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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/. */
+
+/*
+ * The context-wide source hook allows the source of scripts/functions to be
+ * discarded, if that source is constant and readily-reloadable if it's needed
+ * in the future.
+ *
+ * Ordinarily, functions and scripts store a copy of their underlying source, to
+ * support |Function.prototype.toString| and debuggers.  Some scripts, however,
+ * might be constant and retrievable on demand -- perhaps burned into the binary
+ * or in a readonly file provided by the embedding.  Why not just ask the
+ * embedding for a copy of the source?
+ *
+ * The context-wide |SourceHook| gives embedders a way to respond to these
+ * requests.  The source of scripts/functions compiled with the compile option
+ * |JS::CompileOptions::setSourceIsLazy(true)| is eligible to be discarded.
+ * (The exact conditions under which source is discarded are unspecified.)  *If*
+ * source is discarded, performing an operation that requires source uses the
+ * source hook to load the source.
+ *
+ * The source hook must return the *exact* same source for every call.  (This is
+ * why the source hook is unsuitable for use with scripts loaded from the web:
+ * in general, their contents can change over time.)  If the source hook doesn't
+ * return the exact same source, Very Bad Things may happen.  (For example,
+ * previously-valid indexes into the source will no longer be coherent: they
+ * might index out of bounds, into the middle of multi-unit code points, &c.)
+ *
+ * These APIs are experimental because they shouldn't provide a per-*context*
+ * mechanism, rather something that's per-compilation.
+ */
+
+#ifndef js_experimental_SourceHook_h
+#define js_experimental_SourceHook_h
+
+#include "mozilla/UniquePtr.h"  // mozilla::UniquePtr
+
+#include <stddef.h>  // size_t
+
+#include "jstypes.h"  // JS_FRIEND_API
+
+struct JS_PUBLIC_API JSContext;
+
+namespace js {
+
+/**
+ * A class of objects that return source code on demand.
+ *
+ * When code is compiled with setSourceIsLazy(true), SpiderMonkey doesn't
+ * retain the source code (and doesn't do lazy bytecode generation). If we ever
+ * need the source code, say, in response to a call to Function.prototype.
+ * toSource or Debugger.Source.prototype.text, then we call the 'load' member
+ * function of the instance of this class that has hopefully been registered
+ * with the runtime, passing the code's URL, and hope that it will be able to
+ * find the source.
+ */
+class SourceHook {
+ public:
+  virtual ~SourceHook() = default;
+
+  /**
+   * Attempt to load the source for |filename|.
+   *
+   * On success, return true and store an owning pointer to the UTF-8 or UTF-16
+   * contents of the file in whichever of |twoByteSource| or |utf8Source| is
+   * non-null.  (Exactly one of these will be non-null.)  If the stored pointer
+   * is non-null, source was loaded and must be |js_free|'d when it's no longer
+   * needed.  If the stored pointer is null, the JS engine will simply act as if
+   * source was unavailable, and users like |Function.prototype.toString| will
+   * produce fallback results, e.g. "[native code]".
+   *
+   * On failure, return false.  The contents of whichever of |twoByteSource| or
+   * |utf8Source| was initially non-null are unspecified and must not be
+   * |js_free|'d.
+   */
+  virtual bool load(JSContext* cx, const char* filename,
+                    char16_t** twoByteSource, char** utf8Source,
+                    size_t* length) = 0;
+};
+
+/**
+ * Have |cx| use |hook| to retrieve lazily-retrieved source code. See the
+ * comments for SourceHook. The context takes ownership of the hook, and
+ * will delete it when the context itself is deleted, or when a new hook is
+ * set.
+ */
+extern JS_FRIEND_API void SetSourceHook(JSContext* cx,
+                                        mozilla::UniquePtr<SourceHook> hook);
+
+/** Remove |cx|'s source hook, and return it. The caller now owns the hook. */
+extern JS_FRIEND_API mozilla::UniquePtr<SourceHook> ForgetSourceHook(
+    JSContext* cx);
+
+}  // namespace js
+
+#endif  // js_experimental_SourceHook_h