Since POSIX removed rand_r(3), provide fallback (re: af6a32d)

As of the referenced commit, the rand_r(3) function is used for
$RANDOM instead of rand(3). This is necessary to keep repeatable
seeded $RANDOM sequences working in the shell -- particularly in
combination with virtual subshells, which require us to save and
restore the intermediate seed value. Only rand_r will allow us to
access and control that seed value so we can save it.

However, POSIX removed rand_r in the 2024 edition, so we cannot
count on operating systems continuing to provide it forever.

src/lib/libast/features/{lib,sys}:
- Add lib test for rand_r, checking if it exists in OS libraries.
  Defines _lib_rand_r as 1 if found.
- Add extern test for rand_r, checking if the extern declaration is
  found in the system headers and emitting one if not. This is
  needed because some operating systems (e.g., NetBSD) keep
  deprecated functions in their libraries for ABI compatibility
  purposes while hiding or removing their header declarations.

src/cmd/ksh93/sh/{init,subshell}.c:
- Add rand_r fallback implementation (nicked from FreeBSD libc) if
  !_lib_rand_r (i.e., rand_r is not found in system libraries).
- Since this is a static function (no need for extern here as it's
  only used in init.c), use a #define to rename it to _ksh_rand_r
  to avoid a conflict with the extern declaration.
- Eliminate pointless srand(3) calls. It seeds rand(3) sequences;
  it's irrelevant to rand_r(3) because we give it a pointer to our
  own seed variable! All we need to do is assign to that.
- Eliminate an init-time loop that calls rand(3) to check whether
  it returns large values. This is not inefficient, and now invalid
  in case we provide our own rand_r whose range may differ from the
  OS's rand. Determine this at compile time instead, using the
  RAND_MAX macro (which we redefine when providing our own rand_r).

Resolves: #806

src/cmd/ksh93/sh/init.c

@@ -216,7 +216,6 @@ static Init_t		*nv_init(void);
 #if SHOPT_STATS
 static void		stat_init(void);
 #endif
-static int		rand_shift;
 
 /*
  * Exception callback routine for stk(3) and sh_*alloc wrappers.
@@ -624,6 +623,39 @@ 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.
+ */
+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 */
+
 /*
  * These four functions are used to get and set the RANDOM variable
  */
@@ -644,7 +676,7 @@ static void put_rand(Namval_t* np,const char *val,int flags,Namfun_t *fp)
 		n = *(Sfdouble_t*)val;
 	else
 		n = sh_arith(val);
-	srand(rp->rand_seed = (unsigned int)n);
+	rp->rand_seed = (unsigned int)n;
 	rp->rand_last = -1;
 	if(!np->nvalue)
 		np->nvalue = &rp->rand_last;
@@ -662,7 +694,14 @@ static Sfdouble_t nget_rand(Namval_t* np, Namfun_t *fp)
 	int32_t last = *lp;
 	sh_save_rand_seed(rp, 1);
 	do
-		cur = (rand_r(&rp->rand_seed)>>rand_shift)&RANDMASK;
+#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
 	while(cur==last);
 	*lp = cur;
 	return (Sfdouble_t)cur;
@@ -676,7 +715,7 @@ static char* get_rand(Namval_t* np, Namfun_t *fp)
 
 void sh_reseed_rand(struct rand *rp)
 {
-	srand(rp->rand_seed = arc4random());
+	rp->rand_seed = arc4random();
 	rp->rand_last = -1;
 }
 
@@ -1254,15 +1293,6 @@ Shell_t *sh_init(int argc,char *argv[], Shinit_f userinit)
 	error_info.catalog = e_dict;
 	sh.cpipe[0] = -1;
 	sh.coutpipe = -1;
-	for(n=0;n < 10; n++)
-	{
-		/* don't use lower bits when rand() generates large numbers */
-		if(rand() > RANDMASK)
-		{
-			rand_shift = 3;
-			break;
-		}
-	}
 	sh_ioinit();
 	/* initialize signal handling */
 	sh_siginit();

src/cmd/ksh93/sh/subshell.c

@@ -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)
 		{
-			srand(rp->rand_seed = sp->rand_seed);
+			rp->rand_seed = sp->rand_seed;
 			rp->rand_last = sp->rand_last;
 		}
 		/* restore $SRANDOM upper bound */

src/lib/libast/features/lib

@@ -32,6 +32,7 @@ lib	glob,iswblank,iswctype,killpg,link,localeconv,madvise
 lib	mbtowc,mbrtowc,memalign,memdup
 lib	mkdir,mkfifo,mktemp,mktime
 lib	mount,opendir,openat,pathconf
+lib	rand_r
 lib	readlink,remove,rename,rewinddir,rmdir,setlocale
 lib	setpgrp,setpgrp2,setreuid,setuid
 lib	socketpair

src/lib/libast/features/sys

@@ -176,7 +176,7 @@ extern	pause		int		(void)
 extern	pipe		int		(int[])
 extern	pvalloc		void*		(size_t)
 extern	qsort		void		(void*, size_t, size_t, int(*)(const void*, const void*))
-extern	rand		int		(void)
+extern	rand_r		int		(unsigned int*)
 extern	read		ssize_t		(int, void*, size_t)
 extern	realloc		void*		(void*, size_t)
 extern	realpath	char*		(const char*, char*)
@@ -187,7 +187,6 @@ extern	setpgid		int		(pid_t, pid_t)
 extern	setsid		pid_t		(void)
 extern	setuid		int		(uid_t)
 extern	sleep		unsigned		(unsigned int)
-extern	srand		void		(unsigned int)
 extern	strcasecmp	int		(const char*, const char*)
 extern	strcoll		int		(const char*, const char*)
 extern	strcspn		size_t		(const char*, const char*)