src/trace_processor/process_tracker.cc - third_party/perfetto - Git at Google

 /*
  * Copyright (C) 2018 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  *      http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */

 #include "src/trace_processor/process_tracker.h"
 #include "src/trace_processor/stats.h"

 #include <utility>

 #include <inttypes.h>

 namespace perfetto {
 namespace trace_processor {

 ProcessTracker::ProcessTracker(TraceProcessorContext* context)
     : context_(context) {
   // Create a mapping from (t|p)id 0 -> u(t|p)id 0 for the idle process.
   tids_.emplace(0, std::vector<UniqueTid>{0});
   pids_.emplace(0, 0);
 }

 ProcessTracker::~ProcessTracker() = default;

 UniqueTid ProcessTracker::StartNewThread(int64_t timestamp,
                                          uint32_t tid,
                                          StringId thread_name_id) {
   tables::ThreadTable::Row row;
   row.tid = tid;
   row.name = thread_name_id;
   row.start_ts = timestamp;

   auto* thread_table = context_->storage->mutable_thread_table();
   UniqueTid new_utid = thread_table->Insert(row).value;
   tids_[tid].emplace_back(new_utid);
   return new_utid;
 }

 void ProcessTracker::EndThread(int64_t timestamp, uint32_t tid) {
   auto* thread_table = context_->storage->mutable_thread_table();
   auto* process_table = context_->storage->mutable_process_table();

   UniqueTid utid = GetOrCreateThread(tid);
   thread_table->mutable_end_ts()->Set(utid, timestamp);

   // Remove the thread from the list of threads being tracked as any event after
   // this one should be ignored.
   auto& vector = tids_[tid];
   vector.erase(std::remove(vector.begin(), vector.end(), utid));

   auto opt_upid = thread_table->upid()[utid];
   if (opt_upid.has_value()) {
     // If the process pid and thread tid are equal, then this is the main thread
     // of the process.
     if (process_table->pid()[*opt_upid] == thread_table->tid()[utid]) {
       process_table->mutable_end_ts()->Set(*opt_upid, timestamp);
     }
   }
 }

 base::Optional<UniqueTid> ProcessTracker::GetThreadOrNull(uint32_t tid) {
   auto vector_it = tids_.find(tid);
   if (vector_it == tids_.end() || vector_it->second.empty()) {
     return base::nullopt;
   }

   // If the thread is being tracked by the process tracker, it should not be
   // known to have ended.
   auto* thread_table = context_->storage->mutable_thread_table();
   UniqueTid utid = vector_it->second.back();
   PERFETTO_DCHECK(thread_table->tid()[utid] == tid);
   PERFETTO_DCHECK(!thread_table->end_ts()[utid].has_value());
   return utid;
 }

 UniqueTid ProcessTracker::GetOrCreateThread(uint32_t tid) {
   auto utid = GetThreadOrNull(tid);
   return utid ? utid.value() : StartNewThread(0, tid, 0);
 }

 UniqueTid ProcessTracker::UpdateThreadName(uint32_t tid,
                                            StringId thread_name_id) {
   auto* thread_table = context_->storage->mutable_thread_table();
   auto utid = GetOrCreateThread(tid);
   if (!thread_name_id.is_null())
     thread_table->mutable_name()->Set(utid, thread_name_id);
   return utid;
 }

 void ProcessTracker::SetThreadNameIfUnset(UniqueTid utid,
                                           StringId thread_name_id) {
   auto* thread_table = context_->storage->mutable_thread_table();
   if (thread_table->name()[utid].is_null())
     thread_table->mutable_name()->Set(utid, thread_name_id);
 }

 UniqueTid ProcessTracker::UpdateThread(uint32_t tid, uint32_t pid) {
   auto* thread_table = context_->storage->mutable_thread_table();
   auto* process_table = context_->storage->mutable_process_table();

   auto vector_it = tids_.find(tid);

   // Try looking for a thread that matches both tid and thread group id (pid).
   base::Optional<UniqueTid> opt_utid;
   if (vector_it != tids_.end()) {
     const auto& vector = vector_it->second;

     // Iterate backwards through the threads so ones later in the trace are more
     // likely to be picked.
     for (auto it = vector.rbegin(); it != vector.rend(); it++) {
       UniqueTid it_utid = *it;

