Fix assert in background compression and encryption. (#1366)

* use assert to find whether, the async compression/encryption able to fit the
destination block in the allocated space. Allocated space is calculated using
(item.end-item.start) + padding +1

table/builder.go

@@ -149,13 +149,13 @@ func (b *Builder) handleBlock() {
 			blockBuf = eBlock
 		}
 
-		// The newend should always be less than or equal to the original end
-		// plus the padding. If the new end is greater than item.end+padding
-		// that means the data from this block cannot be stored in its existing
-		// location and trying to copy it over would mean we would over-write
-		// some data of the next block.
-		y.AssertTruef(uint32(len(blockBuf)) <= item.end+padding,
-			"newend: %d item.end: %d padding: %d", len(blockBuf), item.end, padding)
+		// BlockBuf should always less than or equal to allocated space. If the blockBuf is greater
+		// than allocated space that means the data from this block cannot be stored in its
+		// existing location and trying to copy it over would mean we would over-write some data
+		// of the next block.
+		allocatedSpace := (item.end - item.start) + padding + 1
+		y.AssertTruef(uint32(len(blockBuf)) <= allocatedSpace, "newend: %d oldend: %d padding: %d",
+			item.start+uint32(len(blockBuf)), item.end, padding)
 
 		// Acquire the buflock here. The builder.grow function might change
 		// the b.buf while this goroutine was running.