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gefs: fix undercount in reserved space computation

when counting the space we needed to reserve in a pivot
node, we could undercount by up to 8 bytes; this would
only affect us when inserting a maximally sized entry
into a node, but could cause crashes.

--- a/sys/src/cmd/gefs/tree.c

+++ b/sys/src/cmd/gefs/tree.c

@@ -308,10 +308,14 @@

 	/* 

 	 * We need to guarantee there's room for one message

 	 * at all times, so that splits along the whole path

-	 * have somewhere to go as they propagate up.

+	 * have somewhere to go as they propagate up. For

+	 * each key-ptr pair we have 8 bytes of overhead,

+	 * so remember to account for that: 2 bytes of fill,

+	 * 2 bytes of offset table, and 2 bytes of length

+	 * for both key and value.

 	 */

 	bassert(b, b->type == Tpivot);

-	return 2*(b->nval+1) + b->valsz + reserve*Kpmax > Pivspc;

+	return 2*(b->nval+1) + b->valsz + reserve*(8+Keymax+Ptrsz) > Pivspc;

 }

 static void

-- 

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