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diff --git a/js/public/WaitCallbacks.h b/js/public/WaitCallbacks.h
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+/* -*- Mode: C++; tab-width: 8; 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 js_WaitCallbacks_h
+#define js_WaitCallbacks_h
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include "jstypes.h"
+
+struct JS_PUBLIC_API JSRuntime;
+
+namespace JS {
+
+/**
+ * When the JSRuntime is about to block in an Atomics.wait() JS call or in a
+ * `wait` instruction in WebAssembly, it can notify the host by means of a call
+ * to BeforeWaitCallback.  After the wait, it can notify the host by means of a
+ * call to AfterWaitCallback.  Both callbacks must be null, or neither.
+ *
+ * (If you change the callbacks from null to not-null or vice versa while some
+ * thread on the runtime is in a wait, you will be sorry.)
+ *
+ * The argument to the BeforeWaitCallback is a pointer to uninitialized
+ * stack-allocated working memory of size WAIT_CALLBACK_CLIENT_MAXMEM bytes.
+ * The caller of SetWaitCallback() must pass the amount of memory it will need,
+ * and this amount will be checked against that limit and the process will crash
+ * reliably if the check fails.
+ *
+ * The value returned by the BeforeWaitCallback will be passed to the
+ * AfterWaitCallback.
+ *
+ * The AfterWaitCallback will be called even if the wakeup is spurious and the
+ * thread goes right back to waiting again.  Of course the thread will call the
+ * BeforeWaitCallback once more before it goes to sleep in this situation.
+ */
+
+static constexpr size_t WAIT_CALLBACK_CLIENT_MAXMEM = 32;
+
+using BeforeWaitCallback = void* (*)(uint8_t* memory);
+using AfterWaitCallback = void (*)(void* cookie);
+
+extern JS_PUBLIC_API void SetWaitCallback(JSRuntime* rt,
+                                          BeforeWaitCallback beforeWait,
+                                          AfterWaitCallback afterWait,
+                                          size_t requiredMemory);
+
+}  // namespace JS
+
+#endif  // js_WaitCallbacks_h