[sdk-platform-java] Race condition in TracedUnaryCallable causes flaky integration tests

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难度
3/5
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1-2 天
新手友好度
68/100
Issue 类型
缺陷
描述清晰度
描述清楚
活跃度
冷清
技术栈
grpc, java
领域
observability, testing

调研方向

从 TracedUnaryCallable 和 TracedBatchingCallable 开始，然后检查 issue 中描述的 ApiFutures 回调用法。运行 ITOtelGoldenMetrics 以重现空指标失败。完成的标准是：只有在 TraceFinisher 记录成功或失败之后，返回的 future 才完成，并且集成测试不再出现间歇性失败。

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描述

priority: p3 type: feature request

Description

In gax-java, observability tracers (such as TracedUnaryCallable and TracedBatchingCallable) use TraceFinisher attached via ApiFutures.addCallback to record metrics and spans upon completion.

ApiFutures.addCallback(future, finisher, MoreExecutors.directExecutor());

This pattern creates a race condition in synchronous environments, such as integration tests (ITOtelGoldenMetrics), where the test thread calls future.get() and immediately asserts state (e.g., checks OpenTelemetry metrics).

Root Cause

When the underlying RPC future completes, the Guava/ApiFuture infrastructure notifies listeners. addCallback attaches a listener that runs independently of the future's resolution state for the blocking caller.

In ITOtelGoldenMetrics:

  1. client.echo() blocks the test thread.
  2. The gRPC network thread completes the RPC.
  3. The test thread unblocks and calls metricReader.collectAllMetrics().
  4. Concurrently (or slightly after), the TraceFinisher runs to record the metric in the OTel meter.

If the test thread executes before the TraceFinisher completes its work, the test fails with "empty metrics".

Proposed Solution

Convert the detached addCallback into a chained future transformation. This guarantees that the returned future only resolves after the tracer has finished recording metrics/spans.

Example for TracedUnaryCallable:

ApiFuture<ResponseT> onSuccessFuture =
    ApiFutures.transform(
        future,
        response -> {
            finisher.onSuccess(response);
            return response;
        },
        MoreExecutors.directExecutor());

return ApiFutures.catchingAsync(
    onSuccessFuture,
    Throwable.class,
    t -> {
        finisher.onFailure(t);
        return ApiFutures.immediateFailedFuture(t);
    },
    MoreExecutors.directExecutor());

This approach ensures structural LIFO ordering: the metric is recorded before any downstream consumer can yield from a blocking .get() on the returned future.

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