Improve detection of postgresql conf dir and data dir

Redhat conf_dir detection was regressed in 57d7275 which inadvertently
removed the setting of @conf_dir. Any attempt to use the postgres
resource on RHEL would rain an exception:

    inspec> postgres.data_dir
    TypeError: no implicit conversion of nil into String

Further, the redhat detection code appears to assume that RHEL always
uses versioned data directories. This however, does not appear to be the
case:

    $ cat /etc/redhat-release
    CentOS release 6.7 (Final)
    $ sudo ls /var/lib/pgsql/
    backups  data  pgstartup.log

The code now can handle both versioned and un-versioned directory
formats on RHEL. Further, it provides diagnostic warnings about
uncertainty in the discovered data directories and configuration
directories.

Signed-off-by: Steven Danna <steve@chef.io>

lib/resources/postgres.rb

@@ -12,29 +12,111 @@ module Inspec::Resources
     def initialize
       os = inspec.os
       if os.debian?
-        @service = 'postgresql'
-        @data_dir = '/var/lib/postgresql'
-        @version = inspec.command('ls /etc/postgresql/').stdout.chomp
-        @conf_dir = "/etc/postgresql/#{@version}/main"
+        #
+        # https://wiki.debian.org/PostgreSql
+        #
+        # Debian allows multiple versions of postgresql to be
+        # installed as well as multiple "clusters" to be configured.
+        #
+        version = version_from_dir('/etc/postgresql')
+        cluster = cluster_from_dir("/etc/postgresql/#{version}")
+        @conf_dir = "/etc/postgresql/#{version}/#{cluster}"
+        @data_dir = "/var/lib/postgresql/#{version}/#{cluster}"
       elsif os.redhat?
-        @service = 'postgresql'
-        @version = inspec.command('ls /var/lib/pgsql/').stdout.chomp
-        @data_dir = "/var/lib/pgsql/#{@version}/data"
+        #
+        # /var/lib/pgsql/data is the default data directory on RHEL6
+        # and RHEL7. However, PR #824 explicitly added version-based
+        # directories. Thus, we call #version_from_dir unless it looks
+        # like we are using unversioned directories.
+        #
+        # TODO(ssd): This has the potential to be noisy because of the
+        # warning in version_from_dir. We should determine which case
+        # is more common and only warn in the less common case.
+        #
+        version = if inspec.directory('/var/lib/pgsql/data').exist?
+                    warn 'Found /var/lib/pgsql/data. Assuming postgresql install uses un-versioned directories.'
+                    nil
+                  else
+                    version_from_dir('/var/lib/pgsql/')
+                  end
+
+        @data_dir = File.join('/var/lib/pgsql/', version.to_s, 'data')
       elsif os[:name] == 'arch'
-        @service = 'postgresql'
+        #
+        # https://wiki.archlinux.org/index.php/PostgreSQL
+        #
+        # The archlinux wiki points to /var/lib/postgresql/data as the
+        # main data directory.
+        #
         @data_dir = '/var/lib/postgres/data'
-        @conf_dir = '/var/lib/postgres/data'
       else
-        @service = 'postgresql'
-        @data_dir = '/var/lib/postgresql'
-        @conf_dir = '/var/lib/pgsql/data'
+        #
+        # According to https://www.postgresql.org/docs/9.5/static/creating-cluster.html
+        #
+        # > There is no default, although locations such as
+        # > /usr/local/pgsql/data or /var/lib/pgsql/data are popular.
+        #
+        @data_dir = '/var/lib/pgsql/data'
       end
 
+      @service = 'postgresql'
+      @conf_dir ||= @data_dir
+      verify_dirs
       @conf_path = File.join @conf_dir, 'postgresql.conf'
     end
 
     def to_s
       'PostgreSQL'
     end
+
+    private
+
+    def verify_dirs
+      if !inspec.directory(@conf_dir).exist?
+        warn "Default postgresql configuration directory: #{@conf_dir} does not exist. Postgresql may not be installed or we've misidentified the configuration directory."
+      end
+
+      if !inspec.directory(@data_dir).exist?
+        warn "Default postgresql data directory: #{@data_dir} does not exist. Postgresql may not be installed or we've misidentified the data directory."
+      end
+    end
+
+    def version_from_dir(dir)
+      dirs = inspec.command("ls -d #{dir}/*/").stdout
+      entries = dirs.lines.count
+      case entries
+      when 0
+        warn "Could not determine version of installed postgresql by inspecting #{dir}"
+        nil
+      when 1
+        warn "Using #{dirs}: #{dir_to_version(dirs)}"
+        dir_to_version(dirs)
+      else
+        warn "Multiple versions of postgresql installed or incorrect base dir #{dir}"
+        first = dir_to_version(dirs.lines.first)
+        warn "Using the first version found: #{first}"
+        first
+      end
+    end
+
+    def dir_to_version(dir)
+      dir.chomp.split('/').last
+    end
+
+    def cluster_from_dir(dir)
+      # Main is the default cluster name on debian use it if it
+      # exists.
+      if inspec.directory("#{dir}/main").exist?
+        'main'
+      else
+        dirs = inspec.command("ls -d #{dir}/*/").stdout.lines
+        first = dirs.first.chomp.split('/').last
+        if dirs.count > 1
+          warn "Multiple postgresql clusters configured or incorrect base dir #{dir}"
+          warn "Using the first directory found: #{first}"
+        end
+        first
+      end
+    end
   end
 end