* Start of the setuid helper (the program that performs the operations

that have to be done as root: running builders under different uids,
  changing ownership of build results, and deleting paths in the store
  with the wrong ownership).

configure.ac

@@ -271,6 +271,7 @@ AC_CONFIG_FILES([Makefile
    src/nix-instantiate/Makefile
    src/nix-env/Makefile
    src/nix-worker/Makefile
+   src/nix-setuid-helper/Makefile
    src/nix-log2xml/Makefile
    src/bsdiff-4.3/Makefile
    scripts/Makefile

src/Makefile.am

@@ -1,4 +1,5 @@
 SUBDIRS = bin2c boost libutil libstore libmain nix-store nix-hash \
- libexpr nix-instantiate nix-env nix-worker nix-log2xml bsdiff-4.3
+ libexpr nix-instantiate nix-env nix-worker nix-setuid-helper \
+ nix-log2xml bsdiff-4.3
 
 EXTRA_DIST = aterm-helper.pl

src/libstore/build.cc

@@ -454,11 +454,12 @@ static void killUser(uid_t uid)
 		if (kill(-1, SIGKILL) == 0) break;
 		if (errno == ESRCH) break; /* no more processes */
 		if (errno != EINTR)
-		    throw SysError(format("cannot kill processes for UID `%1%'") % uid);
+		    throw SysError(format("cannot kill processes for uid `%1%'") % uid);
 	    }
         
         } catch (std::exception & e) {
-            std::cerr << format("killing build users: %1%\n") % e.what();
+            std::cerr << format("killing processes beloging to uid `%1%': %1%\n")
+                % uid % e.what();
             quickExit(1);
         }
         quickExit(0);
@@ -466,7 +467,7 @@ static void killUser(uid_t uid)
     
     /* parent */
     if (pid.wait(true) != 0)
-        throw Error(format("cannot kill processes for UID `%1%'") % uid);
+        throw Error(format("cannot kill processes for uid `%1%'") % uid);
 
     /* !!! We should really do some check to make sure that there are
        no processes left running under `uid', but there is no portable

src/nix-setuid-helper/Makefile.am

@@ -0,0 +1,8 @@
+libexec_PROGRAMS = nix-setuid-helper
+
+nix_setuid_helper_SOURCES = main.cc
+nix_setuid_helper_LDADD = ../libutil/libutil.la \
+ ../boost/format/libformat.la ${aterm_lib}
+
+AM_CXXFLAGS = \
+ -I$(srcdir)/.. $(aterm_include) -I$(srcdir)/../libutil

src/nix-setuid-helper/main.cc

@@ -0,0 +1,3 @@
+int main(int argc, char * * argv)
+{
+}