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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"zlib: Brotli reset() drops quality and dictionary","articleBody":"### Version\n\nv27.0.0-pre (built from `main` at `a2a051cd42`; `src/node_zlib.cc` is unchanged on current `main`)\n\n### Platform\n\n```text\nLinux x86_64 (Ubuntu 22.04, built with clang 20)\n```\n\n### Subsystem\n\nzlib\n\n### What steps will reproduce the bug?\n\nCreate a Brotli compressor with a custom quality or a dictionary, call `.reset()` **before writing anything**, then compress as usual.\n\nThe quality/dictionary you passed in is gone. Nothing throws.\n\n```js\nimport * as zlib from 'node:zlib';\n\nconst input = Buffer.from('the quick brown fox jumps over the lazy dog '.repeat(100));\nconst dict = Buffer.from('the quick brown fox jumps over the lazy dog '.repeat(40));\n\nfunction compress(options, reset) {\n  return new Promise((resolve, reject) => {\n    const s = zlib.createBrotliCompress(options);\n    const chunks = [];\n    s.on('data', (d) => chunks.push(d));\n    s.on('error', reject);\n    if (reset) s.reset();\n    s.end(input);\n    s.on('end', () => resolve(Buffer.concat(chunks).length));\n  });\n}\n\nconst q0 = { params: { [zlib.constants.BROTLI_PARAM_QUALITY]: 0 } };\nconsole.log('quality 0, no reset:', await compress(q0, false));  // 109\nconsole.log('quality 0, then reset:', await compress(q0, true)); // 55  (same as default quality 11)\nconsole.log('with dictionary, no reset:', await compress({ dictionary: dict }, false));  // 18\nconsole.log('with dictionary, then reset:', await compress({ dictionary: dict }, true)); // 55 (same as no dictionary)\n\nconst compressed = zlib.brotliCompressSync(input, { dictionary: dict });\nconst dec = zlib.createBrotliDecompress({ dictionary: dict });\ndec.reset();\ndec.end(compressed);\ndec.on('error', (err) => console.log('decompress after reset:', err.code));\n// ERR__ERROR_FORMAT_DICTIONARY\n```\n\n### How often does it reproduce? Is there a required condition?\n\nAlways. You do not need to write any data first. Just construct the stream with options and call `reset()`.\n\n### What is the expected behavior? Why is that the expected behavior?\n\n`reset()` should start a new Brotli session on the **same** stream, keeping the quality/params and dictionary you already set.\n\nThat is what the same API already documents for Zstd (`zlib.reset()`: \"start a new session while preserving the configured parameters and dictionary\"), and it is what `test/parallel/test-zlib-zstd-reset.js` asserts. Brotli goes through the same JS `reset()` method.\n\nIf the dictionary is dropped, a decoder cannot read a dictionary-compressed frame at all.\n\n### What do you see instead?\n\n- Quality 0 after `reset()` compresses as if you had used the default quality 11 (109 bytes become 55).\n- A dictionary after `reset()` compresses as if you had used no dictionary (18 bytes become 55).\n- `createBrotliDecompress({ dictionary })` then `reset()` fails with `ERR__ERROR_FORMAT_DICTIONARY` on a frame that was compressed with that dictionary.\n\nThe compressor never reports an error. The settings just disappear.\n\n### Additional information\n\nC++: `BrotliEncoderContext::ResetStream` / `BrotliDecoderContext::ResetStream` both do `return Init();` with no arguments. `Init()` immediately does `dictionary_.clear()` and creates a brand-new Brotli instance. It never replays `SetParams`.\n\nThis is not [#66087](https://github.com/nodejs/node/issues/66087) / [#66088](https://github.com/nodejs/node/pull/66088). Those are about `flush()` then `reset()` leaving a broken **frame**. Here there is no write at all — only the **configuration** is thrown away.\n\nLLDB on `src/node_zlib.cc` at `a2a051cd42` (same `ResetStream`/`Init` as current `main`):\n\n1. Stopped in `BrotliEncoderContext::ResetStream` at line 1514. Watch: `this->dictionary_` still has size **1760**. Next statement is `return Init();` with no dictionary argument.\n\n\u003cimg width=\"2620\" height=\"1596\" alt=\"ResetStream at line 1514: dictionary still size 1760, about to call empty Init()\" src=\"https://github.com/user-attachments/assets/2d03d57a-4e30-44e7-a931-33a173947bb0\" />\n\n2. That call lands in `Init` at line 1474, on `dictionary_.clear();`. Watch: the member `this->dictionary_` is **still 1760** (clear has not run yet), but the `dictionary` **parameter** is already size **0**, because `ResetStream` did not pass it in.\n\n\u003cimg width=\"3062\" height=\"1606\" alt=\"Init at line 1474: member dictionary still 1760, parameter dictionary already 0\" src=\"https://github.com/user-attachments/assets/c695f462-f74a-4902-98a9-a966a6521e0f\" />\n\n3. Continue to line 1483, `if (!dictionary.empty())`. Watch: both `this->dictionary_` and the `dictionary` parameter are now size **0**, so the attach-dictionary branch is skipped.\n\n\u003cimg width=\"3008\" height=\"1590\" alt=\"Init at line 1483: both dictionaries size 0, attach branch skipped\" src=\"https://github.com/user-attachments/assets/6c0d650e-05a9-44ed-a235-3366493b71c6\" />","author":{"url":"https://github.com/xia-chao","@type":"Person","name":"xia-chao"},"datePublished":"2026-09-20T12:42:16Z","url":"https://github.com/nodejs/node/issues/66156"}