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diff --git a/tools/profiler/public/ProfilerThreadRegistry.h b/tools/profiler/public/ProfilerThreadRegistry.h
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+++ b/tools/profiler/public/ProfilerThreadRegistry.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 ProfilerThreadRegistry_h
+#define ProfilerThreadRegistry_h
+
+#include "mozilla/BaseProfilerDetail.h"
+#include "mozilla/ProfilerThreadRegistration.h"
+#include "mozilla/Vector.h"
+
+namespace mozilla::profiler {
+
+class ThreadRegistry {
+ private:
+  using RegistryMutex = baseprofiler::detail::BaseProfilerSharedMutex;
+  using RegistryLockExclusive =
+      baseprofiler::detail::BaseProfilerAutoLockExclusive;
+  using RegistryLockShared = baseprofiler::detail::BaseProfilerAutoLockShared;
+
+ public:
+  // Aliases to data accessors (removing the ThreadRegistration prefix).
+
+  using UnlockedConstReader = ThreadRegistrationUnlockedConstReader;
+  using UnlockedConstReaderAndAtomicRW =
+      ThreadRegistrationUnlockedConstReaderAndAtomicRW;
+  using UnlockedRWForLockedProfiler =
+      ThreadRegistrationUnlockedRWForLockedProfiler;
+  using UnlockedReaderAndAtomicRWOnThread =
+      ThreadRegistrationUnlockedReaderAndAtomicRWOnThread;
+  using LockedRWFromAnyThread = ThreadRegistrationLockedRWFromAnyThread;
+  using LockedRWOnThread = ThreadRegistrationLockedRWOnThread;
+
+  // Off-thread access through the registry, providing the following data
+  // accessors: UnlockedConstReader, UnlockedConstReaderAndAtomicRW,
+  // UnlockedRWForLockedProfiler, and LockedRWFromAnyThread.
+  // (See ThreadRegistration class for ON-thread access.)
+
+  // Reference-like class pointing at a ThreadRegistration.
+  // It should only exist while sRegistryMutex is locked.
+  class OffThreadRef {
+   public:
+    // const UnlockedConstReader
+
+    [[nodiscard]] const UnlockedConstReader& UnlockedConstReaderCRef() const {
+      return mThreadRegistration->mData;
+    }
+
+    template <typename F>
+    auto WithUnlockedConstReader(F&& aF) const {
+      return std::forward<F>(aF)(UnlockedConstReaderCRef());
+    }
+
+    // const UnlockedConstReaderAndAtomicRW
+
+    [[nodiscard]] const UnlockedConstReaderAndAtomicRW&
+    UnlockedConstReaderAndAtomicRWCRef() const {
+      return mThreadRegistration->mData;
+    }
+
+    template <typename F>
+    auto WithUnlockedConstReaderAndAtomicRW(F&& aF) const {
+      return std::forward<F>(aF)(UnlockedConstReaderAndAtomicRWCRef());
+    }
+
+    // UnlockedConstReaderAndAtomicRW
+
+    [[nodiscard]] UnlockedConstReaderAndAtomicRW&
+    UnlockedConstReaderAndAtomicRWRef() {
+      return mThreadRegistration->mData;
+    }
+
+    template <typename F>
+    auto WithUnlockedConstReaderAndAtomicRW(F&& aF) {
+      return std::forward<F>(aF)(UnlockedConstReaderAndAtomicRWRef());
+    }
+
+    // const UnlockedRWForLockedProfiler
+
+    [[nodiscard]] const UnlockedRWForLockedProfiler&
+    UnlockedRWForLockedProfilerCRef() const {
+      return mThreadRegistration->mData;
+    }
+
+    template <typename F>
+    auto WithUnlockedRWForLockedProfiler(F&& aF) const {
+      return std::forward<F>(aF)(UnlockedRWForLockedProfilerCRef());
+    }
+
+    // UnlockedRWForLockedProfiler
+
+    [[nodiscard]] UnlockedRWForLockedProfiler&
+    UnlockedRWForLockedProfilerRef() {
+      return mThreadRegistration->mData;
+    }
+
+    template <typename F>
+    auto WithUnlockedRWForLockedProfiler(F&& aF) {
+      return std::forward<F>(aF)(UnlockedRWForLockedProfilerRef());
+    }
+
+    // const LockedRWFromAnyThread through ConstRWFromAnyThreadWithLock
+
+    class ConstRWFromAnyThreadWithLock {
+     public:
+      [[nodiscard]] const LockedRWFromAnyThread& DataCRef() const {
+        return mLockedRWFromAnyThread;
+      }
+      [[nodiscard]] const LockedRWFromAnyThread* operator->() const {
+        return &mLockedRWFromAnyThread;
+      }
+
+      ConstRWFromAnyThreadWithLock(
