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src: trace_events: fix race with metadata events #25235

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src: trace_events: fix race with metadata events #25235

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12 changes: 10 additions & 2 deletions src/tracing/agent.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -207,9 +207,17 @@ void Agent::AppendTraceEvent(TraceObject* trace_event) {
id_writer.second->AppendTraceEvent(trace_event);
}

void Agent::AddMetadataEvent(std::unique_ptr<TraceObject> event) {
Mutex::ScopedLock lock(metadata_events_mutex_);
metadata_events_.push_back(std::move(event));
}

void Agent::Flush(bool blocking) {
for (const auto& event : metadata_events_)
AppendTraceEvent(event.get());
{
Mutex::ScopedLock lock(metadata_events_mutex_);
for (const auto& event : metadata_events_)
AppendTraceEvent(event.get());
}

for (const auto& id_writer : writers_)
id_writer.second->Flush(blocking);
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6 changes: 3 additions & 3 deletions src/tracing/agent.h
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Original file line number Diff line number Diff line change
Expand Up @@ -104,9 +104,7 @@ class Agent {
// Writes to all writers registered through AddClient().
void AppendTraceEvent(TraceObject* trace_event);

void AddMetadataEvent(std::unique_ptr<TraceObject> event) {
metadata_events_.push_back(std::move(event));
}
void AddMetadataEvent(std::unique_ptr<TraceObject> event);
// Flushes all writers registered through AddClient().
void Flush(bool blocking);

Expand Down Expand Up @@ -145,6 +143,8 @@ class Agent {
ConditionVariable initialize_writer_condvar_;
uv_async_t initialize_writer_async_;
std::set<AsyncTraceWriter*> to_be_initialized_;

Mutex metadata_events_mutex_;
std::list<std::unique_ptr<TraceObject>> metadata_events_;
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Maybe a dumb question, but when does this list get cleared? Does it ever?

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It doesn't and shouldn't really. These need to be cached and re-emitted whenever the trace file rolls over to a new file. There should only ever be a limited number of these.

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Right. Metadata events are meta information about the process and environment rather than being events at particular points in time. In production scenarios, the trace events may be streamed across the network where only a trailing buffer is retained. The idea is that these metadata events should be periodically remitted to ensure that they are still available even if the start of the log is truncated.

};

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