[v1.1] Replace usage of rand_r with nrand48 (#817)

This replaces the call to rand_r with nrand48 and changes rand_seed
to be an array instead of an unsigned int.

With rand_r being deprecated in POSIX 2024, the best replacement is
nrand48, which stores its intermediate values as a 48-bit integer
in an array of shorts instead of an unsigned int and returns a
32-bit value not constrained by RAND_MAX.

nrand48 has been in the POSIX spec. since 2004 and on the large
UNIX systems since SVr4. Only things that could be affected are
BSDs older than ~2004, but those are presumably old enough that we
can ignore them.

bin/package test returned with all tests passing (particularly
tests/subshell.sh and tests/variables.sh) and monkey testing shows
getting and setting RANDOM works.

ANNOUNCE

@@ -90,3 +90,8 @@ New features in shell variables:
 
 - TIMEFORMAT now supports a precision of up to six places after the decimal
   point. The default precision for format directives remains 3.
+
+- The $RANDOM pseudorandom numbers are now generated by nrand48(3) instead
+  of rand_r(3). Since POSIX specifies a particular pseudorandom generator
+  for nrand48, this means the same pseudorandom sequence should now be
+  generated on all platforms if RANDOM is assigned the same seed value.

NEWS

@@ -2,6 +2,13 @@ This documents significant changes in the dev branch of ksh 93u+m.
 For full details, see the git log at: https://github.com/ksh93/ksh
 Uppercase BUG_* IDs are shell bug IDs as used by the Modernish shell library.
 
+2025-01-15:
+
+- [v1.1] The $RANDOM pseudorandom numbers are now generated by nrand48(3)
+  instead of rand_r(3). Since POSIX specifies a particular pseudorandom
+  generator for nrand48, this means the same pseudorandom sequence should now
+  be generated on all platforms if RANDOM is assigned the same seed value.
+
 2025-01-08:
 
 - Fixed buggy emacs mode behaviour on exceeding the maximum line length.

src/cmd/ksh93/include/variables.h

@@ -26,7 +26,7 @@
 struct rand
 {
 	Namfun_t	hdr;
-	unsigned int	rand_seed;
+	unsigned short	rand_seed[3];
 	int32_t		rand_last;
 };
 extern void sh_reseed_rand(struct rand *);

src/cmd/ksh93/include/version.h

@@ -18,7 +18,7 @@
 #include <ast_release.h>
 #include "git.h"
 
-#define SH_RELEASE_DATE	"2025-01-08"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2025-01-15"	/* must be in this format for $((.sh.version)) */
 /*
  * This comment keeps SH_RELEASE_DATE a few lines away from SH_RELEASE_SVER to avoid
  * merge conflicts when cherry-picking dev branch commits onto a release branch.

src/cmd/ksh93/sh/init.c

@@ -623,38 +623,15 @@ static Sfdouble_t nget_seconds(Namval_t* np, Namfun_t *fp)
 	return dtime(&tp) - offset;
 }
 
-#if !_lib_rand_r
-#undef RAND_MAX
-#define RAND_MAX 0x7fffffff
-#define rand_r _ksh_rand_r
 /*
- * rand_r(3) fallback nicked from FreeBSD libc.
- * License: BSD 3-clause. See the COPYRIGHT file.
- * Not to be confused with actual randomness!
- *
- * Compute x = (7^5 * x) mod (2^31 - 1)
- * without overflowing 31 bits:
- *      (2^31 - 1) = 127773 * (7^5) + 2836
- * From "Random number generators: good ones are hard to find",
- * Park and Miller, Communications of the ACM, vol. 31, no. 10,
- * October 1988, p. 1195.
+ * Seeds the rand stucture using the same algorithm as srand48()
  */
-static int rand_r(unsigned int *seed)
-{
-	long hi, lo, x;
-	/* Transform to [1, 0x7ffffffe] range. */
-	x = (*seed % 0x7ffffffe) + 1;
-	hi = x / 127773;
-	lo = x % 127773;
-	x = 16807 * lo - 2836 * hi;
-	if (x < 0)
-		x += 0x7fffffff;
-	/* Transform to [0, 0x7ffffffd] range. */
-	x--;
-	*seed = x;
-	return x;
-}
-#endif /* !_lib_rand_r */
+static void seed_rand_uint(struct rand *rp, unsigned int seed)
+{
+  	rp->rand_seed[0] = 0x330e; /* Constant from POSIX spec. */
+	rp->rand_seed[1] = (unsigned short)seed;
+	rp->rand_seed[2] = (unsigned short)(seed >> 16);
+}
 
