Flake: HedgedObjectReadSourceTest.HedgeOpenFailureReleasesSlot

https://storage.googleapis.com/cloud-cpp-community-publiclogs/logs/google-cloud-cpp/16412/acd756cac86341327010db887393348eaabb9831/ubuntu-22.04-cmake-oldest-deps-__default__/log-412127b2-3ff0-4b33-a2dc-363f228a0a1b.txt

```
Step #2 - "build.sh": [ RUN ] HedgedObjectReadSourceTest.HedgeOpenFailureReleasesSlot
Step #2 - "build.sh":
Step #2 - "build.sh": GMOCK WARNING:
Step #2 - "build.sh": Uninteresting mock function call - returning default value.
Step #2 - "build.sh": Function call: Close()
Step #2 - "build.sh": Returns: 104-byte object
Step #2 - "build.sh": NOTE: You can safely ignore the above warning unless this call should not happen. Do not suppress it by blindly adding an EXPECT_CALL() if you don't mean to enforce the call. See https://github.com/google/googletest/blob/main/docs/gmock_cook_book.md#knowing-when-to-expect-useoncall for details.
Step #2 - "build.sh": /workspace/google/cloud/storage/internal/hedged_object_read_source_test.cc:316: Failure
Step #2 - "build.sh": Value of: result
Step #2 - "build.sh": Expected: code is equal to OK and message is anything
Step #2 - "build.sh": Actual: 304-byte object <90-78 6E-CB 76-55 00-00 00-00 00-00 00-00 00-00 A8-7E 8B-CA 76-55 00-00 28-C1 DA-32 FF-7F 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 ... 30-10 00-7C 21-7F 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00>, whose status is UNAVAILABLE: open failed, with a code that isn't equal to OK, but a message that is anything
Step #2 - "build.sh":
Step #2 - "build.sh": [ FAILED ] HedgedObjectReadSourceTest.HedgeOpenFailureReleasesSlot (30 ms)

```

internal/hedged_object_read_source_test.cc の316行目付近から始め、リンクされたビルドログを比較に使って HedgedObjectReadSourceTest.HedgeOpenFailureReleasesSlot を実行します。テストが OK ではなく UNAVAILABLE を受け取る理由を追跡し、断続的な失敗の原因を特定します。テストがフレークしなくなり、期待される結果が確実になれば完了です。

索引モデルが issue の本文から書いたものです。