Add function pg_xlog_location_diff to help comparisons

Comparing two xlog locations are useful for example when calculating
replication lag.

Euler Taveira de Oliveira, reviewed by Fujii Masao, and some cleanups
from me

doc/src/sgml/func.sgml

@@ -14475,11 +14475,15 @@ SELECT set_config('log_statement_stats', 'off', false);
    <indexterm>
     <primary>pg_xlogfile_name_offset</primary>
    </indexterm>
+   <indexterm>
+    <primary>pg_xlog_location_diff</primary>
+   </indexterm>
 
    <para>
     The functions shown in <xref
     linkend="functions-admin-backup-table"> assist in making on-line backups.
-    These functions cannot be executed during recovery.
+    These functions cannot be executed during recovery (except
+    <function>pg_xlog_location_diff</function>).
    </para>
 
    <table id="functions-admin-backup-table">
@@ -14547,6 +14551,13 @@ SELECT set_config('log_statement_stats', 'off', false);
        <entry><type>text</>, <type>integer</></entry>
        <entry>Convert transaction log location string to file name and decimal byte offset within file</entry>
       </row>
+      <row>
+       <entry>
+        <literal><function>pg_xlog_location_diff(<parameter>location</> <type>text</>, <parameter>location</> <type>text</>)</function></literal>
+       </entry>
+       <entry><type>numeric</></entry>
+       <entry>Calculate the difference between two transaction log locations</entry>
+      </row>
      </tbody>
     </tgroup>
    </table>
@@ -14639,6 +14650,13 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
     needs to be archived.
    </para>
 
+   <para>
+    <function>pg_xlog_location_diff</> calculates the difference in bytes
+    between two transaction log locations. It can be used with
+    <structname>pg_stat_replication</structname> or some functions shown in
+    <xref linkend="functions-admin-backup-table"> to get the replication lag.
+   </para>
+
    <para>
     For details about proper usage of these functions, see
     <xref linkend="continuous-archiving">.

src/backend/access/transam/xlogfuncs.c

@@ -26,6 +26,7 @@
 #include "replication/walreceiver.h"
 #include "storage/smgr.h"
 #include "utils/builtins.h"
+#include "utils/numeric.h"
 #include "utils/guc.h"
 #include "utils/timestamp.h"
 
