Cheaper and safe caching for py_capsule! and py_capsule_fn! #268
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The `py_capsule!` and `py_capsule_fn!` macros generate `retrieve` functions that cache their results for subsequent calls. Prior to this commit, caching is done with a generated unsafe `static mut` item whose access is made thread-safe by a `std::sync::Once` on the side. However this synchronization is unnecessary since `retreive` takes a `Python<'_>` parameter that indicates that the GIL is held. This changes the cache to use safe `static` item instead, with `GILProtected` for synchronization-free thread-safety and `OnceCell` for interior mutability. As a bonus, this removes the need for cache-related `unsafe` code in generated `retrieve` functions. This adds a dependency to the `once_cell` crate, which can be removed when `OnceCell` becomes stable in the standard library: rust-lang/rust#74465 Alternatively `OnceCell` could be replaced with `UnsafeCell` plus some `unsafe` code, effectively inlining the core of the logic of `OnceCell`. Fixes dgrunwald#263
SimonSapin
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@markbt, any thought on this? |
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The
py_capsule!andpy_capsule_fn!macros generateretrievefunctions that cache their results for subsequent calls.Prior to this commit, caching is done with a generated unsafe
static mutitem whose access is made thread-safe by astd::sync::Onceon the side. However this synchronization is unnecessary sinceretreivetakes aPython<'_>parameter that indicates that the GIL is held.This changes the cache to use safe
staticitem instead, withGILProtectedfor synchronization-free thread-safety andOnceCellfor interior mutability. As a bonus, this removes the need for cache-relatedunsafecode in generatedretrievefunctions.This adds a dependency to the
once_cellcrate, which can be removed whenOnceCellbecomes stable in the standard library: rust-lang/rust#74465Alternatively
OnceCellcould be replaced withUnsafeCellplus someunsafecode, effectively inlining the core of the logic ofOnceCell.Fixes #263