76 lines
3.5 KiB
Markdown
76 lines
3.5 KiB
Markdown
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% Send and Sync
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Not everything obeys inherited mutability, though. Some types allow you to multiply
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alias a location in memory while mutating it. Unless these types use synchronization
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to manage this access, they are absolutely not thread safe. Rust captures this with
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through the `Send` and `Sync` traits.
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* A type is Send if it is safe to send it to another thread.
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* A type is Sync if it is safe to share between threads (`&T` is Send).
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Send and Sync are *very* fundamental to Rust's concurrency story. As such, a
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substantial amount of special tooling exists to make them work right. First and
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foremost, they're *unsafe traits*. This means that they are unsafe *to implement*,
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and other unsafe code can *trust* that they are correctly implemented. Since
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they're *marker traits* (they have no associated items like methods), correctly
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implemented simply means that they have the intrinsic properties an implementor
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should have. Incorrectly implementing Send or Sync can cause Undefined Behaviour.
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Send and Sync are also what Rust calls *opt-in builtin traits*.
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This means that, unlike every other trait, they are *automatically* derived:
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if a type is composed entirely of Send or Sync types, then it is Send or Sync.
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Almost all primitives are Send and Sync, and as a consequence pretty much
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all types you'll ever interact with are Send and Sync.
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Major exceptions include:
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* raw pointers are neither Send nor Sync (because they have no safety guards)
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* `UnsafeCell` isn't Sync (and therefore `Cell` and `RefCell` aren't)
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* `Rc` isn't Send or Sync (because the refcount is shared and unsynchronized)
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`Rc` and `UnsafeCell` are very fundamentally not thread-safe: they enable
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unsynchronized shared mutable state. However raw pointers are, strictly speaking,
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marked as thread-unsafe as more of a *lint*. Doing anything useful
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with a raw pointer requires dereferencing it, which is already unsafe. In that
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sense, one could argue that it would be "fine" for them to be marked as thread safe.
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However it's important that they aren't thread safe to prevent types that
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*contain them* from being automatically marked as thread safe. These types have
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non-trivial untracked ownership, and it's unlikely that their author was
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necessarily thinking hard about thread safety. In the case of Rc, we have a nice
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example of a type that contains a `*mut` that is *definitely* not thread safe.
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Types that aren't automatically derived can *opt-in* to Send and Sync by simply
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implementing them:
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```rust
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struct MyBox(*mut u8);
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unsafe impl Send for MyBox {}
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unsafe impl Sync for MyBox {}
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```
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In the *incredibly rare* case that a type is *inappropriately* automatically
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derived to be Send or Sync, then one can also *unimplement* Send and Sync:
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```rust
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struct SpecialThreadToken(u8);
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impl !Send for SpecialThreadToken {}
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impl !Sync for SpecialThreadToken {}
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```
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Note that *in and of itself* it is impossible to incorrectly derive Send and Sync.
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Only types that are ascribed special meaning by other unsafe code can possible cause
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trouble by being incorrectly Send or Sync.
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Most uses of raw pointers should be encapsulated behind a sufficient abstraction
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that Send and Sync can be derived. For instance all of Rust's standard
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collections are Send and Sync (when they contain Send and Sync types)
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in spite of their pervasive use raw pointers to
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manage allocations and complex ownership. Similarly, most iterators into these
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collections are Send and Sync because they largely behave like an `&` or `&mut`
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into the collection.
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TODO: better explain what can or can't be Send or Sync. Sufficient to appeal
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only to data races?
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