Optimize response header handling for HTTP/2 (P2)

[xinlian12]

Option A: Skip redundant toLowerCase() for HTTP/2 headers

• Add keysAlreadyLowerCase flag to Cosmos HttpHeaders
• When true, set() skips the toLowerCase() call on header names
• Detect HTTP/2 in ReactorNettyHttpResponse.headers() via version check
• Saves ~15-25 toLowerCase calls + String allocations per response
• HTTP/2 headers are guaranteed lowercase per RFC 7540 §8.1.2

[xinlian12]

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[github-actions]

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Generated by sdkReviewAgent for issue #21

[github-actions]

🔴 Blocking · Correctness: set() skips normalization but getHeader() always normalizes — silent lookup miss

When isHttp2Headers=true, set() stores the key verbatim (e.g. "content-type"). However, getHeader() unconditionally calls headerName.toLowerCase(Locale.ROOT) before looking up in the map. This is symmetric and safe only as long as every caller passes lowercase names. The moment any caller does headers.set("Content-Type", ...) on an isHttp2Headers=true instance (perfectly valid via the public API), the key is stored as "Content-Type" but every lookup attempts "content-type" — a silent null return with no error.

Two complementary fixes are needed:

1. Guard in set() (fail-fast):

public HttpHeaders set(String name, String value) {
    if (isHttp2Headers && !name.equals(name.toLowerCase(Locale.ROOT))) {
        throw new IllegalArgumentException(
            "isHttp2Headers=true requires lowercase header names, got: " + name);
    }
    final String headerKey = isHttp2Headers ? name : name.toLowerCase(Locale.ROOT);
    ...
}

2. Skip normalization in getHeader() when flag is set (complete the optimization):

private HttpHeader getHeader(String headerName) {
    final String headerKey = isHttp2Headers ? headerName : headerName.toLowerCase(Locale.ROOT);
    return headers.get(headerKey);
}

These two changes together make the optimization correct and complete.

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_{⚠️ AI-generated review — may be incorrect. Agree? → resolve the conversation. Disagree? → reply with your reasoning.}

[github-actions]

🟡 Recommendation · Test Coverage: Test bypasses the actual lookup path — bug in getHeader() would go undetected

toMap() iterates headers.values() directly and never calls getHeader(). This means the test does not exercise the set() → getHeader() round-trip that the optimization affects. If getHeader() had a normalization bug when isHttp2Headers=true, this test would still pass.

Change the assertion to use value() (which goes through getHeader()):

// Before:
Map(String, String) map = headers.toMap();
assertThat(map.get(headerName)).isEqualTo(headerValue);

// After:
assertThat(headers.value(headerName)).isEqualTo(headerValue);

Also add a test for the mixed-case negative case once the guard from the set() fix is in place:

`@Test`(groups = "unit", expectedExceptions = IllegalArgumentException.class)
public void http2HeadersRejectsMixedCaseName() {
    HttpHeaders headers = new HttpHeaders(4, true);
    headers.set("Content-Type", "application/json"); // must throw
}

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_{⚠️ AI-generated review — may be incorrect. Agree? → resolve the conversation. Disagree? → reply with your reasoning.}

[github-actions]

🔴 Blocking · Correctness: Optimization is dead code — HTTP/2 detection never wired up

The new HttpHeaders(int size, boolean isHttp2Headers) constructor is never called from the response-handling code path. The PR description says "Detect HTTP/2 in ReactorNettyHttpResponse.headers() via version check", but that detection is absent from the diff. Every response — HTTP/1.1 or HTTP/2 — continues to use new HttpHeaders(reactorNettyResponse.responseHeaders().size()), paying the full toLowerCase() cost. The optimization is entirely bypassed in production.

The integration point should be ReactorNettyHttpResponse.headers(). Roughly:

`@Override`
public HttpHeaders headers() {
    boolean isHttp2 = /* check reactor-netty version API, e.g.: */
        reactorNettyResponse.version() == HttpVersion.HTTP_2_0;
    HttpHeaders headers = new HttpHeaders(reactorNettyResponse.responseHeaders().size(), isHttp2);
    reactorNettyResponse.responseHeaders().forEach(e -> headers.set(e.getKey(), e.getValue()));
    return headers;
}

Verify the exact version-detection API available on the HttpClientResponse type used in this module's reactor-netty version, as the API varies across releases.

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_{⚠️ AI-generated review — may be incorrect. Agree? → resolve the conversation. Disagree? → reply with your reasoning.}

[xinlian12]

/sdkReviewAgent

[xinlian12]

/sdkReviewAgent