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