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diff --git a/mozglue/baseprofiler/core/ProfiledThreadData.h b/mozglue/baseprofiler/core/ProfiledThreadData.h
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+++ b/mozglue/baseprofiler/core/ProfiledThreadData.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef ProfiledThreadData_h
+#define ProfiledThreadData_h
+
+#include "BaseProfilingStack.h"
+#include "platform.h"
+#include "ProfileBufferEntry.h"
+#include "ThreadInfo.h"
+
+#include "mozilla/RefPtr.h"
+#include "mozilla/TimeStamp.h"
+#include "mozilla/UniquePtr.h"
+
+#include <string>
+
+namespace mozilla {
+namespace baseprofiler {
+
+class ProfileBuffer;
+
+// This class contains information about a thread that is only relevant while
+// the profiler is running, for any threads (both alive and dead) whose thread
+// name matches the "thread filter" in the current profiler run.
+// ProfiledThreadData objects may be kept alive even after the thread is
+// unregistered, as long as there is still data for that thread in the profiler
+// buffer.
+//
+// Accesses to this class are protected by the profiler state lock.
+//
+// Created as soon as the following are true for the thread:
+//  - The profiler is running, and
+//  - the thread matches the profiler's thread filter, and
+//  - the thread is registered with the profiler.
+// So it gets created in response to either (1) the profiler being started (for
+// an existing registered thread) or (2) the thread being registered (if the
+// profiler is already running).
+//
+// The thread may be unregistered during the lifetime of ProfiledThreadData.
+// If that happens, NotifyUnregistered() is called.
+//
+// This class is the right place to store buffer positions. Profiler buffer
+// positions become invalid if the profiler buffer is destroyed, which happens
+// when the profiler is stopped.
+class ProfiledThreadData final {
+ public:
+  explicit ProfiledThreadData(ThreadInfo* aThreadInfo);
+  ~ProfiledThreadData();
+
+  void NotifyUnregistered(uint64_t aBufferPosition) {
+    mLastSample = Nothing();
+    MOZ_ASSERT(!mBufferPositionWhenReceivedJSContext,
+               "JSContext should have been cleared before the thread was "
+               "unregistered");
+    mUnregisterTime = TimeStamp::Now();
+    mBufferPositionWhenUnregistered = Some(aBufferPosition);
+  }
+  Maybe<uint64_t> BufferPositionWhenUnregistered() {
+    return mBufferPositionWhenUnregistered;
+  }
+
+  Maybe<uint64_t>& LastSample() { return mLastSample; }
+
+  void StreamJSON(const ProfileBuffer& aBuffer, SpliceableJSONWriter& aWriter,
+                  const std::string& aProcessName,
+                  const std::string& aETLDplus1,
+                  const TimeStamp& aProcessStartTime, double aSinceTime);
+
+  const RefPtr<ThreadInfo> Info() const { return mThreadInfo; }
+
+  void NotifyReceivedJSContext(uint64_t aCurrentBufferPosition) {
+    mBufferPositionWhenReceivedJSContext = Some(aCurrentBufferPosition);
+  }
+
+ private:
+  // Group A:
+  // The following fields are interesting for the entire lifetime of a
+  // ProfiledThreadData object.
+
+  // This thread's thread info.
+  const RefPtr<ThreadInfo> mThreadInfo;
+
+  // Group B:
+  // The following fields are only used while this thread is alive and
+  // registered. They become Nothing() once the thread is unregistered.
+
+  // When sampling, this holds the position in ActivePS::mBuffer of the most
+  // recent sample for this thread, or Nothing() if there is no sample for this
+  // thread in the buffer.
+  Maybe<uint64_t> mLastSample;
+
+  // Only non-Nothing() if the thread currently has a JSContext.
+  Maybe<uint64_t> mBufferPositionWhenReceivedJSContext;
+
+  // Group C:
+  // The following fields are only used once this thread has been unregistered.
+
+  Maybe<uint64_t> mBufferPositionWhenUnregistered;
+  TimeStamp mUnregisterTime;
+};
+
+// Stream all samples and markers from aBuffer with the given aThreadId (or 0
+// for everything, which is assumed to be a single backtrace sample.)
+// Returns the thread id of the output sample(s), or 0 if none was present.
+BaseProfilerThreadId StreamSamplesAndMarkers(
+    const char* aName, BaseProfilerThreadId aThreadId,
+    const ProfileBuffer& aBuffer, SpliceableJSONWriter& aWriter,
+    const std::string& aProcessName, const std::string& aETLDplus1,
+    const TimeStamp& aProcessStartTime, const TimeStamp& aRegisterTime,
+    const TimeStamp& aUnregisterTime, double aSinceTime,
+    UniqueStacks& aUniqueStacks);
+
+}  // namespace baseprofiler
+}  // namespace mozilla
+
+#endif  // ProfiledThreadData_h