[davidfowl]

Use exact-size header arrays for output cache entries

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Allocate output cache header arrays at their exact size instead of renting them.

Description

OutputCacheEntryFormatter.Deserialize (every cache hit) and OutputCacheEntry.CopyHeadersFrom (every stored response) rent their header array from ArrayPool<(string, StringValues)>.Shared, but the arrays are never returned: OutputCacheEntry.Dispose is a no-op, and the entry keeps the array. The shared pool therefore never has an array to hand back, so each call allocates a new pool-sized array with at least 16 elements (16 B each) plus the pool lookup overhead.

This change allocates an array of exactly the needed length. The stored format, cache keys and the headers written to the response are unchanged. Because nothing is rented, no array can be shared between entries.

Benchmark: the existing EndToEndBenchmarks.ReadAsync, which deserializes a cached entry with 6 headers. Three interleaved A/B rounds:

PayloadLength	Before (main)	After
10	154-170 ns / 488 B	136-143 ns / 328 B
1000	150-154 ns / 488 B	137-139 ns / 328 B
65553	151-162 ns / 488 B	138-139 ns / 328 B
262161	149-158 ns / 488 B	137-142 ns / 328 B

The store path (CopyHeadersFrom) follows the same pattern: before, it rented a 16-element array for the 7 response headers; now it allocates a 7-element array.

Tests: Microsoft.AspNetCore.OutputCaching.Tests, 403/403. The existing formatter and middleware tests cover header round-tripping, including ignored headers.

🤖 Posted by ASP.NET Core Perf 2 (AI Agent)

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🟢 Approval recommended

There are no active review issues for the header-array allocation changes.

No open issues 🎉

What changed in this PR

This PR reduces output-cache header allocation overhead by replacing pooled arrays that were not returned with directly allocated arrays, while preserving serialization and response-header behavior.

Changes:

• Allocate header arrays using the requested count during deserialization and response storage.
• Remove pool-return handling and simplify header-memory assignment.

File	Description
src/​Middleware/​OutputCaching/​src/​OutputCacheEntryFormatter.cs	Allocates an exact-length array for deserialized headers and passes it directly to SetHeaders.
src/​Middleware/​OutputCaching/​src/​OutputCacheEntry.cs	Replaces array rental with direct allocation while retaining ignored-header filtering and slicing.

_{Review effort: Balanced}