       // If we finished this thread, we should have removed it from the vector
       // entirely.
       PERFETTO_DCHECK(!thread_table->end_ts()[it_utid].has_value());

       auto opt_it_upid = thread_table->upid()[it_utid];
       if (!opt_it_upid.has_value()) {
         // We haven't discovered the parent process for the thread. Assign it
         // now and use this thread.
         opt_utid = *it;
         break;
       }

       if (process_table->end_ts()[*opt_it_upid].has_value()) {
         // If the process is already dead, don't bother choosing the associated
         // thread.
         continue;
       }
       if (process_table->pid()[*opt_it_upid] == pid) {
         // We found a thread that matches both the tid and its parent pid.
         opt_utid = *it;
         break;
       }
     }  // for(tids).
   }

   // If no matching thread was found, create a new one.
   UniqueTid utid = opt_utid ? *opt_utid : StartNewThread(0, tid, 0);
   PERFETTO_DCHECK(thread_table->tid()[utid] == tid);

   // Find matching process or create new one.
   if (!thread_table->upid()[utid].has_value()) {
     thread_table->mutable_upid()->Set(utid, GetOrCreateProcess(pid));
   }

   ResolvePendingAssociations(utid, *thread_table->upid()[utid]);

   return utid;
 }

 UniquePid ProcessTracker::StartNewProcess(int64_t timestamp,
                                           uint32_t parent_tid,
                                           uint32_t pid,
                                           StringId main_thread_name) {
   pids_.erase(pid);

   // Create a new UTID for the main thread, so we don't end up reusing an old
   // entry in case of TID recycling.
   StartNewThread(timestamp, /*tid=*/pid, 0);

   // Note that we erased the pid above so this should always return a new
   // process.
   UniquePid upid = GetOrCreateProcess(pid);

   auto* process_table = context_->storage->mutable_process_table();
   auto* thread_table = context_->storage->mutable_thread_table();

   PERFETTO_DCHECK(process_table->name()[upid] == 0);
   process_table->mutable_start_ts()->Set(upid, timestamp);
   process_table->mutable_name()->Set(upid, main_thread_name);

   UniqueTid parent_utid = GetOrCreateThread(parent_tid);
   auto opt_parent_upid = thread_table->upid()[parent_utid];
   if (opt_parent_upid.has_value()) {
     process_table->mutable_parent_upid()->Set(upid, *opt_parent_upid);
   } else {
     pending_parent_assocs_.emplace_back(parent_utid, upid);
   }
   return upid;
 }

 UniquePid ProcessTracker::SetProcessMetadata(uint32_t pid,
                                              base::Optional<uint32_t> ppid,
                                              base::StringView name) {
   auto proc_name_id = context_->storage->InternString(name);

   base::Optional<UniquePid> pupid;
   if (ppid.has_value()) {
     pupid = GetOrCreateProcess(ppid.value());
   }

   UniquePid upid = GetOrCreateProcess(pid);

   auto* process_table = context_->storage->mutable_process_table();
   process_table->mutable_name()->Set(upid, proc_name_id);

   if (pupid)
     process_table->mutable_parent_upid()->Set(upid, *pupid);

   return upid;
 }

 void ProcessTracker::SetProcessUid(UniquePid upid, uint32_t uid) {
   auto* process_table = context_->storage->mutable_process_table();
   process_table->mutable_uid()->Set(upid, uid);
 }

 void ProcessTracker::SetProcessNameIfUnset(UniquePid upid,
                                            StringId process_name_id) {
   auto* process_table = context_->storage->mutable_process_table();
   if (process_table->name()[upid].is_null())
     process_table->mutable_name()->Set(upid, process_name_id);
 }

 void ProcessTracker::UpdateProcessNameFromThreadName(uint32_t tid,
                                                      StringId thread_name) {
   auto* thread_table = context_->storage->mutable_thread_table();
   auto* process_table = context_->storage->mutable_process_table();

   auto utid = GetOrCreateThread(tid);
   auto opt_upid = thread_table->upid()[utid];
   if (opt_upid.has_value()) {
     if (process_table->pid()[*opt_upid] == tid) {
       process_table->mutable_name()->Set(*opt_upid, thread_name);
     }
   }
 }

