Use "ssize_t" not "long" in max_stack_depth-related code.

This change adapts these functions to the machine's address width
without depending on "long" to be the right size.  (It isn't on
Win64, for example.)  While it seems unlikely anyone would care
to run with a stack depth limit exceeding 2GB, this is part of a
general push to avoid using type "long" to represent memory sizes.

It's convenient to use ssize_t rather than the perhaps-more-obvious
choice of size_t/Size, because the code involved depends on working
with a signed data type.  Our MAX_KILOBYTES limit already ensures
that ssize_t will be sufficient to represent the maximum value of
max_stack_depth.

Extracted from a larger patch by Vladlen, plus additional hackery
by me.

Author: Vladlen Popolitov <v.popolitov@postgrespro.ru>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/1a01f0-66ec2d80-3b-68487680@27595217

Author: tglsfdc

src/backend/utils/misc/guc.c

@@ -1589,7 +1589,7 @@ static void
 InitializeGUCOptionsFromEnvironment(void)
 {
 	char	   *env;
-	long		stack_rlimit;
+	ssize_t		stack_rlimit;
 
 	env = getenv("PGPORT");
 	if (env != NULL)
@@ -1613,7 +1613,7 @@ InitializeGUCOptionsFromEnvironment(void)
 	stack_rlimit = get_stack_depth_rlimit();
 	if (stack_rlimit > 0)
 	{
-		long		new_limit = (stack_rlimit - STACK_DEPTH_SLOP) / 1024L;
+		ssize_t		new_limit = (stack_rlimit - STACK_DEPTH_SLOP) / 1024;
 
 		if (new_limit > 100)
 		{
@@ -1627,7 +1627,7 @@ InitializeGUCOptionsFromEnvironment(void)
 				new_limit = 2048;
 				source = PGC_S_DYNAMIC_DEFAULT;
 			}
-			snprintf(limbuf, sizeof(limbuf), "%ld", new_limit);
+			snprintf(limbuf, sizeof(limbuf), "%d", (int) new_limit);
 			SetConfigOption("max_stack_depth", limbuf,
 							PGC_POSTMASTER, source);
 		}

src/backend/utils/misc/stack_depth.c

@@ -26,7 +26,7 @@
 int			max_stack_depth = 100;
 
 /* max_stack_depth converted to bytes for speed of checking */
-static long max_stack_depth_bytes = 100 * 1024L;
+static ssize_t max_stack_depth_bytes = 100 * (ssize_t) 1024;
 
 /*
  * Stack base pointer -- initialized by set_stack_base(), which
@@ -109,12 +109,12 @@ bool
 stack_is_too_deep(void)
 {
 	char		stack_top_loc;
-	long		stack_depth;
+	ssize_t		stack_depth;
 
 	/*
 	 * Compute distance from reference point to my local variables
 	 */
-	stack_depth = (long) (stack_base_ptr - &stack_top_loc);
+	stack_depth = (ssize_t) (stack_base_ptr - &stack_top_loc);
 
 	/*
 	 * Take abs value, since stacks grow up on some machines, down on others
@@ -141,13 +141,13 @@ stack_is_too_deep(void)
 bool
 check_max_stack_depth(int *newval, void **extra, GucSource source)
 {
-	long		newval_bytes = *newval * 1024L;
-	long		stack_rlimit = get_stack_depth_rlimit();
+	ssize_t		newval_bytes = *newval * (ssize_t) 1024;
+	ssize_t		stack_rlimit = get_stack_depth_rlimit();
 
 	if (stack_rlimit > 0 && newval_bytes > stack_rlimit - STACK_DEPTH_SLOP)
 	{
-		GUC_check_errdetail("\"max_stack_depth\" must not exceed %ldkB.",
-							(stack_rlimit - STACK_DEPTH_SLOP) / 1024L);
+		GUC_check_errdetail("\"max_stack_depth\" must not exceed %zdkB.",
+							(stack_rlimit - STACK_DEPTH_SLOP) / 1024);
 		GUC_check_errhint("Increase the platform's stack depth limit via \"ulimit -s\" or local equivalent.");
 		return false;
 	}
@@ -158,7 +158,7 @@ check_max_stack_depth(int *newval, void **extra, GucSource source)
 void
 assign_max_stack_depth(int newval, void *extra)
 {
-	long		newval_bytes = newval * 1024L;
+	ssize_t		newval_bytes = newval * (ssize_t) 1024;
 
 	max_stack_depth_bytes = newval_bytes;
 }
@@ -167,12 +167,16 @@ assign_max_stack_depth(int newval, void *extra)
  * Obtain platform stack depth limit (in bytes)
  *
  * Return -1 if unknown
+ *
+ * Note: we choose to use ssize_t not size_t as the result type because
+ * callers compute values that could theoretically go negative,
+ * such as "result - STACK_DEPTH_SLOP".
  */
-long
+ssize_t
 get_stack_depth_rlimit(void)
 {
 #if defined(HAVE_GETRLIMIT)
-	static long val = 0;
+	static ssize_t val = 0;
 
 	/* This won't change after process launch, so check just once */
 	if (val == 0)
@@ -182,10 +186,10 @@ get_stack_depth_rlimit(void)
 		if (getrlimit(RLIMIT_STACK, &rlim) < 0)
 			val = -1;
 		else if (rlim.rlim_cur == RLIM_INFINITY)
-			val = LONG_MAX;
+			val = SSIZE_MAX;
 		/* rlim_cur is probably of an unsigned type, so check for overflow */
-		else if (rlim.rlim_cur >= LONG_MAX)
-			val = LONG_MAX;
+		else if (rlim.rlim_cur >= SSIZE_MAX)
+			val = SSIZE_MAX;
 		else
 			val = rlim.rlim_cur;
 	}

src/include/miscadmin.h

@@ -293,15 +293,15 @@ extern PGDLLIMPORT bool VacuumCostActive;
 extern PGDLLIMPORT int max_stack_depth;
 
 /* Required daylight between max_stack_depth and the kernel limit, in bytes */
-#define STACK_DEPTH_SLOP (512 * 1024L)
+#define STACK_DEPTH_SLOP (512 * 1024)
 
 typedef char *pg_stack_base_t;
 
 extern pg_stack_base_t set_stack_base(void);
 extern void restore_stack_base(pg_stack_base_t base);
 extern void check_stack_depth(void);
 extern bool stack_is_too_deep(void);
-extern long get_stack_depth_rlimit(void);
+extern ssize_t get_stack_depth_rlimit(void);
 
 /* in tcop/utility.c */
 extern void PreventCommandIfReadOnly(const char *cmdname);