+          const LockedRWFromAnyThread& aLockedRWFromAnyThread,
+          ThreadRegistration::DataMutex& aDataMutex)
+          : mLockedRWFromAnyThread(aLockedRWFromAnyThread),
+            mDataLock(aDataMutex) {}
+
+     private:
+      const LockedRWFromAnyThread& mLockedRWFromAnyThread;
+      ThreadRegistration::DataLock mDataLock;
+    };
+
+    [[nodiscard]] ConstRWFromAnyThreadWithLock ConstLockedRWFromAnyThread()
+        const {
+      return ConstRWFromAnyThreadWithLock{mThreadRegistration->mData,
+                                          mThreadRegistration->mDataMutex};
+    }
+
+    template <typename F>
+    auto WithConstLockedRWFromAnyThread(F&& aF) const {
+      ConstRWFromAnyThreadWithLock lockedData = ConstLockedRWFromAnyThread();
+      return std::forward<F>(aF)(lockedData.DataCRef());
+    }
+
+    // LockedRWFromAnyThread through RWFromAnyThreadWithLock
+
+    class RWFromAnyThreadWithLock {
+     public:
+      [[nodiscard]] const LockedRWFromAnyThread& DataCRef() const {
+        return mLockedRWFromAnyThread;
+      }
+      [[nodiscard]] LockedRWFromAnyThread& DataRef() {
+        return mLockedRWFromAnyThread;
+      }
+      [[nodiscard]] const LockedRWFromAnyThread* operator->() const {
+        return &mLockedRWFromAnyThread;
+      }
+      [[nodiscard]] LockedRWFromAnyThread* operator->() {
+        return &mLockedRWFromAnyThread;
+      }
+
+      // In some situations, it may be useful to do some on-thread operations if
+      // we are indeed on this thread now. The lock is still held here; caller
+      // should not use this pointer longer than this RWFromAnyThreadWithLock.
+      [[nodiscard]] LockedRWOnThread* GetLockedRWOnThread() {
+        if (mLockedRWFromAnyThread.Info().ThreadId() ==
+            profiler_current_thread_id()) {
+          // mLockedRWFromAnyThread references a subclass of the
+          // ThreadRegistration's mData, so it's safe to downcast it to another
+          // hierarchy level of the object.
+          return &static_cast<LockedRWOnThread&>(mLockedRWFromAnyThread);
+        }
+        return nullptr;
+      }
+
+     private:
+      friend class OffThreadRef;
+      RWFromAnyThreadWithLock(LockedRWFromAnyThread& aLockedRWFromAnyThread,
+                              ThreadRegistration::DataMutex& aDataMutex)
+          : mLockedRWFromAnyThread(aLockedRWFromAnyThread),
+            mDataLock(aDataMutex) {}
+
+      LockedRWFromAnyThread& mLockedRWFromAnyThread;
+      ThreadRegistration::DataLock mDataLock;
+    };
+
+    [[nodiscard]] RWFromAnyThreadWithLock LockedRWFromAnyThread() {
+      return RWFromAnyThreadWithLock{mThreadRegistration->mData,
+                                     mThreadRegistration->mDataMutex};
+    }
+
+    template <typename F>
+    auto WithLockedRWFromAnyThread(F&& aF) {
+      RWFromAnyThreadWithLock lockedData = LockedRWFromAnyThread();
+      return std::forward<F>(aF)(lockedData.DataRef());
+    }
+
+   private:
+    // Only ThreadRegistry should construct an OnThreadRef.
+    friend class ThreadRegistry;
+    explicit OffThreadRef(ThreadRegistration& aThreadRegistration)
+        : mThreadRegistration(&aThreadRegistration) {}
+
+    // If we have an ON-thread ref, it's safe to convert to an OFF-thread ref.
+    explicit OffThreadRef(ThreadRegistration::OnThreadRef aOnThreadRef)
+        : mThreadRegistration(aOnThreadRef.mThreadRegistration) {}
+
+    [[nodiscard]] bool IsPointingAt(
+        ThreadRegistration& aThreadRegistration) const {
+      return mThreadRegistration == &aThreadRegistration;
+    }
+
+    // Guaranted to be non-null by construction.
+    ThreadRegistration* mThreadRegistration;
+  };
+
+  // Lock the registry non-exclusively and allow iteration. E.g.:
+  // `for (OffThreadRef thread : LockedRegistry{}) { ... }`
+  // Do *not* export copies/references, as they could become dangling.
+  // Locking order: Profiler, ThreadRegistry, ThreadRegistration.
