[Isabelle (ibrandes)]

Related Core PRs:
#50312
#50311

Related Storage PRs:
#50094

Summary

Adds opt-in support for the Netty HTTP client to observe the Expect: 100-continue handshake and surface it to callers. When a caller places an AtomicBoolean in the request Context under the key azure-http-client-expect-continue-received, the client sets it to true if the service answers with an interim 100 Continue response. This does not change how or when the request body is sent.

Why

This is the azure-core-http-netty piece of the broader Expect: 100-continue effort for Storage (see #50094). In that design, the behavior lives in the pipeline-policy layer, which applies the Expect: 100-continue header and installs an observation holder in the request Context; the HTTP client's only responsibility is to report whether the interim 100 Continue was actually received so the policy can act/log on it.

The approach deliberately differs from the vertx client:

• vertx must manually withhold the body (sendHead() + continueHandler() + fallback timer) because it otherwise sends headers and body together.
• reactor-netty already tolerates the interim 100 Continue on the wire and fuses headers+body for in-memory bodies, so manual withholding is infeasible and unnecessary. The only gap was surfacing the observation — which is what this change does.

Changes

• NettyAsyncHttpClient — reads the opt-in AtomicBoolean from the request Context and threads it through attemptAsync (including proxy-retry paths) into the per-request context. The context is still attached only when needed (response-timeout override, progress reporter, or the new holder).
• AzureNettyHttpClientContext — carries the optional caller-provided observation holder.
• AzureSdkHandler — in channelRead, sets the holder to true when it observes an interim response scoped exactly to status 100, before reactor-netty consumes it. Body sending is untouched.
• NettyExpectContinueObservationTests — new test using a minimal raw HTTP/1.1 server; covers continue / no-continue × sync / async.
• CHANGELOG.md — Features Added entry under 1.17.0-beta.1.

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🟢 Approval recommended

The implementation is narrowly scoped, preserves body-sending behavior, and includes adequate sync and async coverage.

0 open findings

What changed in this PR

Adds opt-in reporting of interim HTTP 100 Continue responses in the Netty transport.

Changes:

• Propagates an AtomicBoolean observation holder through request context.
• Detects exact 100 Continue responses in the Netty handler.
• Adds sync/async tests and release notes.

File	Description
NettyAsyncHttpClient.java	Extracts and propagates the observation holder.
AzureSdkHandler.java	Records interim 100 Continue responses.
AzureNettyHttpClientContext.java	Carries the per-request holder.
NettyExpectContinueObservationTests.java	Tests positive and negative observation paths.
CHANGELOG.md	Documents the opt-in behavior.

🧠 Review effort: Balanced