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diff --git a/js/public/MemoryCallbacks.h b/js/public/MemoryCallbacks.h
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+++ b/js/public/MemoryCallbacks.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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 js_MemoryCallbacks_h
+#define js_MemoryCallbacks_h
+
+#include "jstypes.h"
+
+struct JS_PUBLIC_API JSContext;
+
+namespace JS {
+
+/**
+ * If a large allocation fails when calling pod_{calloc,realloc}CanGC, the JS
+ * engine may call the large-allocation-failure callback, if set, to allow the
+ * embedding to flush caches, possibly perform shrinking GCs, etc. to make some
+ * room. The allocation will then be retried (and may still fail.) This callback
+ * can be called on any thread and must be set at most once in a process.
+ */
+
+using LargeAllocationFailureCallback = void (*)();
+
+extern JS_PUBLIC_API void SetProcessLargeAllocationFailureCallback(
+    LargeAllocationFailureCallback afc);
+
+/**
+ * Unlike the error reporter, which is only called if the exception for an OOM
+ * bubbles up and is not caught, the OutOfMemoryCallback is called immediately
+ * at the OOM site to allow the embedding to capture the current state of heap
+ * allocation before anything is freed. If the large-allocation-failure callback
+ * is called at all (not all allocation sites call the large-allocation-failure
+ * callback on failure), it is called before the out-of-memory callback; the
+ * out-of-memory callback is only called if the allocation still fails after the
+ * large-allocation-failure callback has returned.
+ */
+
+using OutOfMemoryCallback = void (*)(JSContext*, void*);
+
+extern JS_PUBLIC_API void SetOutOfMemoryCallback(JSContext* cx,
+                                                 OutOfMemoryCallback cb,
+                                                 void* data);
+
+}  // namespace JS
+
+#endif  // js_MemoryCallbacks_h