 /*
  * These four functions are used to get and set the RANDOM variable
@@ -676,7 +653,7 @@ static void put_rand(Namval_t* np,const char *val,int flags,Namfun_t *fp)
 		n = *(Sfdouble_t*)val;
 	else
 		n = sh_arith(val);
-	rp->rand_seed = (unsigned int)n;
+	seed_rand_uint(rp, (unsigned int)n);
 	rp->rand_last = -1;
 	if(!np->nvalue)
 		np->nvalue = &rp->rand_last;
@@ -694,14 +671,8 @@ static Sfdouble_t nget_rand(Namval_t* np, Namfun_t *fp)
 	int32_t last = *lp;
 	sh_save_rand_seed(rp, 1);
 	do
-#if RAND_MAX > (RANDMASK << 3)
-		/* don't use lower bits when rand_r() generates large numbers */
-		cur = (rand_r(&rp->rand_seed) >> 3) & RANDMASK;
-#elif RAND_MAX > RANDMASK
-		cur = rand_r(&rp->rand_seed) & RANDMASK;
-#else
-		cur = rand_r(&rp->rand_seed);
-#endif
+		/* don't use lower bits when generating large numbers */
+		cur = (nrand48(rp->rand_seed) >> 3) & RANDMASK;
 	while(cur==last);
 	*lp = cur;
 	return (Sfdouble_t)cur;
@@ -715,7 +686,7 @@ static char* get_rand(Namval_t* np, Namfun_t *fp)
 
 void sh_reseed_rand(struct rand *rp)
 {
-	rp->rand_seed = arc4random();
+	seed_rand_uint(rp, arc4random());
 	rp->rand_last = -1;
 }
 

src/cmd/ksh93/sh/subshell.c

@@ -80,7 +80,7 @@ static struct subshell
 	int		cpipe;
 	char		subshare;
 	char		comsub;
-	unsigned int	rand_seed;          /* parent shell $RANDOM seed */
+	unsigned short	rand_seed[3];       /* parent shell $RANDOM seed */
 	int		rand_last;          /* last random number from $RANDOM in parent shell */
 	char		rand_state;         /* 0 means sp->rand_seed hasn't been set, 1 is the opposite */
 	uint32_t	srand_upper_bound;  /* parent shell's upper bound for $SRANDOM */
@@ -226,7 +226,7 @@ void sh_save_rand_seed(struct rand *rp, int reseed)
 	struct subshell	*sp = subshell_data;
 	if(!sh.subshare && sp && !sp->rand_state)
 	{
-		sp->rand_seed = rp->rand_seed;
+		memcpy(sp->rand_seed, rp->rand_seed, sizeof sp->rand_seed);
 		sp->rand_last = rp->rand_last;
 		sp->rand_state = 1;
 		if(reseed)
@@ -886,7 +886,7 @@ Sfio_t *sh_subshell(Shnode_t *t, volatile int flags, int comsub)
 		rp = (struct rand*)RANDNOD->nvfun;
 		if(sp->rand_state)
 		{
-			rp->rand_seed = sp->rand_seed;
+			memcpy(rp->rand_seed, sp->rand_seed, sizeof rp->rand_seed);
 			rp->rand_last = sp->rand_last;
 		}
 		/* restore $SRANDOM upper bound */