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diff --git a/xpcom/io/nsIOutputStream.idl b/xpcom/io/nsIOutputStream.idl
new file mode 100644
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+++ b/xpcom/io/nsIOutputStream.idl
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+/* -*- Mode: C++; tab-width: 2; 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/. */
+
+#include "nsISupports.idl"
+
+interface nsIOutputStream;
+interface nsIInputStream;
+
+%{C++
+/**
+ * The signature for the reader function passed to WriteSegments. This
+ * is the "provider" of data that gets written into the stream's buffer.
+ *
+ * @param aOutStream stream being written to
+ * @param aClosure opaque parameter passed to WriteSegments
+ * @param aToSegment pointer to memory owned by the output stream
+ * @param aFromOffset amount already written (since WriteSegments was called)
+ * @param aCount length of toSegment
+ * @param aReadCount number of bytes written
+ *
+ * Implementers should return the following:
+ *
+ * @throws <any-error> if not interested in providing any data
+ *
+ * Errors are never passed to the caller of WriteSegments.
+ */
+typedef nsresult (*nsReadSegmentFun)(nsIOutputStream *aOutStream,
+                                     void *aClosure,
+                                     char *aToSegment,
+                                     uint32_t aFromOffset,
+                                     uint32_t aCount,
+                                     uint32_t *aReadCount);
+%}
+
+native nsReadSegmentFun(nsReadSegmentFun);
+
+/**
+ * nsIOutputStream
+ *
+ * An interface describing a writable stream of data.  An output stream may be
+ * "blocking" or "non-blocking" (see the IsNonBlocking method).  A blocking
+ * output stream may suspend the calling thread in order to satisfy a call to
+ * Close, Flush, Write, WriteFrom, or WriteSegments.  A non-blocking output
+ * stream, on the other hand, must not block the calling thread of execution.
+ *
+ * NOTE: blocking output streams are often written to on a background thread to
+ * avoid locking up the main application thread.  For this reason, it is
+ * generally the case that a blocking output stream should be implemented using
+ * thread- safe AddRef and Release.
+ */
+[scriptable, builtinclass, uuid(0d0acd2a-61b4-11d4-9877-00c04fa0cf4a)]
+interface nsIOutputStream : nsISupports
+{
+    /**
+     * Close the stream. Forces the output stream to flush any buffered data.
+     * Any subsequent calls to StreamStatus should throw NS_BASE_STREAM_CLOSED.
+     * Succeeds without effect if already closed.
+     *
+     * @throws NS_BASE_STREAM_WOULD_BLOCK if unable to flush without blocking
+     *   the calling thread (non-blocking mode only)
+     */
+    void close();
+
+    /**
+     * Flush the stream.
+     *
+     * @throws NS_BASE_STREAM_WOULD_BLOCK if unable to flush without blocking
+     *   the calling thread (non-blocking mode only)
+     */
+    void flush();
+
+    /**
+     * Check the current status of the stream.  A stream that is closed will
+     * throw an exception when this method is called.  That enables the caller
+     * to know the condition of the stream before attempting to write into it.
+     *
+     * This method will not throw NS_BASE_STREAM_WOULD_BLOCK, even if the stream
+     * is a non-blocking stream with no available space. A non-blocking stream
+     * which has not been closed, but has no available room should return NS_OK.
+     *
+     * NOTE: This method should not block the calling thread (e.g. to query the
+     * state of a file descriptor), even when called on a blocking stream.
+     *
+     * @throws NS_BASE_STREAM_CLOSED if the stream closed normally
+     * @throws <other-error> if the stream closed with a different status
+     */
+    void streamStatus();
+
+    /**
+     * Write data into the stream.
+     *
+     * @param aBuf the buffer containing the data to be written
+     * @param aCount the maximum number of bytes to be written
+     *
+     * @return number of bytes written (may be less than aCount)
+     *
+     * @throws NS_BASE_STREAM_WOULD_BLOCK if writing to the output stream would
+     *   block the calling thread (non-blocking mode only)
+     * @throws <other-error> on failure
+     */
+    unsigned long write(in string aBuf, in unsigned long aCount);
+
+    /**
+     * Writes data into the stream from an input stream.
+     *
+     * @param aFromStream the stream containing the data to be written
+     * @param aCount the maximum number of bytes to be written
+     *
+     * @return number of bytes written (may be less than aCount)
+     *
+     * @throws NS_BASE_STREAM_WOULD_BLOCK if writing to the output stream would
+     *    block the calling thread (non-blocking mode only). This failure
+     *    means no bytes were transferred.
+     * @throws <other-error> on failure
+     *
+     * NOTE: This method is defined by this interface in order to allow the
+     * output stream to efficiently copy the data from the input stream into
+     * its internal buffer (if any). If this method was provided as an external
+     * facility, a separate char* buffer would need to be used in order to call
+     * the output stream's other Write method.
+     */
+    unsigned long writeFrom(in nsIInputStream aFromStream,
+                            in unsigned long aCount);
+
+    /**
+     * Low-level write method that has access to the stream's underlying buffer.
+     * The reader function may be called multiple times for segmented buffers.
+     * WriteSegments is expected to keep calling the reader until either there
+     * is nothing left to write or the reader returns an error.  WriteSegments
+     * should not call the reader with zero bytes to provide.
+     *
+     * @param aReader the "provider" of the data to be written
+     * @param aClosure opaque parameter passed to reader
+     * @param aCount the maximum number of bytes to be written
+     *
+     * @return number of bytes written (may be less than aCount)
+     *
+     * @throws NS_BASE_STREAM_WOULD_BLOCK if writing to the output stream would
+     *    block the calling thread (non-blocking mode only). This failure
+     *    means no bytes were transferred.
+     * @throws NS_ERROR_NOT_IMPLEMENTED if the stream has no underlying buffer
+     * @throws <other-error> on failure
+     *
+     * NOTE: this function may be unimplemented if a stream has no underlying
+     * buffer (e.g., socket output stream).
+     */
+    [noscript] unsigned long writeSegments(in nsReadSegmentFun aReader,
+                                           in voidPtr aClosure,
+                                           in unsigned long aCount);
+
+    /**
+     * @return true if stream is non-blocking
+     *
+     * NOTE: writing to a blocking output stream will block the calling thread
+     * until all given data can be consumed by the stream.
+     *
+     * NOTE: a non-blocking output stream may implement nsIAsyncOutputStream to
+     * provide consumers with a way to wait for the stream to accept more data
+     * once its write method is unable to accept any data without blocking.
+     */
+    boolean isNonBlocking();
+};