Streaming Zip/ZipDeflate produces corrupt deflate output for specific small binary files (regression in 0.8.x)

[venkat-kaluva]

Description

corrupt-when-streamed.zip

The streaming Zip + ZipDeflate API produces invalid compressed data for certain small binary files. This is not batch-dependent — it reproduces with a single 691-byte file.

Reproduction

const fs = require('fs');
const { Zip, ZipDeflate, unzipSync } = require('fflate');

// 691-byte binary file (attached)
const fileData = fs.readFileSync('corrupt-when-streamed.eot');

const chunks = [];
const z = new Zip();
z.ondata = (err, data, final) => {
    if (err) throw err;
    chunks.push(data);
    if (final) {
        const combined = Buffer.concat(chunks);
        unzipSync(new Uint8Array(combined)); // throws "invalid distance"
    }
};

const zf = new ZipDeflate('font.eot', { level: 6 });
z.add(zf);
zf.push(fileData, true);
z.end();

Findings

Compression level	Streaming (ZipDeflate)	Synchronous (zipSync)
0 (store)	PASS	PASS
1	PASS	PASS
2	FAIL — invalid distance	PASS
3	FAIL — invalid distance	PASS
4	FAIL — invalid distance	PASS
5	FAIL — invalid distance	PASS
6	FAIL — invalid distance	PASS
7	FAIL — invalid distance	PASS
8	FAIL — invalid distance	PASS
9	FAIL — invalid distance	PASS

• Reproduces with a single 691-byte binary file — no large batch needed
• Affects compression levels 2–9. Levels 0 (store) and 1 pass
• Only the streaming Zip/ZipDeflate path is broken; zipSync works at all levels
• The deflate stream itself is invalid (decompression fails with "invalid distance too far back")
• ZIP structure (local file headers, CRC32, data descriptors, central directory) is correct
• Tested on fflate 0.8.2

Attached

corrupt-when-streamed.eot — 691-byte binary file that triggers the bug.

Workaround

Switch from streaming Zip/ZipDeflate to zipSync.

[cyfung1031]

Actually the big letter "Zip" and "Unzip" (streaming APIs) in fflate both are unstable for production use. You can see there are so many open issues regarding the streaming APIs.