58d7231b2f
golang.org/cl/37258 was committed to fix issue #16146. This patch seemed intent to allow at most one dangling byte. But, as implemented, many more bytes may actually slip through. This is because the LZW layer creates a bufio.Reader which will itself consume data beyond the end of the LZW stream, and this isn't accounted for anywhere. This change means to avoid the allocation of the bufio.Reader by making blockReader implement io.ByteReader. Further, it adds a close() method which detects extra data in the block sequence. To avoid any regressions with poorly encoded GIFs which may have worked accidentally, there are no restrictions on how many extra bytes may exist in the final full sub-block that contained LZW data. If the end of the LZW stream happened to align with the end of a sub-block, at most one more sub-block with a length of 1 byte may exist before the block terminator. This change aims to be at least as performant as the prior implementation. But the primary gain is avoiding the allocation of a bufio.Reader per frame: name old time/op new time/op delta Decode-8 276µs ± 0% 275µs ± 2% ~ (p=0.690 n=5+5) name old speed new speed delta Decode-8 55.9MB/s ± 0% 56.3MB/s ± 2% ~ (p=0.690 n=5+5) name old alloc/op new alloc/op delta Decode-8 49.2kB ± 0% 44.8kB ± 0% -9.10% (p=0.008 n=5+5) name old allocs/op new allocs/op delta Decode-8 269 ± 0% 267 ± 0% -0.74% (p=0.008 n=5+5) Change-Id: Iec4f9b895561ad52266313fbc73ec82c070c3349 Reviewed-on: https://go-review.googlesource.com/68350 Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com> Reviewed-by: Nigel Tao <nigeltao@golang.org> |
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