tablefunc: Reject negative number of tuples passed to normal_rand()

The function converted the first argument i.e. the number of tuples to
return into an unsigned integer which turns out to be huge number when
a negative value is passed.  This causes the function to take much
longer time to execute.  Instead, reject a negative value.

(If someone really wants to generate many more result rows, they
should consider adding a bigint or numeric variant.)

While at it, improve SQL test to test the number of tuples returned by
this function.

Author: Ashutosh Bapat <ashutosh.bapat@2ndquadrant.com>
Discussion: https://www.postgresql.org/message-id/CAG-ACPW3PUUmSnM6cLa9Rw4BEC5cEMKjX8Gogc8gvQcT3cYA1A@mail.gmail.com

contrib/tablefunc/expected/tablefunc.out

@@ -3,12 +3,15 @@ CREATE EXTENSION tablefunc;
 -- normal_rand()
 -- no easy way to do this for regression testing
 --
-SELECT avg(normal_rand)::int FROM normal_rand(100, 250, 0.2);
- avg 
------
- 250
+SELECT avg(normal_rand)::int, count(*) FROM normal_rand(100, 250, 0.2);
+ avg | count 
+-----+-------
+ 250 |   100
 (1 row)
 
+-- negative number of tuples
+SELECT avg(normal_rand)::int, count(*) FROM normal_rand(-1, 250, 0.2);
+ERROR:  number of rows cannot be negative
 --
 -- crosstab()
 --

contrib/tablefunc/sql/tablefunc.sql

@@ -4,7 +4,9 @@ CREATE EXTENSION tablefunc;
 -- normal_rand()
 -- no easy way to do this for regression testing
 --
-SELECT avg(normal_rand)::int FROM normal_rand(100, 250, 0.2);
+SELECT avg(normal_rand)::int, count(*) FROM normal_rand(100, 250, 0.2);
+-- negative number of tuples
+SELECT avg(normal_rand)::int, count(*) FROM normal_rand(-1, 250, 0.2);
 
 --
 -- crosstab()

contrib/tablefunc/tablefunc.c

@@ -184,6 +184,8 @@ normal_rand(PG_FUNCTION_ARGS)
 	/* stuff done only on the first call of the function */
 	if (SRF_IS_FIRSTCALL())
 	{
+		int32		num_tuples;
+
 		/* create a function context for cross-call persistence */
 		funcctx = SRF_FIRSTCALL_INIT();
 
@@ -193,7 +195,12 @@ normal_rand(PG_FUNCTION_ARGS)
 		oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
 
 		/* total number of tuples to be returned */
-		funcctx->max_calls = PG_GETARG_UINT32(0);
+		num_tuples = PG_GETARG_INT32(0);
+		if (num_tuples < 0)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+					 errmsg("number of rows cannot be negative")));
+		funcctx->max_calls = num_tuples;
 
 		/* allocate memory for user context */
 		fctx = (normal_rand_fctx *) palloc(sizeof(normal_rand_fctx));