Cache empty chunked responses

[bneradt]

Empty chunked responses never start a cache write VIO, so ATS aborts
the cache entry and sends later requests back to the origin.

This starts a zero-byte cache write and preserves the empty-document
state through cache setup. It also adds coverage for negative and
successful responses.

Fixes: #11313

[Copilot]

Pull request overview

This PR fixes a cache correctness gap where chunked responses with an empty body could fail to commit cache metadata (leading to later requests going back to origin), and adds replay-based coverage to prevent regressions. The change is implemented by ensuring cache writes create a VIO even for zero-byte bodies and by teaching the cache write path to treat an explicit zero-byte write as an empty document.

Changes:

• Start a zero-byte cache write VIO for empty-body cache writes so closing the cache VC commits (rather than aborts) the entry.
• Mark zero-byte cache writes as “empty document” in CacheVC and ensure object size is set to 0 in that case.
• Extend the negative-caching replay to cover empty chunked bodies for both negative (404) and successful (200) responses.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
tests/gold_tests/cache/replay/negative-caching-300-second-timeout.replay.yaml	Adds replay coverage for caching empty chunked 404 and 200 responses (issue #11313).
src/proxy/http/HttpTunnel.cc	Ensures cache-write consumers get a VIO even when the computed write size is 0.
src/iocore/cache/CacheVC.cc	Treats do_io_write(..., nbytes=0, ...) as an empty-document write and propagates that into cache metadata handling.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

Suppressed comments (3)

tests/gold_tests/cache/replay/negative-caching-300-second-timeout.replay.yaml:124

• This transaction asserts proxy-request: expect: absent (origin should not be contacted), but it still specifies a server-response. In the replay framework this can make the test inconsistent (it may try to drive/validate an origin exchange that must not happen). Fix by removing the server-response block for this transaction, or, if you intend to verify behavior when the origin returns 502, drop expect: absent and explicitly assert the origin request.

    proxy-request:
      expect: absent

    server-response:
      status: 502
      reason: Bad Gateway
      headers:
        fields:
        - [ Content-Length, 0 ]

src/iocore/cache/CacheVC.cc:223

• The same state update (f.allow_empty_doc = 1 plus alternate.object_size_set(0)) is duplicated in both do_io_write and do_io_close. To reduce the chance of these paths drifting over time, factor this into a small helper (e.g., mark_empty_doc()) and call it from both places.

  if (nbytes == 0) {
    // A zero-byte write represents an empty document, while closing without a
    // write represents a header-only update.
    f.allow_empty_doc = 1;
    if (alternate.valid()) {
      alternate.object_size_set(0);
    }
  }

src/iocore/cache/CacheVC.cc:241

• The same state update (f.allow_empty_doc = 1 plus alternate.object_size_set(0)) is duplicated in both do_io_write and do_io_close. To reduce the chance of these paths drifting over time, factor this into a small helper (e.g., mark_empty_doc()) and call it from both places.

  if (alerrno == -1 && vio.op == VIO::WRITE && vio.get_reader() != nullptr && vio.nbytes == 0) {
    // The write may have started with an unknown length and been finalized
    // at zero after the response framing was parsed.
    f.allow_empty_doc = 1;
    if (alternate.valid()) {
      alternate.object_size_set(0);
    }
  }

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Suppressed comments (1)

tests/gold_tests/cache/replay/negative-caching-300-second-timeout.replay.yaml:124

• This transaction asserts proxy-request: expect: absent (no origin request), but still defines a server-response. In replay tests, specifying a server-response while asserting no upstream request can be contradictory and may cause the harness to wait for or validate an origin exchange that should not occur. Prefer removing the server-response block here (or changing it to an explicit expect: absent equivalent if supported by the replay schema) so the transaction strictly verifies cache hit behavior.

    proxy-request:
      expect: absent

    server-response:
      status: 502
      reason: Bad Gateway
      headers:
        fields:
        - [ Content-Length, 0 ]

[cmcfarlen]

Cherry-picked to the 10.2.x branch as e7a6e44 for the 10.2.0 release.