traced: preserve UUID LSB when cloning a session

The LSB is used to correlate tracing sessions with statsd subscriptions.
Retain the LSB to avoid breaking downstream pipelines that depend on this.

Bug: b/260112703
Bug: 287071990
Test: TracingServiceImplTest.CloneSession
(cherry picked from https://android-review.googlesource.com/q/commit:8755774a3fbe4e8c664a6436eb4fd9ea5e34a297)
Merged-In: I4c512701143a21ac1b01a2f785ebe005e9983556
Change-Id: I4c512701143a21ac1b01a2f785ebe005e9983556

2 files changed, 14 insertions, 3 deletions

diff --git a/src/tracing/core/tracing_service_impl.cc b/src/tracing/core/tracing_service_impl.cc
index b32a65854..16ef09e7c 100644
--- a/src/tracing/core/tracing_service_impl.cc
+++ b/src/tracing/core/tracing_service_impl.cc
@@ -3614,8 +3614,14 @@ base::Status TracingServiceImpl::DoCloneSession(ConsumerEndpointImpl* consumer,
                std::forward_as_tuple(tsid, consumer, src->config, task_runner_))
            .first->second;
 
+  // Generate a new UUID for the cloned session, but preserve the LSB. In some
+  // contexts the LSB is used to tie the trace back to the statsd subscription
+  // that triggered it. See the corresponding code in perfetto_cmd.cc which
+  // reads at triggering_subscription_id().
+  const int64_t orig_uuid_lsb = src->trace_uuid.lsb();
   cloned_session->state = TracingSession::CLONED_READ_ONLY;
-  cloned_session->trace_uuid = base::Uuidv4();  // Generate a new UUID.
+  cloned_session->trace_uuid = base::Uuidv4();
+  cloned_session->trace_uuid.set_lsb(orig_uuid_lsb);
   *new_uuid = cloned_session->trace_uuid;
 
   for (auto& kv : buf_snaps) {
diff --git a/src/tracing/core/tracing_service_impl_unittest.cc b/src/tracing/core/tracing_service_impl_unittest.cc
index fb175e990..dcfcfa6c1 100644
--- a/src/tracing/core/tracing_service_impl_unittest.cc
+++ b/src/tracing/core/tracing_service_impl_unittest.cc
@@ -4419,6 +4419,8 @@ TEST_F(TracingServiceImplTest, CloneSession) {
   TraceConfig trace_config;
   trace_config.add_buffers()->set_size_kb(32);  // Buf 0.
   trace_config.add_buffers()->set_size_kb(32);  // Buf 1.
+  trace_config.set_trace_uuid_lsb(4242);
+  trace_config.set_trace_uuid_msb(3737);
   auto* ds_cfg = trace_config.add_data_sources()->mutable_config();
   ds_cfg->set_name("ds_1");
   ds_cfg->set_target_buffer(0);
@@ -4469,8 +4471,11 @@ TEST_F(TracingServiceImplTest, CloneSession) {
           [clone_done, &clone_uuid](const Consumer::OnSessionClonedArgs& args) {
             ASSERT_TRUE(args.success);
             ASSERT_TRUE(args.error.empty());
-            ASSERT_NE(args.uuid.msb(), 0);
-            ASSERT_NE(args.uuid.lsb(), 0);
+            // Ensure the LSB is preserved, but the MSB is different. See
+            // comments in tracing_service_impl.cc and perfetto_cmd.cc around
+            // triggering_subscription_id().
+            ASSERT_EQ(args.uuid.lsb(), 4242);
+            ASSERT_NE(args.uuid.msb(), 3737);
             clone_uuid = args.uuid;
             clone_done();
           }));