image/gif: make blockReader a ByteReader, harden tests
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)