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diff --git a/xpcom/io/SlicedInputStream.h b/xpcom/io/SlicedInputStream.h
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+++ b/xpcom/io/SlicedInputStream.h
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+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* 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 SlicedInputStream_h
+#define SlicedInputStream_h
+
+#include "mozilla/Attributes.h"
+#include "nsCOMPtr.h"
+#include "nsIAsyncInputStream.h"
+#include "nsICloneableInputStream.h"
+
+// A wrapper for a slice of an underlying input stream.
+
+class SlicedInputStream final : public nsIAsyncInputStream
+                              , public nsICloneableInputStream
+{
+public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIINPUTSTREAM
+  NS_DECL_NSICLONEABLEINPUTSTREAM
+  NS_DECL_NSIASYNCINPUTSTREAM
+
+  // Create an input stream whose data comes from a slice of aInputStream.  The
+  // slice begins at aStart bytes beyond aInputStream's current position, and
+  // extends for a maximum of aLength bytes.  If aInputStream contains fewer
+  // than aStart bytes, reading from SlicedInputStream returns no data.  If
+  // aInputStream contains more than aStart bytes, but fewer than aStart +
+  // aLength bytes, reading from SlicedInputStream returns as many bytes as can
+  // be consumed from aInputStream after reading aLength bytes.
+  //
+  // aInputStream should not be read from after constructing a
+  // SlicedInputStream wrapper around it.
+
+  SlicedInputStream(nsIInputStream* aInputStream,
+                    uint64_t aStart, uint64_t aLength);
+
+private:
+  ~SlicedInputStream();
+
+  nsCOMPtr<nsIInputStream> mInputStream;
+  uint64_t mStart;
+  uint64_t mLength;
+  uint64_t mCurPos;
+
+  bool mClosed;
+};
+
+#endif // SlicedInputStream_h