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

1 files changed, 90 insertions, 0 deletions

diff --git a/src/backend/access/transam/xlogfuncs.c b/src/backend/access/transam/xlogfuncs.c
index 2e10d4d15f7..08b5724b97e 100644
--- a/src/backend/access/transam/xlogfuncs.c
+++ b/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);
+}