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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"zlib: BrotliCompress.flush() with Z_FINISH or Z_BLOCK hangs at 100% CPU","articleBody":"### Version\n\nv26.3.0 (also reproduces on v22.20.0 and v27.0.0-nightly20260601aa444b2f58)\n\n### Platform\n\n```text\nDarwin arm64 but not platform-specific\n```\n\n### Subsystem\n\nzlib\n\n### What steps will reproduce the bug?\n\n```ts\nconst zlib = require(\"zlib\");\nzlib.createBrotliCompress().flush(zlib.constants.Z_FINISH);\n```\nThe process spins at ~100% CPU and never exits\n\n### How often does it reproduce? Is there a required condition?\n\nEvery time. flush(Z_FINISH) (4) and flush(Z_BLOCK) (5) both hang. flush(Z_TREES) (6) throws, and 0–3 are the valid brotli operations.\n\n### What is the expected behavior? Why is that the expected behavior?\n\nIt should throw (as flush(6) already does) or be a no-op, not loop forever. Z_FINISH and Z_BLOCK are zlib flush modes with no corresponding brotli operation, so they reach a state the flush loop never satisfies.\n\n### What do you see instead?\n\nThe call never returns. CPU stays pinned and the event loop makes no progress. Calling .end() afterwards does not drain it.\n\n### Additional information\n\n_No response_","author":{"url":"https://github.com/alii","@type":"Person","name":"alii"},"datePublished":"2026-06-01T18:17:26.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/63701/node/issues/63701"}