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Reimplement the Lock middleware with tokio::sync
As described in tokio-rs/tokio#2237, the `tokio::sync::Semaphore` can hold unbounded memory, especially when the semaphor is being contested and consumers drop interest. Unfortunately, this use case is common in the proxy, especially when a destination service is unavailable and the proxy is timing out requests. This change reimplements the Lock middleware without using `tokio::sync::Semaphore`. This implementation is in some ways more naive and inefficient, but it appears to be better suited for the proxy's needs. Specifically, waiters are stored in a LIFO stack, which optimizes for minimizing latency. Under certain high-load scenarios, this Lock could be forced to grow its waiters set without cleaning up expired watchers. If this becomes a more serious concern, we could change the implementation to use a FIFO queue of waiters.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,44 @@ | ||
| pub use linkerd2_error::Error; | ||
| use std::sync::Arc; | ||
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| #[derive(Debug)] | ||
| pub struct Poisoned(()); | ||
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| #[derive(Debug)] | ||
| pub struct ServiceError(Arc<Error>); | ||
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| // === impl POisoned === | ||
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| impl Poisoned { | ||
| pub fn new() -> Self { | ||
| Poisoned(()) | ||
| } | ||
| } | ||
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| impl std::fmt::Display for Poisoned { | ||
| fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result { | ||
| write!(f, "poisoned") | ||
| } | ||
| } | ||
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| impl std::error::Error for Poisoned {} | ||
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| // === impl ServiceError === | ||
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| impl ServiceError { | ||
| pub(crate) fn new(e: Arc<Error>) -> Self { | ||
| ServiceError(e) | ||
| } | ||
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| pub fn inner(&self) -> &Error { | ||
| self.0.as_ref() | ||
| } | ||
| } | ||
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| impl std::fmt::Display for ServiceError { | ||
| fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result { | ||
| self.0.fmt(f) | ||
| } | ||
| } | ||
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| impl std::error::Error for ServiceError {} |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,20 @@ | ||
| use super::Lock; | ||
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| #[derive(Clone, Debug, Default)] | ||
| pub struct LockLayer(()); | ||
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| // === impl Layer === | ||
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| impl LockLayer { | ||
| pub fn new() -> Self { | ||
| LockLayer(()) | ||
| } | ||
| } | ||
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| impl<S> tower::layer::Layer<S> for LockLayer { | ||
| type Service = Lock<S>; | ||
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| fn layer(&self, inner: S) -> Self::Service { | ||
| Self::Service::new(inner) | ||
| } | ||
| } |
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