Fix corner case bug in numeric to_char().

Trailing-zero stripping applied by the FM specifier could strip zeroes
to the left of the decimal point, for a format with no digit positions
after the decimal point (such as "FM999.").

Reported and diagnosed by Marti Raudsepp, though I didn't use his patch.

src/backend/utils/adt/formatting.c

@@ -3902,6 +3902,9 @@ NUM_prepare_locale(NUMProc *Np)
 /* ----------
  * Return pointer of last relevant number after decimal point
  *	12.0500 --> last relevant is '5'
+ *	12.0000 --> last relevant is '.'
+ * If there is no decimal point, return NULL (which will result in same
+ * behavior as if FM hadn't been specified).
  * ----------
  */
 static char *
@@ -3915,7 +3918,8 @@ get_last_relevant_decnum(char *num)
 #endif
 
 	if (!p)
-		p = num;
+		return NULL;
+
 	result = p;
 
 	while (*(++p))
@@ -4452,13 +4456,22 @@ NUM_processor(FormatNode *node, NUMDesc *Num, char *inout, char *number,
 	{
 		Np->num_pre = plen;
 
-		if (IS_FILLMODE(Np->Num))
+		if (IS_FILLMODE(Np->Num) && IS_DECIMAL(Np->Num))
 		{
-			if (IS_DECIMAL(Np->Num))
-				Np->last_relevant = get_last_relevant_decnum(
-															 Np->number +
-									 ((Np->Num->zero_end - Np->num_pre > 0) ?
-									  Np->Num->zero_end - Np->num_pre : 0));
+			Np->last_relevant = get_last_relevant_decnum(Np->number);
+
+			/*
+			 * If any '0' specifiers are present, make sure we don't strip
+			 * those digits.
+			 */
+			if (Np->last_relevant && Np->Num->zero_end > Np->num_pre)
+			{
+				char   *last_zero;
+
+				last_zero = Np->number + (Np->Num->zero_end - Np->num_pre);
+				if (Np->last_relevant < last_zero)
+					Np->last_relevant = last_zero;
+			}
 		}
 
 		if (Np->sign_wrote == FALSE && Np->num_pre == 0)

src/test/regress/expected/numeric.out

@@ -1154,6 +1154,24 @@ SELECT '' AS to_char_23, to_char(val, '9.999EEEE')				FROM num_data;
             | -2.493e+07
 (10 rows)
 
+SELECT '' AS to_char_24, to_char('100'::numeric, 'FM999.9');
+ to_char_24 | to_char 
+------------+---------
+            | 100.
+(1 row)
+
+SELECT '' AS to_char_25, to_char('100'::numeric, 'FM999.');
+ to_char_25 | to_char 
+------------+---------
+            | 100
+(1 row)
+
+SELECT '' AS to_char_26, to_char('100'::numeric, 'FM999');
+ to_char_26 | to_char 
+------------+---------
+            | 100
+(1 row)
+
 -- TO_NUMBER()
 --
 SELECT '' AS to_number_1,  to_number('-34,338,492', '99G999G999');

src/test/regress/sql/numeric.sql

@@ -764,6 +764,10 @@ SELECT '' AS to_char_21, to_char(val, '999999SG9999999999')			FROM num_data;
 SELECT '' AS to_char_22, to_char(val, 'FM9999999999999999.999999999999999')	FROM num_data;
 SELECT '' AS to_char_23, to_char(val, '9.999EEEE')				FROM num_data;
 
+SELECT '' AS to_char_24, to_char('100'::numeric, 'FM999.9');
+SELECT '' AS to_char_25, to_char('100'::numeric, 'FM999.');
+SELECT '' AS to_char_26, to_char('100'::numeric, 'FM999');
+
 -- TO_NUMBER()
 --
 SELECT '' AS to_number_1,  to_number('-34,338,492', '99G999G999');