tracing: migrate opentracing-cpp → opentelemetry-cpp (API only)

[chen3feng]

opentracing-cpp is deprecated and superseded by OpenTelemetry. flare ships no real tracer — it exposes a pluggable abstraction (TracingOpsProvider / QuickerSpan / TracingOps) over the tracing-API types, with concrete providers plugged in by users. This re-bases that abstraction on the opentelemetry-cpp API (header-only, v1.27.0) and removes opentracing-cpp from both build systems.

Scope / key decisions

• API only — no SDK/exporter, so no protobuf/grpc is pulled in and the deliberately-pinned, abseil-free protobuf 3.21.12 stays intact. (The SDK+OTLP path would conflict with it.)
• Keep the abstraction (perf buffering + pluggability), rewriting only the types/bodies.
• Keep the wire format: rpc_meta.proto's bytes tracing_context stays an opaque, provider-serialized blob (no W3C text-map propagators).

Source (flare/rpc/tracing/)

• tracing_ops_provider.h: virtuals re-typed to OTel; new flare-owned SpanStartOptions (parent SpanContext + SpanKind + timestamps + start attrs) replaces opentracing::StartSpanOptions/ChildOf; Extract → std::optional<SpanContext>.
• tracing_ops.h/.cc: span_ is nostd::shared_ptr<trace::Span>; SetAttribute/AddEvent/End; SpanContext is a value type; buffered ops keep flare's owning variant and convert to the non-owning AttributeValue only at flush (avoids a use-after-free).
• framework_tags.h → constexpr std::string_view; string_view_interop.h → ToStd/ToOtel. Standard-tag table is a literal allowlist; span.kind moves from a tag to SpanStartOptions::kind.
• Call sites rpc_channel.cc / normal_connection_handler.cc updated. rpc_meta.proto field unchanged (comment updated).
• Tests/benchmark rewritten onto a recording fake otel::trace::Span (OTel's noop span discards everything and can't be asserted on). The integration test asserts span kind via a provider-recorded span.kind attribute.

Build

• blade: BLADE_ROOT vcpkg_config gains opentelemetry-cpp (default features → no exporters); thin wrapper //thirdparty/opentelemetry-cpp:api. The opentelemetry_api component is header-only (no .a), so the wrapper references opentelemetry_common for the include dir (which carries both the otel + absl headers); flare still consumes only API headers. Vendored thirdparty/opentracing-cpp/ removed.
• bazel: deps.bzl http_archive for opentelemetry-cpp 1.27.0 + api-only bazel/opentelemetry-cpp.BUILD (header-only; bundled nostd → no abseil needed). patch_cmds drop the archive's nested BUILD files so the api headers can be globbed from one package. opentracing BUILD/patch + .bazelignore/.gitattributes entries removed.

Verification

Both build systems build //flare/rpc/tracing/... and pass tracing_ops_test + integration_test (full RPC flow: client/server spans, context propagation through the bytes field, framework-tag translation, logs).

🤖 Generated with Claude Code

[chen3feng]

不要提交

[chen3feng]

不要提交

[chen3feng]

Code review 意见(非阻塞)

整体方向正确:API-only 迁移克制到位(不引入 protobuf/grpc、保留 bytes tracing_context 不透明 blob),缓冲层"owning variant → flush 时才转 non-owning AttributeValue"的生命周期处理正确,无 UAF,也无阻塞性 bug。以下几点建议留待后续处理。

1. forcibly_sampled 的 attribute 改名可能让现有 provider 失效

flare/rpc/tracing/tracing_ops.h:358

span_->SetAttribute("flare.sampling_priority", 1);

旧代码设的是 opentracing 标准 tag sampling.priority。新名字带了 flare. 前缀,但没有走 provider_->SetFrameworkTag 翻译(直接 SetAttribute)。结果是个混合体:监听标准 sampling.priority 的 provider 收不到这个信号;而带 flare. 前缀本应被翻译,这里却绕过了。"直接 set 不翻译"本身与旧代码一致(不是回归),但 key 改名 + 前缀语义不一致这点,建议在 flare/doc/tracing.md 里向 provider 实现者说明,或考虑让它走 framework-tag 通道。

2. Log → Event 的映射:空 event name

flare/rpc/tracing/tracing_ops.cc:136

span_->AddEvent(e.GetKey(), {{"value", attr}});

flare log 的 key 变成 OTel event 名,value 放在固定属性名 "value" 下。对常见的单参 AddTracingLog(value)(key 为空,见 integration_test.cc 断言 pair("", kLogValue))会产生空名 event。很多 OTel 后端按 event name 分组/展示,空名 event 渲染会比较难看。语义上可接受,但建议给空 key 一个默认 event 名(如 "log"),并在 doc 里说明:旧 opentracing 把多个 field 归到一条带时间戳的 log record,现在每条 log 是独立 event。

3. variant 对 C++ 标准的隐式依赖

flare/rpc/tracing/tracing_ops.h:174-176

QuickerValue variant 的 bool 在前、std::string 在后。调用方传 const char*(如 rpc_channel.cc 的 full_name().c_str())。当前两套构建都是 C++20(-std=gnu++2a / c++2a),P0608R3 保证 const char* 正确选中 std::string,没问题。但在 C++17 下这会静默选中 bool(经典陷阱)。当前安全,只是这层正确性隐式依赖编译标准且无显式保护——可考虑在调用点用 std::string(...) 代替 .c_str(),或加注释/static_assert 防御未来降级。

4. nostd / provider ABI(信息性)

当前保留 OTel nostd:: 类型(经讨论确认:与 blade 侧预编译 vcpkg 库的 bundled nostd 天然一致,故有意不切到 std)。需要注意的硬不变量:TracingOpsProvider 虚接口上的 nostd::shared_ptr/variant/string_view 要求 flare 与任何 provider 插件采用相同的 OTel STL 配置(都不定义 HAVE_ABSEIL / OPENTELEMETRY_STL_VERSION),否则会在接口边界 ABI 不匹配。建议在 doc 里写明这条约束,供未来写 provider 的人参考。

5. 测试中的小问题(可忽略)

• tracing_ops_test.cc 的 DummySpan::tags/logs 是 inline static,跨用例不清空(当前只有一个用例写入,无影响)。
• integration_test.cc 的 reported_spans 在 SleepFor 后无锁读取(测试场景 benign)。

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已核对正确:无残留 opentracing 代码引用(仅注释);ReportViaDpc move 走 span_ 后 FLARE_CHECK(!Tracing()) 成立;非采样路径 span_ = nullptr 不再隐式 End()(符合 OTel 语义);FlushBufferedOps 的 scratch 生命周期覆盖 Set*/AddEvent;IsStandardTag 正确移除 span.kind;bazel patch_cmds 用 -mindepth 2 删嵌套 BUILD 的思路正确;LICENSE/README/.bazelignore 归属更新干净。