Show a sensible value in pg_settings.unit for GUC_UNIT_XSEGS variables.

Commit 88e9823 invented GUC_UNIT_XSEGS for min_wal_size and max_wal_size,
but neglected to make it display sensibly in pg_settings.unit (by adding a
case to the switch in GetConfigOptionByNum).  Fix that, and adjust said
switch to throw a run-time error the next time somebody forgets.

In passing, avoid using a static buffer for the output string --- the rest
of this function pstrdup's from a local buffer, and I see no very good
reason why the units code should do it differently and less safely.

Per report from Otar Shavadze.  Back-patch to 9.5 where the new unit type
was added.

Report: <CAG-jOyA=iNFhN+yB4vfvqh688B7Tr5SArbYcFUAjZi=0Exp-Lg@mail.gmail.com>

Author: tglsfdc

src/backend/utils/misc/guc.c

@@ -8016,20 +8016,23 @@ GetConfigOptionByNum(int varnum, const char **values, bool *noshow)
 	/* unit */
 	if (conf->vartype == PGC_INT)
 	{
-		static char buf[8];
-
 		switch (conf->flags & (GUC_UNIT_MEMORY | GUC_UNIT_TIME))
 		{
 			case GUC_UNIT_KB:
 				values[2] = "kB";
 				break;
 			case GUC_UNIT_BLOCKS:
-				snprintf(buf, sizeof(buf), "%dkB", BLCKSZ / 1024);
-				values[2] = buf;
+				snprintf(buffer, sizeof(buffer), "%dkB", BLCKSZ / 1024);
+				values[2] = pstrdup(buffer);
 				break;
 			case GUC_UNIT_XBLOCKS:
-				snprintf(buf, sizeof(buf), "%dkB", XLOG_BLCKSZ / 1024);
-				values[2] = buf;
+				snprintf(buffer, sizeof(buffer), "%dkB", XLOG_BLCKSZ / 1024);
+				values[2] = pstrdup(buffer);
+				break;
+			case GUC_UNIT_XSEGS:
+				snprintf(buffer, sizeof(buffer), "%dMB",
+						 XLOG_SEG_SIZE / (1024 * 1024));
+				values[2] = pstrdup(buffer);
 				break;
 			case GUC_UNIT_MS:
 				values[2] = "ms";
@@ -8040,7 +8043,12 @@ GetConfigOptionByNum(int varnum, const char **values, bool *noshow)
 			case GUC_UNIT_MIN:
 				values[2] = "min";
 				break;
+			case 0:
+				values[2] = NULL;
+				break;
 			default:
+				elog(ERROR, "unrecognized GUC units value: %d",
+					 conf->flags & (GUC_UNIT_MEMORY | GUC_UNIT_TIME));
 				values[2] = NULL;
 				break;
 		}

src/include/utils/guc.h

@@ -219,12 +219,12 @@ typedef enum
 #define GUC_UNIT_BLOCKS			0x2000	/* value is in blocks */
 #define GUC_UNIT_XBLOCKS		0x3000	/* value is in xlog blocks */
 #define GUC_UNIT_XSEGS			0x4000	/* value is in xlog segments */
-#define GUC_UNIT_MEMORY			0xF000	/* mask for KB, BLOCKS, XBLOCKS */
+#define GUC_UNIT_MEMORY			0xF000	/* mask for size-related units */
 
 #define GUC_UNIT_MS			   0x10000	/* value is in milliseconds */
 #define GUC_UNIT_S			   0x20000	/* value is in seconds */
 #define GUC_UNIT_MIN		   0x30000	/* value is in minutes */
-#define GUC_UNIT_TIME		   0xF0000	/* mask for MS, S, MIN */
+#define GUC_UNIT_TIME		   0xF0000	/* mask for time-related units */
 
 #define GUC_UNIT				(GUC_UNIT_MEMORY | GUC_UNIT_TIME)