searchPath)
-            execvp(program.c_str(), (char * *) &cargs[0]);
+            execvp(program.c_str(), stringsToCharPtrs(args_).data());
         else
-            execv(program.c_str(), (char * *) &cargs[0]);
+            execv(program.c_str(), stringsToCharPtrs(args_).data());
 
         throw SysError(format("executing `%1%'") % program);
     });
diff --git a/nix/libutil/util.hh b/nix/libutil/util.hh
index 6a84ed8851..24e16ba36a 100644
--- a/nix/libutil/util.hh
+++ b/nix/libutil/util.hh
@@ -284,7 +284,7 @@ MakeError(ExecError, Error)
 /* Convert a list of strings to a null-terminated vector of char
    *'s. The result must not be accessed beyond the lifetime of the
    list of strings. */
-std::vector<const char *> stringsToCharPtrs(const Strings & ss);
+std::vector<char *> stringsToCharPtrs(const Strings & ss);
 
 /* Close all file descriptors except stdin, stdout, stderr, and those
    listed in the given set.  Good practice in child processes. */