 UniquePid ProcessTracker::GetOrCreateProcess(uint32_t pid) {
   UniquePid upid;
   auto it = pids_.find(pid);
   if (it != pids_.end()) {
     upid = it->second;
   } else {
     tables::ProcessTable::Row row;
     row.pid = pid;
     upid = context_->storage->mutable_process_table()->Insert(row).value;

     pids_.emplace(pid, upid);

     // Create an entry for the main thread.
     // We cannot call StartNewThread() here, because threads for this process
     // (including the main thread) might have been seen already prior to this
     // call. This call usually comes from the ProcessTree dump which is delayed.
     UpdateThread(/*tid=*/pid, pid);
   }
   return upid;
 }

 void ProcessTracker::AssociateThreads(UniqueTid utid1, UniqueTid utid2) {
   auto* tt = context_->storage->mutable_thread_table();

   // First of all check if one of the two threads is already bound to a process.
   // If that is the case, map the other thread to the same process and resolve
   // recursively any associations pending on the other thread.

   auto opt_upid1 = tt->upid()[utid1];
   auto opt_upid2 = tt->upid()[utid2];

   if (opt_upid1.has_value() && !opt_upid2.has_value()) {
     tt->mutable_upid()->Set(utid2, *opt_upid1);
     ResolvePendingAssociations(utid2, *opt_upid1);
     return;
   }

   if (opt_upid2.has_value() && !opt_upid1.has_value()) {
     tt->mutable_upid()->Set(utid1, *opt_upid2);
     ResolvePendingAssociations(utid1, *opt_upid2);
     return;
   }

   if (opt_upid1.has_value() && opt_upid1 != opt_upid2) {
     // Cannot associate two threads that belong to two different processes.
     PERFETTO_ELOG("Process tracker failure. Cannot associate threads %u, %u",
                   tt->tid()[utid1], tt->tid()[utid2]);
     context_->storage->IncrementStats(stats::process_tracker_errors);
     return;
   }

   pending_assocs_.emplace_back(utid1, utid2);
 }

 void ProcessTracker::ResolvePendingAssociations(UniqueTid utid_arg,
                                                 UniquePid upid) {
   auto* tt = context_->storage->mutable_thread_table();
   auto* pt = context_->storage->mutable_process_table();
   PERFETTO_DCHECK(tt->upid()[utid_arg] == upid);

   std::vector<UniqueTid> resolved_utids;
   resolved_utids.emplace_back(utid_arg);

   while (!resolved_utids.empty()) {
     UniqueTid utid = resolved_utids.back();
     resolved_utids.pop_back();
     for (auto it = pending_parent_assocs_.begin();
          it != pending_parent_assocs_.end();) {
       UniqueTid parent_utid = it->first;
       UniquePid child_upid = it->second;

       if (parent_utid != utid) {
         ++it;
         continue;
       }
       PERFETTO_DCHECK(child_upid != upid);

       // Set the parent pid of the other process
       PERFETTO_DCHECK(!pt->parent_upid()[child_upid] ||
                       pt->parent_upid()[child_upid] == upid);
       pt->mutable_parent_upid()->Set(child_upid, upid);

       // Erase the pair. The |pending_parent_assocs_| vector is not sorted and
       // swapping a std::pair<uint32_t, uint32_t> is cheap.
       std::swap(*it, pending_parent_assocs_.back());
       pending_parent_assocs_.pop_back();
     }

     for (auto it = pending_assocs_.begin(); it != pending_assocs_.end();) {
       UniqueTid other_utid;
       if (it->first == utid) {
         other_utid = it->second;
       } else if (it->second == utid) {
         other_utid = it->first;
       } else {
         ++it;
         continue;
       }

       PERFETTO_DCHECK(other_utid != utid);

       // Update the other thread and associated it to the same process.
       PERFETTO_DCHECK(!tt->upid()[other_utid] ||
                       tt->upid()[other_utid] == upid);
       tt->mutable_upid()->Set(other_utid, upid);

       // Erase the pair. The |pending_assocs_| vector is not sorted and swapping
       // a std::pair<uint32_t, uint32_t> is cheap.
       std::swap(*it, pending_assocs_.back());
       pending_assocs_.pop_back();

       // Recurse into the newly resolved thread. Some other threads might have
       // been bound to that.
       resolved_utids.emplace_back(other_utid);
     }
   }  // while (!resolved_utids.empty())
 }

