Add a GUC to report whether data page checksums are enabled.

Backported from master. It was an oversight in the original data checksums
patch to not have a GUC like this.

Author: hlinnaka

doc/src/sgml/config.sgml

@@ -6171,6 +6171,19 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-data-checksums" xreflabel="data_checksums">
+      <term><varname>data_checksums</varname> (<type>boolean</type>)</term>
+      <indexterm>
+       <primary><varname>data_checksums</> configuration parameter</primary>
+      </indexterm>
+      <listitem>
+       <para>
+        Reports whether data checksums are enabled for this cluster.
+        See <xref linkend="app-initdb-data-checksums"> for more information.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-integer-datetimes" xreflabel="integer_datetimes">
       <term><varname>integer_datetimes</varname> (<type>boolean</type>)</term>
       <indexterm>

src/backend/access/transam/xlog.c

@@ -3739,6 +3739,10 @@ ReadControlFile(void)
 				  " but the server was compiled without USE_FLOAT8_BYVAL."),
 				 errhint("It looks like you need to recompile or initdb.")));
 #endif
+
+	/* Make the initdb settings visible as GUC variables, too */
+	SetConfigOption("data_checksums", DataChecksumsEnabled() ? "yes" : "no",
+					PGC_INTERNAL, PGC_S_OVERRIDE);
 }
 
 void

src/backend/utils/misc/guc.c

@@ -464,6 +464,7 @@ static int	max_identifier_length;
 static int	block_size;
 static int	segment_size;
 static int	wal_block_size;
+static bool	data_checksums;
 static int	wal_segment_size;
 static bool integer_datetimes;
 static int	effective_io_concurrency;
@@ -1453,6 +1454,17 @@ static struct config_bool ConfigureNamesBool[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"data_checksums", PGC_INTERNAL, PRESET_OPTIONS,
+			gettext_noop("Shows whether data checksums are turned on for this cluster"),
+			NULL,
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+		},
+		&data_checksums,
+		false,
+		NULL, NULL, NULL
+	},
+
 	/* End-of-list marker */
 	{
 		{NULL, 0, 0, NULL, NULL}, NULL, false, NULL, NULL, NULL