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This requires a new associated type for mapped regions which would break backward compatibility without default associated types, which is currently unstable in Rust. Instead, a new `ShmVfs` trait is introduced with alternate registration methods. Most of the code is shared between the two except for the open/close glue functions for `ShmVfs`, which must initialize and tear down a derived file handle structure. The lifetime semantics of returned mapped regions is a little odd: the region is only accessed once by sqlite immediately upon return, but we don't know for sure when it's done until it explicitly requests a new region, unmaps all regions, or closes the handle. Therefore the API glue obtains a region from the trait as an owned `AsMut<[u8]>` and holds on to it until its known to no longer be referenced, at which point its `Drop` impl can release resources. The previous code which cast the `sqlite3_file` directly to `&mut` looked like it could lead to undefined behavior (it's UB to create an `&mut` to uninitialized memory, and even if sqlite zeroes the memory first, if there were any embedded `NonZero` you're still in UB territory). I instead `ptr::write` and `ptr::read` the whole thing.
Contributor
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Woof, this is a much larger change. I'd like to find time to connect and chat about this one before merging. Are you open to that? |
Contributor
Author
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On Discord? Yeah, hold on. |
Contributor
Author
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This can't work as designed since sqlite does hold multiple mapped regions at once, making a safe Rust interface very challenging. Needs a rework. |
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This requires a new associated type for mapped pages/regions which would break backward compatibility without default associated types, which is currently unstable in Rust. Instead, a new
ShmVfstrait is introduced with alternate registration methods. Most of the code is shared between the two except for the open/close glue functions forShmVfs, which must initialize and tear down a derived file handle structure.The lifetime semantics of returned mapped regions is a little odd: the region is only accessed once by sqlite immediately upon return, but we don't know for sure when it's done until it explicitly requests a new region, unmaps all regions, or closes the handle. Therefore the API glue obtains a region from the trait as an owned
AsMut<[u8]>and holds on to it until its known to no longer be referenced, at which point itsDropimpl can release resources.The previous code which cast the
sqlite3_filedirectly to&mutlooked like it could lead to undefined behavior (it's UB to create an&mutto uninitialized memory, and even if sqlite zeroes the memory first, if there were any embeddedNonZeroyou're still in UB territory). I insteadptr::writeandptr::readthe whole thing.