+  class LockedRegistry {
+   public:
+    LockedRegistry()
+        : mRegistryLock([]() -> RegistryMutex& {
+            MOZ_ASSERT(!IsRegistryMutexLockedOnCurrentThread(),
+                       "Recursive locking detected");
+            // In DEBUG builds, *before* we attempt to lock sRegistryMutex, we
+            // want to check that the ThreadRegistration mutex is *not* locked
+            // on this thread, to avoid inversion deadlocks.
+            MOZ_ASSERT(!ThreadRegistration::IsDataMutexLockedOnCurrentThread());
+            return sRegistryMutex;
+          }()) {
+      ThreadRegistration::WithOnThreadRef(
+          [](ThreadRegistration::OnThreadRef aOnThreadRef) {
+            aOnThreadRef.mThreadRegistration
+                ->mIsRegistryLockedSharedOnThisThread = true;
+          });
+    }
+
+    ~LockedRegistry() {
+      ThreadRegistration::WithOnThreadRef(
+          [](ThreadRegistration::OnThreadRef aOnThreadRef) {
+            aOnThreadRef.mThreadRegistration
+                ->mIsRegistryLockedSharedOnThisThread = false;
+          });
+    }
+
+    [[nodiscard]] const OffThreadRef* begin() const {
+      return sRegistryContainer.begin();
+    }
+    [[nodiscard]] OffThreadRef* begin() { return sRegistryContainer.begin(); }
+    [[nodiscard]] const OffThreadRef* end() const {
+      return sRegistryContainer.end();
+    }
+    [[nodiscard]] OffThreadRef* end() { return sRegistryContainer.end(); }
+
+   private:
+    RegistryLockShared mRegistryLock;
+  };
+
+  // Call `F(OffThreadRef)` for the given aThreadId.
+  template <typename F>
+  static void WithOffThreadRef(ProfilerThreadId aThreadId, F&& aF) {
+    for (OffThreadRef thread : LockedRegistry{}) {
+      if (thread.UnlockedConstReaderCRef().Info().ThreadId() == aThreadId) {
+        std::forward<F>(aF)(thread);
+        break;
+      }
+    }
+  }
+
+  template <typename F, typename FallbackReturn>
+  [[nodiscard]] static auto WithOffThreadRefOr(ProfilerThreadId aThreadId,
+                                               F&& aF,
+                                               FallbackReturn&& aFallbackReturn)
+      -> decltype(std::forward<F>(aF)(std::declval<OffThreadRef>())) {
+    for (OffThreadRef thread : LockedRegistry{}) {
+      if (thread.UnlockedConstReaderCRef().Info().ThreadId() == aThreadId) {
+        return std::forward<F>(aF)(thread);
+      }
+    }
+    return std::forward<FallbackReturn>(aFallbackReturn);
+  }
+
+  static size_t SizeOfExcludingThis(MallocSizeOf aMallocSizeOf) {
+    LockedRegistry lockedRegistry;
+    // "Ex" because we don't count static objects, but we count whatever they
+    // allocated on the heap.
+    size_t bytes = sRegistryContainer.sizeOfExcludingThis(aMallocSizeOf);
+    for (const OffThreadRef& offThreadRef : lockedRegistry) {
+      bytes +=
+          offThreadRef.mThreadRegistration->SizeOfExcludingThis(aMallocSizeOf);
+    }
+    return bytes;
+  }
+
+  static size_t SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) {
+    return SizeOfExcludingThis(aMallocSizeOf);
+  }
+
+  [[nodiscard]] static bool IsRegistryMutexLockedOnCurrentThread() {
+    return sRegistryMutex.IsLockedExclusiveOnCurrentThread() ||
+           ThreadRegistration::WithOnThreadRefOr(
+               [](ThreadRegistration::OnThreadRef aOnThreadRef) {
+                 return aOnThreadRef.mThreadRegistration
+                     ->mIsRegistryLockedSharedOnThisThread;
+               },
+               false);
+  }
+
+ private:
+  using RegistryContainer = Vector<OffThreadRef>;
+
+  static RegistryContainer sRegistryContainer;
+
+  // Mutex protecting the registry.
+  // Locking order: Profiler, ThreadRegistry, ThreadRegistration.
+  static RegistryMutex sRegistryMutex;
+
+  // Only allow ThreadRegistration to (un)register itself.
+  friend class ThreadRegistration;
+  static void Register(ThreadRegistration::OnThreadRef aOnThreadRef);
+  static void Unregister(ThreadRegistration::OnThreadRef aOnThreadRef);
+};
+
+}  // namespace mozilla::profiler
+
+#endif  // ProfilerThreadRegistry_h