consensus/ethash: pr feedback from @mdempsky, makes a copy of dest su…

…ch that is not mutated

consensus/ethash/algorithm.go

@@ -151,10 +151,13 @@ func generateCache(dest []uint32, epoch uint64, seed []byte) {
 		logFn("Generated ethash verification cache", "elapsed", common.PrettyDuration(elapsed))
 	}()
 	// Convert our destination slice to a byte buffer
-	header := (*reflect.SliceHeader)(unsafe.Pointer(&dest))
-	header.Len *= 4
-	header.Cap *= 4
-	cache := *(*[]byte)(unsafe.Pointer(header))
+	var cache []byte
+	cacheHdr := (*reflect.SliceHeader)(unsafe.Pointer(&cache))
+	dstHdr := (*reflect.SliceHeader)(unsafe.Pointer(&dest))
+	cacheHdr.Data = dstHdr.Data
+	cacheHdr.Len = dstHdr.Len * 4
+	cacheHdr.Cap = dstHdr.Cap * 4
+	cache = *(*[]byte)(unsafe.Pointer(cacheHdr))
 
 	// Calculate the number of theoretical rows (we'll store in one buffer nonetheless)
 	size := uint64(len(cache))