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Auto merge of #18810 - asajeffrey:constellation-document-blocking, r=cbrewster

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Document the can-block-on relationship for servo.

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This PR adds some documentation comments describing the can-block-on relation that is used to ensure deadlock-freedom.

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #14704
- [X] These changes do not require tests because it's documentation

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bors-servo 2017-10-13 15:39:20 -05:00 • committed by GitHub
commit 4592e6e53a
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28
components/constellation/constellation.rs
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@ -64,6 +64,34 @@
//!
//! Since there is only one constellation, and its responsibilities include crash reporting,
//! it is very important that it does not panic.
//!
//! It's also important that the constellation not deadlock. In particular, we need
//! to be careful that we don't introduce any cycles in the can-block-on relation.
//! Blocking is typically introduced by `receiver.recv()`, which blocks waiting for the
//! sender to send some data. Servo tries to achieve deadlock-freedom by using the following
//! can-block-on relation:
//!
//! * Layout can block on canvas
//! * Layout can block on font cache
//! * Layout can block on image cache
//! * Constellation can block on compositor
//! * Constellation can block on embedder
//! * Constellation can block on layout
//! * Script can block on anything (other than script)
//! * Blocking is transitive (if T1 can block on T2 and T2 can block on T3 then T1 can block on T3)
//! * Nothing can block on itself!
//!
//! There is a complexity intoduced by IPC channels, since they do not support
//! non-blocking send. This means that as well as `receiver.recv()` blocking,
//! `sender.send(data)` can also block when the IPC buffer is full. For this reason it is
//! very important that all IPC receivers where we depend on non-blocking send
//! use a router to route IPC messages to an mpsc channel. The reason why that solves
//! the problem is that under the hood, the router uses a dedicated thread to forward
//! messages, and:
//!
//! * Anything (other than a routing thread) can block on a routing thread
//!
//! See https://github.com/servo/servo/issues/14704
use backtrace::Backtrace;
use bluetooth_traits::BluetoothRequest;