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pool: more strict checktree() for poolcheck

check that Free.next and Free.prev pointers are not nil.

check that Free.left and Free.right are Poison in non-tree nodes.

check that Free.left and Free.right are *not* Poison in tree nodes.

change Poison to 0xffffffffcafebabe for 64bit machines.

--- a/sys/src/libc/port/pool.c

+++ b/sys/src/libc/port/pool.c

@@ -123,7 +123,7 @@

 static uchar datamagic[] = { 0xFE, 0xF1, 0xF0, 0xFA };

-#define	Poison	(void*)0xCafeBabe

+#define Poison	((void*)-0x35014542)	/* cafebabe */

 #define _B2D(a)	((void*)((uchar*)a+sizeof(Bhdr)))

 #define _D2B(v)	((Alloc*)((uchar*)v-sizeof(Bhdr)))

@@ -197,12 +197,12 @@

 	Free *q;

 	for(q=t->next; q!=t; q=q->next){

-		assert(q->size == t->size);

-		assert(q->next==nil || q->next->prev==q);

-		assert(q->prev==nil || q->prev->next==q);

-	//	assert(q->left==nil);

-	//	assert(q->right==nil);

 		assert(q->magic==FREE_MAGIC);

+		assert(q->size==t->size);

+		assert(q->left==Poison);

+		assert(q->right==Poison);

+		assert(q->next!=nil && q->next!=Poison && q->next->prev==q);

+		assert(q->prev!=nil && q->prev!=Poison && q->prev->next==q);

 	}

 }

@@ -211,8 +211,10 @@

 {

 	assert(t->magic==FREE_MAGIC);

 	assert(a < t->size && t->size < b);

-	assert(t->next==nil || t->next->prev==t);

-	assert(t->prev==nil || t->prev->next==t);

+	assert(t->left!=Poison);

+	assert(t->right!=Poison);

+	assert(t->next!=nil && t->next!=Poison && t->next->prev==t);

+	assert(t->prev!=nil && t->prev!=Poison && t->prev->next==t);

 	checklist(t);

 	if(t->left)

 		checktree(t->left, a, t->size);