Share memory support#76
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The lifetime semantics of returned mapped regions are odd: the regions are cached until all being released at once with `xShmUnmap`, but region 0 (the start of the file) might be fetched more than one. This makes a safe Rust interface nearly impossible, as sqlite wants to mutably alias whatever you return to it.
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russellromney
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Apr 13, 2026
iVersion was set to 3 in PR orbitinghail#76 when SHM support was added, but xFetch and xUnfetch were left as None. SQLite interprets iVersion>=3 as "xFetch is callable" and does not null-check before calling. Under concurrent WAL load (1 writer + 4 readers), SQLite's pager calls xFetch during checkpoint page reads, jumping to address 0x0. ~60% SEGFAULT rate. Fix: iVersion=2 (declares SHM support, not mmap support). SQLite falls back to xRead for all page reads. Includes regression test with a minimal file-backed VFS that reproduces the crash without any external VFS implementation.
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This exposes an inherently unsafe interface, because sqlite demands it. 🤷♂️