98 lines
4.2 KiB
C#
98 lines
4.2 KiB
C#
// Copyright (c) .NET Foundation. All rights reserved.
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// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
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using System;
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using System.Runtime.CompilerServices;
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namespace Microsoft.AspNetCore.Http.Features
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{
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public struct FeatureReferences<TCache>
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{
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public FeatureReferences(IFeatureCollection collection)
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{
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Collection = collection;
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Cache = default(TCache);
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Revision = collection.Revision;
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}
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public IFeatureCollection Collection { get; private set; }
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public int Revision { get; private set; }
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// cache is a public field because the code calling Fetch must
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// be able to pass ref values that "dot through" the TCache struct memory,
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// if it was a Property then that getter would return a copy of the memory
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// preventing the use of "ref"
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public TCache Cache;
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// Careful with modifications to the Fetch method; it is carefully constructed for inlining
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// See: https://github.com/aspnet/HttpAbstractions/pull/704
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// This method is 59 IL bytes and at inline call depth 3 from accessing a property.
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// This combination is enough for the jit to consider it an "unprofitable inline"
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// Aggressively inlining it causes the entire call chain to dissolve:
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//
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// This means this call graph:
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//
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// HttpResponse.Headers -> Response.HttpResponseFeature -> Fetch -> Fetch -> Revision
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// -> Collection -> Collection
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// -> Collection.Revision
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// Has 6 calls eliminated and becomes just: -> UpdateCached
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//
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// HttpResponse.Headers -> Collection.Revision
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// -> UpdateCached (not called on fast path)
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//
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// As this is inlined at the callsite we want to keep the method small, so it only detects
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// if a reset or update is required and all the reset and update logic is pushed to UpdateCached.
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//
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// Generally Fetch is called at a ratio > x4 of UpdateCached so this is a large gain
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[MethodImpl(MethodImplOptions.AggressiveInlining)]
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public TFeature Fetch<TFeature, TState>(
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ref TFeature cached,
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TState state,
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Func<TState, TFeature> factory) where TFeature : class
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{
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var flush = false;
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var revision = Collection.Revision;
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if (Revision != revision)
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{
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// Clear cached value to force call to UpdateCached
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cached = null;
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// Collection changed, clear whole feature cache
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flush = true;
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}
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return cached ?? UpdateCached(ref cached, state, factory, revision, flush);
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}
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// Update and cache clearing logic, when the fast-path in Fetch isn't applicable
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private TFeature UpdateCached<TFeature, TState>(ref TFeature cached, TState state, Func<TState, TFeature> factory, int revision, bool flush) where TFeature : class
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{
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if (flush)
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{
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// Collection detected as changed, clear cache
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Cache = default(TCache);
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}
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cached = Collection.Get<TFeature>();
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if (cached == null)
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{
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// Item not in collection, create it with factory
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cached = factory(state);
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// Add item to IFeatureCollection
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Collection.Set(cached);
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// Revision changed by .Set, update revision to new value
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Revision = Collection.Revision;
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}
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else if (flush)
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{
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// Cache was cleared, but item retrived from current Collection for version
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// so use passed in revision rather than making another virtual call
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Revision = revision;
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}
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return cached;
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}
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public TFeature Fetch<TFeature>(ref TFeature cached, Func<IFeatureCollection, TFeature> factory)
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where TFeature : class => Fetch(ref cached, Collection, factory);
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}
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} |