 }  // namespace trace_processor
 }  // namespace perfetto
	/*
	* Copyright (C) 2018 The Android Open Source Project
	*
	* Licensed under the Apache License, Version 2.0 (the "License");
	* you may not use this file except in compliance with the License.
	* You may obtain a copy of the License at
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*/

	#include "src/trace_processor/process_tracker.h"
	#include "src/trace_processor/stats.h"

	#include <utility>

	#include <inttypes.h>

	namespace perfetto {
	namespace trace_processor {

	ProcessTracker::ProcessTracker(TraceProcessorContext* context)
	: context_(context) {
	// Create a mapping from (t\|p)id 0 -> u(t\|p)id 0 for the idle process.
	tids_.emplace(0, std::vector<UniqueTid>{0});
	pids_.emplace(0, 0);
	}

	ProcessTracker::~ProcessTracker() = default;

	UniqueTid ProcessTracker::StartNewThread(int64_t timestamp,
	uint32_t tid,
	StringId thread_name_id) {
	tables::ThreadTable::Row row;
	row.tid = tid;
	row.name = thread_name_id;
	row.start_ts = timestamp;

	auto* thread_table = context_->storage->mutable_thread_table();
	UniqueTid new_utid = thread_table->Insert(row).value;
	tids_[tid].emplace_back(new_utid);
	return new_utid;
	}

	void ProcessTracker::EndThread(int64_t timestamp, uint32_t tid) {
	auto* thread_table = context_->storage->mutable_thread_table();
	auto* process_table = context_->storage->mutable_process_table();

	UniqueTid utid = GetOrCreateThread(tid);
	thread_table->mutable_end_ts()->Set(utid, timestamp);

	// Remove the thread from the list of threads being tracked as any event after
	// this one should be ignored.
	auto& vector = tids_[tid];
	vector.erase(std::remove(vector.begin(), vector.end(), utid));

	auto opt_upid = thread_table->upid()[utid];
	if (opt_upid.has_value()) {
	// If the process pid and thread tid are equal, then this is the main thread
	// of the process.
	if (process_table->pid()[*opt_upid] == thread_table->tid()[utid]) {
	process_table->mutable_end_ts()->Set(*opt_upid, timestamp);
	}
	}
	}

	base::Optional<UniqueTid> ProcessTracker::GetThreadOrNull(uint32_t tid) {
	auto vector_it = tids_.find(tid);
	if (vector_it == tids_.end() \|\| vector_it->second.empty()) {
	return base::nullopt;
	}

	// If the thread is being tracked by the process tracker, it should not be
	// known to have ended.
	auto* thread_table = context_->storage->mutable_thread_table();
	UniqueTid utid = vector_it->second.back();
	PERFETTO_DCHECK(thread_table->tid()[utid] == tid);
	PERFETTO_DCHECK(!thread_table->end_ts()[utid].has_value());
	return utid;
	}

	UniqueTid ProcessTracker::GetOrCreateThread(uint32_t tid) {
	auto utid = GetThreadOrNull(tid);
	return utid ? utid.value() : StartNewThread(0, tid, 0);
	}

	UniqueTid ProcessTracker::UpdateThreadName(uint32_t tid,
	StringId thread_name_id) {
	auto* thread_table = context_->storage->mutable_thread_table();
	auto utid = GetOrCreateThread(tid);
	if (!thread_name_id.is_null())
	thread_table->mutable_name()->Set(utid, thread_name_id);
	return utid;
	}

	void ProcessTracker::SetThreadNameIfUnset(UniqueTid utid,
	StringId thread_name_id) {
	auto* thread_table = context_->storage->mutable_thread_table();
	if (thread_table->name()[utid].is_null())
	thread_table->mutable_name()->Set(utid, thread_name_id);
	}

	UniqueTid ProcessTracker::UpdateThread(uint32_t tid, uint32_t pid) {
	auto* thread_table = context_->storage->mutable_thread_table();
	auto* process_table = context_->storage->mutable_process_table();

	auto vector_it = tids_.find(tid);

	// Try looking for a thread that matches both tid and thread group id (pid).
	base::Optional<UniqueTid> opt_utid;
	if (vector_it != tids_.end()) {
	const auto& vector = vector_it->second;

	// Iterate backwards through the threads so ones later in the trace are more
	// likely to be picked.
	for (auto it = vector.rbegin(); it != vector.rend(); it++) {
	UniqueTid it_utid = *it;

