Add commentary about Cygwin's broken erand48, per report from Andrew Dunstan.

configure

@@ -19085,7 +19085,8 @@ esac
 fi
-# Cygwin's erand48 sometimes hangs, so force use of ours
+# Cygwin's erand48() is broken (always returns zero) in some releases,
+# so force use of ours.
 if test "$PORTNAME" = "cygwin"; then
   case " $LIBOBJS " in
   *" erand48.$ac_objext "* ) ;;

configure.in

 dnl Process this file with autoconf to produce a configure script.
-dnl $PostgreSQL: pgsql/configure.in,v 1.606 2009/07/23 23:50:29 adunstan Exp $
+dnl $PostgreSQL: pgsql/configure.in,v 1.607 2009/07/24 15:03:07 tgl Exp $
 dnl
 dnl Developers, please strive to achieve this order:
 dnl
@@ -1289,7 +1289,8 @@ if test "$PORTNAME" = "solaris"; then
   AC_LIBOBJ(getopt)
 fi
 
-# Cygwin's erand48 sometimes hangs, so force use of ours
+# Cygwin's erand48() is broken (always returns zero) in some releases,
+# so force use of ours.
 if test "$PORTNAME" = "cygwin"; then
   AC_LIBOBJ(erand48)
 fi

src/backend/optimizer/geqo/geqo_selection.c

@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/backend/optimizer/geqo/geqo_selection.c,v 1.25 2009/07/16 20:55:44 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/optimizer/geqo/geqo_selection.c,v 1.26 2009/07/24 15:03:07 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -42,7 +42,7 @@
 #include "optimizer/geqo_random.h"
 #include "optimizer/geqo_selection.h"
 
-static int	linear(PlannerInfo *root, int max, double bias);
+static int	linear_rand(PlannerInfo *root, int max, double bias);
 
 
 /*
@@ -57,13 +57,21 @@ geqo_selection(PlannerInfo *root, Chromosome *momma, Chromosome *daddy,
 	int			first,
 				second;
 
-	first = linear(root, pool->size, bias);
-	second = linear(root, pool->size, bias);
+	first = linear_rand(root, pool->size, bias);
+	second = linear_rand(root, pool->size, bias);
 
+	/*
+	 * Ensure we have selected different genes, except if pool size is only
+	 * one, when we can't.
+	 *
+	 * This code has been observed to hang up in an infinite loop when the
+	 * platform's implementation of erand48() is broken.  We consider that a
+	 * feature: it lets you know you'd better fix the random-number generator.
+	 */
 	if (pool->size > 1)
 	{
 		while (first == second)
-			second = linear(root, pool->size, bias);
+			second = linear_rand(root, pool->size, bias);
 	}
 
 	geqo_copy(root, momma, &pool->data[first], pool->string_length);
@@ -71,7 +79,7 @@ geqo_selection(PlannerInfo *root, Chromosome *momma, Chromosome *daddy,
 }
 
 /*
- * linear
+ * linear_rand
  *	  generates random integer between 0 and input max number
  *	  using input linear bias
  *
@@ -81,7 +89,7 @@ geqo_selection(PlannerInfo *root, Chromosome *momma, Chromosome *daddy,
  *			 bias = (prob of first rule) / (prob of middle rule)
  */
 static int
-linear(PlannerInfo *root, int pool_size, double bias)
+linear_rand(PlannerInfo *root, int pool_size, double bias)
 {
 	double		index;			/* index between 0 and pop_size */
 	double		max = (double) pool_size;