fix: handle weezl NoProgress on small interlaced GIF output buffers#233
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lilith wants to merge 2 commits intoimage-rs:masterfrom
Open
fix: handle weezl NoProgress on small interlaced GIF output buffers#233lilith wants to merge 2 commits intoimage-rs:masterfrom
lilith wants to merge 2 commits intoimage-rs:masterfrom
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Three valid interlaced GIFs (7x7, 9x9, 47x63) produced by gifsicle 1.95 are rejected with "image truncated". All decode correctly with giftopnm (netpbm) and are valid per the GIF89a spec. Root cause: when decoding interlaced frames, read_into_buffer passes per-row slices to the LZW decoder. For small images (7-47 byte rows), weezl's NoProgress status triggers two bugs in DecodeSubBlock: 1. Input data is skipped instead of retried with a smaller buffer 2. NoProgress during flush is treated as end-of-stream Related: image-rs/weezl#72
When decoding interlaced frames, read_into_buffer passes per-row slices to the LZW decoder. For small images (7-47 byte rows), weezl returns NoProgress because the output buffer is too small for its internal state machine to advance. Two bugs fixed in DecodeSubBlock: 1. Input data skipped (left > 0): When NoProgress with consumed=0 and bytes_len=0, the decoder executed `consumed = n`, skipping all remaining sub-block input without processing it. 2. Premature frame end (left = 0): During flush, NoProgress with 0 output was treated as end-of-stream, but weezl still had buffered data. Fix: retry LZW decode with a 1-byte temp buffer (lzw_drain_one) to unstick weezl, then append the byte to the caller's output. Related: image-rs/weezl#72
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Stacked on #232 (regression tests).
Problem
Small interlaced GIFs (7x7, 9x9, 47x63) produced by gifsicle 1.95 are rejected with "image truncated". All decode correctly with giftopnm (netpbm).
Root cause
When decoding interlaced frames,
read_into_bufferpasses per-row slices to the LZW decoder. For small images (7-47 byte rows), weezl returnsNoProgressbecause the output buffer is too small for its internal state machine to advance.Two bugs in
DecodeSubBlock:left > 0):NoProgresswithconsumed=0, bytes_len=0causedconsumed = n, skipping all remaining sub-block inputleft = 0):NoProgressduring flush treated as end-of-streamFix
lzw_drain_onehelper: retry LZW decode with a 1-byte temp buffer to unstick weezl, append the byte to the caller's output viaOutputBuffer::append.Related: image-rs/weezl#72
Test plan