@@ -465,3 +466,92 @@ pg_is_in_recovery(PG_FUNCTION_ARGS)
 {
 	PG_RETURN_BOOL(RecoveryInProgress());
 }
+
+/*
+ * Validate the text form of a transaction log location.
+ * (Just using sscanf() input allows incorrect values such as
+ * negatives, so we have to be a bit more careful about that).
+ */
+static void
+validate_xlog_location(char *str)
+{
+#define MAXLSNCOMPONENT		8
+
+	int			len1,
+				len2;
+
+	len1 = strspn(str, "0123456789abcdefABCDEF");
+	if (len1 < 1 || len1 > MAXLSNCOMPONENT || str[len1] != '/')
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
+				 errmsg("invalid input syntax for transaction log location: \"%s\"", str)));
+
+	len2 = strspn(str + len1 + 1, "0123456789abcdefABCDEF");
+	if (len2 < 1 || len2 > MAXLSNCOMPONENT || str[len1 + 1 + len2] != '\0')
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
+				 errmsg("invalid input syntax for transaction log location: \"%s\"", str)));
+}
+
+/*
+ * Compute the difference in bytes between two WAL locations.
+ */
+Datum
+pg_xlog_location_diff(PG_FUNCTION_ARGS)
+{
+	text	   *location1 = PG_GETARG_TEXT_P(0);
+	text	   *location2 = PG_GETARG_TEXT_P(1);
+	char	   *str1,
+			   *str2;
+	XLogRecPtr	loc1,
+				loc2;
+	Numeric		result;
+
+	/*
+	 * Read and parse input
+	 */
+	str1 = text_to_cstring(location1);
+	str2 = text_to_cstring(location2);
+
+	validate_xlog_location(str1);
+	validate_xlog_location(str2);
+
+	if (sscanf(str1, "%X/%X", &loc1.xlogid, &loc1.xrecoff) != 2)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+		   errmsg("could not parse transaction log location \"%s\"", str1)));
+	if (sscanf(str2, "%X/%X", &loc2.xlogid, &loc2.xrecoff) != 2)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+		   errmsg("could not parse transaction log location \"%s\"", str2)));
+
+	/*
+	 * Sanity check
+	 */
+	if (loc1.xrecoff > XLogFileSize)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("xrecoff \"%X\" is out of valid range, 0..%X", loc1.xrecoff, XLogFileSize)));
+	if (loc2.xrecoff > XLogFileSize)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("xrecoff \"%X\" is out of valid range, 0..%X", loc2.xrecoff, XLogFileSize)));
+
+	/*
+	 * result = XLogFileSize * (xlogid1 - xlogid2) + xrecoff1 - xrecoff2
+	 */
+	result = DatumGetNumeric(DirectFunctionCall2(numeric_sub,
+	   DirectFunctionCall1(int8_numeric, Int64GetDatum((int64) loc1.xlogid)),
+	 DirectFunctionCall1(int8_numeric, Int64GetDatum((int64) loc2.xlogid))));
+	result = DatumGetNumeric(DirectFunctionCall2(numeric_mul,
+	  DirectFunctionCall1(int8_numeric, Int64GetDatum((int64) XLogFileSize)),
+												 NumericGetDatum(result)));
+	result = DatumGetNumeric(DirectFunctionCall2(numeric_add,
+												 NumericGetDatum(result),
+	DirectFunctionCall1(int8_numeric, Int64GetDatum((int64) loc1.xrecoff))));
+	result = DatumGetNumeric(DirectFunctionCall2(numeric_sub,
+												 NumericGetDatum(result),
+	DirectFunctionCall1(int8_numeric, Int64GetDatum((int64) loc2.xrecoff))));
+
+	PG_RETURN_NUMERIC(result);
+}

src/include/access/xlog_internal.h

@@ -281,5 +281,6 @@ extern Datum pg_is_in_recovery(PG_FUNCTION_ARGS);
 extern Datum pg_xlog_replay_pause(PG_FUNCTION_ARGS);
 extern Datum pg_xlog_replay_resume(PG_FUNCTION_ARGS);
 extern Datum pg_is_xlog_replay_paused(PG_FUNCTION_ARGS);
+extern Datum pg_xlog_location_diff(PG_FUNCTION_ARGS);
 
 #endif   /* XLOG_INTERNAL_H */

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201203031
+#define CATALOG_VERSION_NO	201203041
 
 #endif

src/include/catalog/pg_proc.h

@@ -2936,6 +2936,9 @@ DESCR("xlog filename and byte offset, given an xlog location");
 DATA(insert OID = 2851 ( pg_xlogfile_name			PGNSP PGUID 12 1 0 0 0 f f f f t f i 1 0 25 "25" _null_ _null_ _null_ _null_ pg_xlogfile_name _null_ _null_ _null_ ));
 DESCR("xlog filename, given an xlog location");
 
+DATA(insert OID = 3165 ( pg_xlog_location_diff		PGNSP PGUID 12 1 0 0 0 f f f f t f i 2 0 1700 "25 25" _null_ _null_ _null_ _null_ pg_xlog_location_diff _null_ _null_ _null_ ));
+DESCR("difference in bytes, given two xlog locations");
+
 DATA(insert OID = 3809 ( pg_export_snapshot		PGNSP PGUID 12 1 0 0 0 f f f f t f v 0 0 25 "" _null_ _null_ _null_ _null_ pg_export_snapshot _null_ _null_ _null_ ));
 DESCR("export a snapshot");