STJ avoid full document string allocation#5908
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Both read paths in CosmosSystemTextJsonSerializer materialize the entire document as a System.String before parsing: 1. Binary path: cosmosObject.ToString() builds a UTF-16 JSON string from the UTF-8 bytes produced by IJsonWriter.GetResult() (via Utf8StringHelpers), only for JsonSerializer.Deserialize<T>(string, ...) to re-parse that UTF-16. Fix: feed the UTF-8 ReadOnlyMemory<byte> directly into JsonSerializer.Deserialize<T>(ReadOnlySpan<byte>, ...). 2. DeserializeStream<T> helper: StreamReader.ReadToEnd() transcodes UTF-8 to a full UTF-16 string before Deserialize<T>(string, ...). Fix: call JsonSerializer.Deserialize<T>(Stream, ...) which feeds Utf8JsonReader directly from the UTF-8 byte stream. Both changes are semantically equivalent for Cosmos response bodies (UTF-8 JSON) and eliminate full-document string allocations that land on the Large Object Heap for non-trivial documents. Refs: Azure#4652 (comment) Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Refs: #4652 (comment)
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Description
Both read paths in
CosmosSystemTextJsonSerializercurrently materialize the entire response document as a UTF-16System.Stringbefore parsing. This PR feeds the UTF-8 bytes directly intoUtf8JsonReaderinstead, eliminating the allocation and the redundant UTF-8 ↔ UTF-16 transcoding on every read.Binary path: replace
cosmosObject.ToString()(which produces a UTF-16string) withcosmosObject.WriteTo(textJsonWriter)+JsonSerializer.Deserialize<T>(ReadOnlySpan<byte>, ...).Text path (
DeserializeStream<T>): replaceStreamReader.ReadToEnd()+Deserialize<T>(string, ...)withJsonSerializer.Deserialize<T>(Stream, ...).Both changes are semantically equivalent for Cosmos response bodies (UTF-8 JSON), no public API impact. Reduces CPU, allocations, and GC/LOH pressure on every read.
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### Unreleasedinchangelog.mdfor the user-facing impact of this change.