* Kill a build if it has gone for more than a certain number of

seconds without producing output on stdout or stderr (NIX-65).  This
  timeout can be specified using the `--max-silent-time' option or the
  `build-max-silent-time' configuration setting.  The default is
  infinity (0).

* Fix a tricky race condition: if we kill the build user before the
  child has done its setuid() to the build user uid, then it won't be
  killed, and we'll potentially lock up in pid.wait().  So also send a
  conventional kill to the child.

nix.conf.example

@@ -78,6 +78,23 @@
 #build-max-jobs = 1
 
 
+### Option `build-max-silent-time'
+#
+# This option defines the maximum number of seconds that builder can
+# go without producing any data on standard output or standard error.
+# This is useful (for instance in a automated build system) to catch
+# builds that are stuck in an infinite loop, or to catch remote builds
+# that are hanging due to network problems.  It can be overriden using
+# the `--max-silent-time' command line switch.
+#
+# The value 0 means that there is no timeout.  This is also the
+# default.
+#
+# Example:
+#   build-max-silent-time = 600 # = 10 minutes
+#build-max-silent-time = 0
+
+
 ### Option `build-users-group'
 #
 # This options specifies the Unix group containing the Nix build user