	// If we finished this thread, we should have removed it from the vector
	// entirely.
	PERFETTO_DCHECK(!thread_table->end_ts()[it_utid].has_value());

	auto opt_it_upid = thread_table->upid()[it_utid];
	if (!opt_it_upid.has_value()) {
	// We haven't discovered the parent process for the thread. Assign it
	// now and use this thread.
	opt_utid = *it;
	break;
	}

	if (process_table->end_ts()[*opt_it_upid].has_value()) {
	// If the process is already dead, don't bother choosing the associated
	// thread.
	continue;
	}
	if (process_table->pid()[*opt_it_upid] == pid) {
	// We found a thread that matches both the tid and its parent pid.
	opt_utid = *it;
	break;
	}
	} // for(tids).
	}

	// If no matching thread was found, create a new one.
	UniqueTid utid = opt_utid ? *opt_utid : StartNewThread(0, tid, 0);
	PERFETTO_DCHECK(thread_table->tid()[utid] == tid);

	// Find matching process or create new one.
	if (!thread_table->upid()[utid].has_value()) {
	thread_table->mutable_upid()->Set(utid, GetOrCreateProcess(pid));
	}

	ResolvePendingAssociations(utid, *thread_table->upid()[utid]);

	return utid;
	}

	UniquePid ProcessTracker::StartNewProcess(int64_t timestamp,
	uint32_t parent_tid,
	uint32_t pid,
	StringId main_thread_name) {
	pids_.erase(pid);

	// Create a new UTID for the main thread, so we don't end up reusing an old
	// entry in case of TID recycling.
	StartNewThread(timestamp, /tid=/pid, 0);

	// Note that we erased the pid above so this should always return a new
	// process.
	UniquePid upid = GetOrCreateProcess(pid);

	auto* process_table = context_->storage->mutable_process_table();
	auto* thread_table = context_->storage->mutable_thread_table();

	PERFETTO_DCHECK(process_table->name()[upid] == 0);
	process_table->mutable_start_ts()->Set(upid, timestamp);
	process_table->mutable_name()->Set(upid, main_thread_name);

	UniqueTid parent_utid = GetOrCreateThread(parent_tid);
	auto opt_parent_upid = thread_table->upid()[parent_utid];
	if (opt_parent_upid.has_value()) {
	process_table->mutable_parent_upid()->Set(upid, *opt_parent_upid);
	} else {
	pending_parent_assocs_.emplace_back(parent_utid, upid);
	}
	return upid;
	}

	UniquePid ProcessTracker::SetProcessMetadata(uint32_t pid,
	base::Optional<uint32_t> ppid,
	base::StringView name) {
	auto proc_name_id = context_->storage->InternString(name);

	base::Optional<UniquePid> pupid;
	if (ppid.has_value()) {
	pupid = GetOrCreateProcess(ppid.value());
	}

	UniquePid upid = GetOrCreateProcess(pid);

	auto* process_table = context_->storage->mutable_process_table();
	process_table->mutable_name()->Set(upid, proc_name_id);

	if (pupid)
	process_table->mutable_parent_upid()->Set(upid, *pupid);

	return upid;
	}

	void ProcessTracker::SetProcessUid(UniquePid upid, uint32_t uid) {
	auto* process_table = context_->storage->mutable_process_table();
	process_table->mutable_uid()->Set(upid, uid);
	}

	void ProcessTracker::SetProcessNameIfUnset(UniquePid upid,
	StringId process_name_id) {
	auto* process_table = context_->storage->mutable_process_table();
	if (process_table->name()[upid].is_null())
	process_table->mutable_name()->Set(upid, process_name_id);
	}

	void ProcessTracker::UpdateProcessNameFromThreadName(uint32_t tid,
	StringId thread_name) {
	auto* thread_table = context_->storage->mutable_thread_table();
	auto* process_table = context_->storage->mutable_process_table();

	auto utid = GetOrCreateThread(tid);
	auto opt_upid = thread_table->upid()[utid];
	if (opt_upid.has_value()) {
	if (process_table->pid()[*opt_upid] == tid) {
	process_table->mutable_name()->Set(*opt_upid, thread_name);
	}
	}
	}

