Improve ereports for VACUUM's BUFFER_USAGE_LIMIT option

There's no need to check if opt->arg is NULL since defGetString() already
does that and raises an ERROR if it is.  Let's just remove that check.

Also, combine the two remaining ERRORs into a single check.  It seems
better to give an indication about what sort of values we're looking for
rather than just to state that the value given isn't valid.  Make
BUFFER_USAGE_LIMIT uppercase in this ERROR message too.  It's already
upper case in one other error message, so make that consistent.

Reported-by: Kyotaro Horiguchi
Discussion: https://postgr.es/m/20230411.102335.1643720544536884844.horikyota.ntt@gmail.com

src/backend/commands/vacuum.c

@@ -195,38 +195,21 @@ ExecVacuum(ParseState *pstate, VacuumStmt *vacstmt, bool isTopLevel)
 			int			result;
 			char	   *vac_buffer_size;
 
-			if (opt->arg == NULL)
-			{
-				ereport(ERROR,
-						(errcode(ERRCODE_SYNTAX_ERROR),
-						 errmsg("buffer_usage_limit option requires a valid value"),
-						 parser_errposition(pstate, opt->location)));
-			}
-
 			vac_buffer_size = defGetString(opt);
 
-			if (!parse_int(vac_buffer_size, &result, GUC_UNIT_KB, &hintmsg))
-			{
-				ereport(ERROR,
-						(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-						 errmsg("value: \"%s\": is invalid for buffer_usage_limit",
-								vac_buffer_size),
-						 hintmsg ? errhint("%s", _(hintmsg)) : 0));
-			}
-
 			/*
-			 * Check that the specified size falls within the hard upper and
-			 * lower limits if it is not 0.  We explicitly disallow -1 since
-			 * that behavior can be obtained by not specifying
-			 * BUFFER_USAGE_LIMIT.
+			 * Check that the specified value is valid and the size falls
+			 * within the hard upper and lower limits if it is not 0.
 			 */
-			if (result != 0 &&
-				(result < MIN_BAS_VAC_RING_SIZE_KB || result > MAX_BAS_VAC_RING_SIZE_KB))
+			if (!parse_int(vac_buffer_size, &result, GUC_UNIT_KB, &hintmsg) ||
+				(result != 0 &&
+				 (result < MIN_BAS_VAC_RING_SIZE_KB || result > MAX_BAS_VAC_RING_SIZE_KB)))
 			{
 				ereport(ERROR,
 						(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-						 errmsg("buffer_usage_limit option must be 0 or between %d kB and %d kB",
-								MIN_BAS_VAC_RING_SIZE_KB, MAX_BAS_VAC_RING_SIZE_KB)));
+						 errmsg("BUFFER_USAGE_LIMIT option must be 0 or between %d kB and %d kB",
+								MIN_BAS_VAC_RING_SIZE_KB, MAX_BAS_VAC_RING_SIZE_KB),
+						 hintmsg ? errhint("%s", _(hintmsg)) : 0));
 			}
 
 			ring_size = result;

src/test/regress/expected/vacuum.out

@@ -358,13 +358,13 @@ VACUUM (BUFFER_USAGE_LIMIT 0) vac_option_tab;
 ANALYZE (BUFFER_USAGE_LIMIT 0) vac_option_tab;
 -- value exceeds max size error
 VACUUM (BUFFER_USAGE_LIMIT 16777220) vac_option_tab;
-ERROR:  buffer_usage_limit option must be 0 or between 128 kB and 16777216 kB
+ERROR:  BUFFER_USAGE_LIMIT option must be 0 or between 128 kB and 16777216 kB
 -- value is less than min size error
 VACUUM (BUFFER_USAGE_LIMIT 120) vac_option_tab;
-ERROR:  buffer_usage_limit option must be 0 or between 128 kB and 16777216 kB
+ERROR:  BUFFER_USAGE_LIMIT option must be 0 or between 128 kB and 16777216 kB
 -- integer overflow error
 VACUUM (BUFFER_USAGE_LIMIT 10000000000) vac_option_tab;
-ERROR:  value: "10000000000": is invalid for buffer_usage_limit
+ERROR:  BUFFER_USAGE_LIMIT option must be 0 or between 128 kB and 16777216 kB
 HINT:  Value exceeds integer range.
 -- incompatible with VACUUM FULL error
 VACUUM (BUFFER_USAGE_LIMIT '512 kB', FULL) vac_option_tab;