{"record":{"id":"bc62ed1db80435fa","repo":"clockworklabs/SpacetimeDB","slug":"write-position-overflow","errorCode":null,"errorMessage":"write position overflow","messagePattern":"write position overflow","errorType":"exception","errorClass":"io::Error","httpStatus":null,"severity":"error","filePath":"crates/dst/src/sim/commitlog.rs","lineNumber":278,"sourceCode":"        Self { pos: 0, storage, space }\n    }\n\n    fn len(&self) -> usize {\n        self.storage.read().unwrap().len()\n    }\n}\n\nimpl io::Write for Segment {\n    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {\n        if buf.is_empty() {\n            return Ok(0);\n        }\n\n        let mut storage = self.storage.write().unwrap();\n        let requested_end = self\n            .pos\n            .checked_add(buf.len() as u64)\n            .ok_or_else(|| io::Error::new(io::ErrorKind::InvalidInput, \"write position overflow\"))?;\n\n        if requested_end > storage.alloc {\n            let mut avail = self.space.lock().unwrap();\n\n            if self.pos >= storage.alloc {\n                let minimum_alloc = next_page_multiple(\n                    self.pos\n                        .checked_add(1)\n                        .ok_or_else(|| io::Error::new(io::ErrorKind::InvalidInput, \"write position overflow\"))?,\n                )?;\n                let needed = minimum_alloc - storage.alloc;\n                if *avail < needed {\n                    return Err(enospc());\n                }\n            }\n\n            let target_alloc = next_page_multiple(requested_end)?;\n            let wanted = target_alloc - storage.alloc;","sourceCodeStart":260,"sourceCodeEnd":296,"githubUrl":"https://github.com/clockworklabs/SpacetimeDB/blob/6dee26c6efc2856793e12b148a59742964f5d783/crates/dst/src/sim/commitlog.rs#L260-L296","documentation":"While writing to an in-memory commitlog Segment, the implementation computes requested_end = self.pos + buf.len() as u64. checked_add returns None only when that sum would exceed u64::MAX, and the write fails with ErrorKind::InvalidInput, 'write position overflow'. In practice this is a defensive guard: reaching it requires the write position to be within buf.len() of u64::MAX, which real logs never approach.","triggerScenarios":"Writing with a write position (self.pos) close to u64::MAX — essentially only in adversarial unit tests or after seeking a Segment to near-u64::MAX and then writing; a corrupted/mis-seeded position value in a hand-constructed test fixture.","commonSituations":"Test code that seeks to u64::MAX-1 and writes; essentially unreachable in production because the space budget (enospc) and real data sizes bound pos far below 2^64.","solutions":["Don't seek a Segment to extreme positions before writing in tests; keep positions realistic","If hit in test fixtures, reset/truncate the segment position before large writes","Treat hitting this in production as data corruption — inspect how the position became huge"],"exampleFix":"// before\nseg.seek(io::SeekFrom::Start(u64::MAX - 2))?;\nseg.write_all(&[0u8; 16])?; // InvalidInput: write position overflow\n\n// after\nseg.seek(io::SeekFrom::Start(0))?;\nseg.write_all(&[0u8; 16])?;","handlingStrategy":"validation","validationCode":"const MAX_WRITE_END: u64 = u64::MAX - (1 << 20); // keep 1 MiB headroom\nfn fits(pos: u64, len: usize) -> bool {\n    pos.checked_add(len as u64).map_or(false, |end| end <= MAX_WRITE_END)\n}","typeGuard":null,"tryCatchPattern":"match seg.write(buf) {\n    Err(ref e) if e.kind() == std::io::ErrorKind::InvalidInput\n        && e.to_string().contains(\"write position overflow\") => {\n        // position corrupted / test fixture at u64::MAX: reset position\n        seg.seek(io::SeekFrom::Start(0))?;\n    }\n    r => r?,\n}","preventionTips":["In tests, never seek segments near u64::MAX before writing","Bound accepted record sizes at your API boundary","Treat this error as corruption if seen outside adversarial tests"],"tags":["commitlog","storage","rust","integer-overflow","simulation"],"backgroundTag":"integer-overflow","analyzedSha":"6dee26c6efc2856793e12b148a59742964f5d783","analyzedAt":"2026-08-20T06:08:37.179Z","contentChangedAt":"2026-08-20T06:08:37.179Z","schemaVersion":2},"datasetVersion":"2026-09-14T00:17:10.932Z"}