	UniquePid ProcessTracker::GetOrCreateProcess(uint32_t pid) {
	UniquePid upid;
	auto it = pids_.find(pid);
	if (it != pids_.end()) {
	upid = it->second;
	} else {
	tables::ProcessTable::Row row;
	row.pid = pid;
	upid = context_->storage->mutable_process_table()->Insert(row).value;

	pids_.emplace(pid, upid);

	// Create an entry for the main thread.
	// We cannot call StartNewThread() here, because threads for this process
	// (including the main thread) might have been seen already prior to this
	// call. This call usually comes from the ProcessTree dump which is delayed.
	UpdateThread(/tid=/pid, pid);
	}
	return upid;
	}

	void ProcessTracker::AssociateThreads(UniqueTid utid1, UniqueTid utid2) {
	auto* tt = context_->storage->mutable_thread_table();

	// First of all check if one of the two threads is already bound to a process.
	// If that is the case, map the other thread to the same process and resolve
	// recursively any associations pending on the other thread.

	auto opt_upid1 = tt->upid()[utid1];
	auto opt_upid2 = tt->upid()[utid2];

	if (opt_upid1.has_value() && !opt_upid2.has_value()) {
	tt->mutable_upid()->Set(utid2, *opt_upid1);
	ResolvePendingAssociations(utid2, *opt_upid1);
	return;
	}

	if (opt_upid2.has_value() && !opt_upid1.has_value()) {
	tt->mutable_upid()->Set(utid1, *opt_upid2);
	ResolvePendingAssociations(utid1, *opt_upid2);
	return;
	}

	if (opt_upid1.has_value() && opt_upid1 != opt_upid2) {
	// Cannot associate two threads that belong to two different processes.
	PERFETTO_ELOG("Process tracker failure. Cannot associate threads %u, %u",
	tt->tid()[utid1], tt->tid()[utid2]);
	context_->storage->IncrementStats(stats::process_tracker_errors);
	return;
	}

	pending_assocs_.emplace_back(utid1, utid2);
	}

	void ProcessTracker::ResolvePendingAssociations(UniqueTid utid_arg,
	UniquePid upid) {
	auto* tt = context_->storage->mutable_thread_table();
	auto* pt = context_->storage->mutable_process_table();
	PERFETTO_DCHECK(tt->upid()[utid_arg] == upid);

	std::vector<UniqueTid> resolved_utids;
	resolved_utids.emplace_back(utid_arg);

	while (!resolved_utids.empty()) {
	UniqueTid utid = resolved_utids.back();
	resolved_utids.pop_back();
	for (auto it = pending_parent_assocs_.begin();
	it != pending_parent_assocs_.end();) {
	UniqueTid parent_utid = it->first;
	UniquePid child_upid = it->second;

	if (parent_utid != utid) {
	++it;
	continue;
	}
	PERFETTO_DCHECK(child_upid != upid);

	// Set the parent pid of the other process
	PERFETTO_DCHECK(!pt->parent_upid()[child_upid] \|\|
	pt->parent_upid()[child_upid] == upid);
	pt->mutable_parent_upid()->Set(child_upid, upid);

	// Erase the pair. The \|pending_parent_assocs_\| vector is not sorted and
	// swapping a std::pair<uint32_t, uint32_t> is cheap.
	std::swap(*it, pending_parent_assocs_.back());
	pending_parent_assocs_.pop_back();
	}

	for (auto it = pending_assocs_.begin(); it != pending_assocs_.end();) {
	UniqueTid other_utid;
	if (it->first == utid) {
	other_utid = it->second;
	} else if (it->second == utid) {
	other_utid = it->first;
	} else {
	++it;
	continue;
	}

	PERFETTO_DCHECK(other_utid != utid);

	// Update the other thread and associated it to the same process.
	PERFETTO_DCHECK(!tt->upid()[other_utid] \|\|
	tt->upid()[other_utid] == upid);
	tt->mutable_upid()->Set(other_utid, upid);

	// Erase the pair. The \|pending_assocs_\| vector is not sorted and swapping
	// a std::pair<uint32_t, uint32_t> is cheap.
	std::swap(*it, pending_assocs_.back());
	pending_assocs_.pop_back();

	// Recurse into the newly resolved thread. Some other threads might have
	// been bound to that.
	resolved_utids.emplace_back(other_utid);
	}
	} // while (!resolved_utids.empty())
	}

	} // namespace trace_processor
	} // namespace perfetto