Disable NSS modules when using the leakchecker

The leakchecker will report leaked file descriptors when tests do things
like access `Etc.getgrgid`, for example, if NSS modules (like `sss`)
handle these lookups by connecting to a daemon like `sssd` and leave the
connection open.

To address this, we can call glibc's `__nss_configure_lookup` to
override NSS modules configured in /etc/nsswitch.conf and only use
ordinary file/DNS lookups.

(This is a cherry-pick of a patch applied to ruby/mspec here:
https://github.com/ruby/mspec/pull/62)

spec/mspec/lib/mspec/runner/actions/leakchecker.rb

@@ -301,6 +301,7 @@ class LeakCheckerAction
   end
 
   def start
+    disable_nss_modules
     @checker = LeakChecker.new
   end
 
@@ -316,4 +317,58 @@ class LeakCheckerAction
       end
     end
   end
+
+  private
+
+  # This function is intended to disable all NSS modules when ruby is compiled
+  # against glibc. NSS modules allow the system administrator to load custom
+  # shared objects into all processes using glibc, and use them to customise
+  # the behaviour of username, groupname, hostname, etc lookups. This is
+  # normally configured in the file /etc/nsswitch.conf.
+  # These modules often do things like open cache files or connect to system
+  # daemons like sssd or dbus, which of course means they have open file
+  # descriptors of their own. This can cause the leak-checking functionality
+  # in this file to report that such descriptors have been leaked, and fail
+  # the test suite.
+  # This function uses glibc's __nss_configure_lookup function to override any
+  # configuration in /etc/nsswitch.conf, and just use the built in files/dns
+  # name lookup functionality (which is of course perfectly sufficient for
+  # running ruby/spec).
+  def disable_nss_modules
+    begin
+      require 'fiddle'
+    rescue LoadError
+      # Make sure it's possible to run the test suite on a ruby implementation
+      # which does not (yet?) have Fiddle.
+      return
+    end
+
+    begin
+      libc = Fiddle.dlopen(nil)
+      nss_configure_lookup = Fiddle::Function.new(
+        libc['__nss_configure_lookup'],
+        [Fiddle::Types::CONST_STRING, Fiddle::Types::CONST_STRING],
+        Fiddle::Types::INT
+      )
+    rescue Fiddle::DLError
+      # We're not running with glibc - no need to do this.
+      return
+    end
+
+    nss_configure_lookup.call 'passwd', 'files'
+    nss_configure_lookup.call 'shadow', 'files'
+    nss_configure_lookup.call 'group', 'files'
+    nss_configure_lookup.call 'hosts', 'files dns'
+    nss_configure_lookup.call 'services', 'files'
+    nss_configure_lookup.call 'netgroup', 'files'
+    nss_configure_lookup.call 'automount', 'files'
+    nss_configure_lookup.call 'aliases', 'files'
+    nss_configure_lookup.call 'ethers', 'files'
+    nss_configure_lookup.call 'gshadow', 'files'
+    nss_configure_lookup.call 'initgroups', 'files'
+    nss_configure_lookup.call 'networks', 'files dns'
+    nss_configure_lookup.call 'protocols', 'files'
+    nss_configure_lookup.call 'publickey', 'files'
+    nss_configure_lookup.call 'rpc', 'files'
+  end
 end