daemon: Build `nix-setuid-helper'.

* daemon.am (libexec_PROGRAMS, nix_setuid_helper_SOURCES,
  nix_setuid_helper_CPPFLAGS, nix_setuid_helper_LDADD): New variables.
* test-env.in: Set and export `NIX_SETUID_HELPER'.
* README (Installing Guix as non-root): New section.

.gitignore

@@ -63,3 +63,4 @@ stamp-h[0-9]
 /test-tmp
 /nix/scripts/list-runtime-roots
 /test-env
+/nix/nix-setuid-helper/nix-setuid-helper.cc

README

@@ -59,10 +59,29 @@ the promise of a build; it is stored as a text file under
 `derivation' primitive, as well as higher-level wrappers such as
 `build-expression->derivation'.
 
-Guix does remote procedure calls (RPCs) to the Nix daemon (the
-=nix-worker --daemon= command), which in turn performs builds and
-accesses to the Nix store on its behalf.  The RPCs are implemented in
-the (guix store) module.
+Guix does remote procedure calls (RPCs) to the Guix or Nix daemon (the
+=guix-daemon= or =nix-daemon= command), which in turn performs builds
+and accesses to the Nix store on its behalf.  The RPCs are implemented
+in the (guix store) module.
+
+* Installing Guix as non-root
+
+The Guix daemon allows software builds to be performed under alternate
+user accounts, which are normally created specifically for this
+purpose.  For instance, you may have a pool of accounts in the
+=guixbuild= group, and then you can instruct =guix-daemon= to use them
+like this:
+
+  $ guix-daemon --build-users-group=guixbuild
+
+However, unless it is run as root, =guix-daemon= cannot switch users.
+In that case, it falls back to using a setuid-root helper program call
+=nix-setuid-helper=.  That program is not setuid-root by default when
+you install it; instead you should run a command along these lines
+(assuming Guix is installed under /usr/local):
+
+  # chown root.root /usr/local/libexec/nix-setuid-helper
+  # chmod 4755 /usr/local/libexec/nix-setuid-helper
 
 * Contact
 

daemon.am

@@ -136,6 +136,16 @@ guix_daemon_LDADD =				\
 guix_daemon_headers =				\
   nix/nix-daemon/shared.hh
 
+libexec_PROGRAMS = nix-setuid-helper
+nix_setuid_helper_SOURCES =			\
+  nix/nix-setuid-helper/nix-setuid-helper.cc
+
+nix_setuid_helper_CPPFLAGS =			\
+  $(libutil_a_CPPFLAGS)
+
+nix_setuid_helper_LDADD =			\
+  libutil.a libformat.a
+
 noinst_HEADERS =						\
   $(libformat_headers) $(libutil_headers) $(libstore_headers)	\
   $(guix_daemon_headers)

test-env.in

@@ -27,6 +27,7 @@
 if [ -x "@abs_top_builddir@/guix-daemon" ]
 then
     NIX_SUBSTITUTERS=""		# don't resort to substituters
+    NIX_SETUID_HELPER="@abs_top_builddir@/nix-setuid-helper" # normally unused
     NIX_IGNORE_SYMLINK_STORE=1	# in case the store is a symlink
     NIX_STORE_DIR="@GUIX_TEST_ROOT@/store"
     NIX_LOCALSTATE_DIR="@GUIX_TEST_ROOT@/var"
@@ -42,7 +43,7 @@ then
 
     export NIX_SUBSTITUTERS NIX_IGNORE_SYMLINK_STORE NIX_STORE_DIR	\
 	NIX_LOCALSTATE_DIR NIX_LOG_DIR NIX_STATE_DIR NIX_DB_DIR		\
-	NIX_ROOT_FINDER
+	NIX_ROOT_FINDER NIX_SETUID_HELPER
 
     # Do that because store.scm calls `canonicalize-path' on it.
     mkdir -p "$NIX_STORE_DIR"