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diff --git a/netwerk/base/nsReadLine.h b/netwerk/base/nsReadLine.h
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+/* -*- Mode: C++; tab-width: 2; 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 nsReadLine_h__
+#define nsReadLine_h__
+
+#include "nsIInputStream.h"
+#include "mozilla/Likely.h"
+
+/**
+ * @file
+ * Functions to read complete lines from an input stream.
+ *
+ * To properly use the helper function in here (NS_ReadLine) the caller should
+ * create a nsLineBuffer<T> with new, and pass it to NS_ReadLine every time it
+ * wants a line out.
+ *
+ * When done, the object should be deleted.
+ */
+
+/**
+ * @internal
+ * Buffer size. This many bytes will be buffered. If a line is longer than this,
+ * the partial line will be appended to the out parameter of NS_ReadLine and the
+ * buffer will be emptied.
+ * Note: if you change this constant, please update the regression test in
+ * netwerk/test/unit/test_readline.js accordingly (bug 397850).
+ */
+#define kLineBufferSize 4096
+
+/**
+ * @internal
+ * Line buffer structure, buffers data from an input stream.
+ * The buffer is empty when |start| == |end|.
+ * Invariant: |start| <= |end|
+ */
+template <typename CharT>
+class nsLineBuffer {
+ public:
+  nsLineBuffer() : start(buf), end(buf) {}
+
+  CharT buf[kLineBufferSize + 1];
+  CharT* start;
+  CharT* end;
+};
+
+/**
+ * Read a line from an input stream. Lines are separated by '\r' (0x0D) or '\n'
+ * (0x0A), or "\r\n" or "\n\r".
+ *
+ * @param aStream
+ *        The stream to read from
+ * @param aBuffer
+ *        The line buffer to use.  A single line buffer must not be used with
+ *        different input streams.
+ * @param aLine [out]
+ *        The string where the line will be stored.
+ * @param more [out]
+ *        Whether more data is available in the buffer. If true, NS_ReadLine may
+ *        be called again to read further lines. Otherwise, further calls to
+ *        NS_ReadLine will return an error.
+ *
+ * @retval NS_OK
+ *         Read successful
+ * @retval error
+ *         Input stream returned an error upon read. See
+ *         nsIInputStream::read.
+ */
+template <typename CharT, class StreamType, class StringType>
+nsresult NS_ReadLine(StreamType* aStream, nsLineBuffer<CharT>* aBuffer,
+                     StringType& aLine, bool* more) {
+  CharT eolchar = 0;  // the first eol char or 1 after \r\n or \n\r is found
+
+  aLine.Truncate();
+
+  while (true) {  // will be returning out of this loop on eol or eof
+    if (aBuffer->start == aBuffer->end) {  // buffer is empty.  Read into it.
+      uint32_t bytesRead;
+      nsresult rv = aStream->Read(aBuffer->buf, kLineBufferSize, &bytesRead);
+      if (NS_FAILED(rv) || MOZ_UNLIKELY(bytesRead == 0)) {
+        *more = false;
+        return rv;
+      }
+      aBuffer->start = aBuffer->buf;
+      aBuffer->end = aBuffer->buf + bytesRead;
+      *(aBuffer->end) = '\0';
+    }
+
+    /*
+     * Walk the buffer looking for an end-of-line.
+     * There are 3 cases to consider:
+     *  1. the eol char is the last char in the buffer
+     *  2. the eol char + one more char at the end of the buffer
+     *  3. the eol char + two or more chars at the end of the buffer
+     * we need at least one char after the first eol char to determine if
+     * it's a \r\n or \n\r sequence (and skip over it), and we need one
+     * more char after the end-of-line to set |more| correctly.
+     */
+    CharT* current = aBuffer->start;
+    if (MOZ_LIKELY(eolchar == 0)) {
+      for (; current < aBuffer->end; ++current) {
+        if (*current == '\n' || *current == '\r') {
+          eolchar = *current;
+          *current++ = '\0';
+          aLine.Append(aBuffer->start);
+          break;
+        }
+      }
+    }
+    if (MOZ_LIKELY(eolchar != 0)) {
+      for (; current < aBuffer->end; ++current) {
+        if ((eolchar == '\r' && *current == '\n') ||
+            (eolchar == '\n' && *current == '\r')) {
+          eolchar = 1;
+          continue;
+        }
+        aBuffer->start = current;
+        *more = true;
+        return NS_OK;
+      }
+    }
+
+    if (eolchar == 0) aLine.Append(aBuffer->start);
+    aBuffer->start = aBuffer->end;  // mark the buffer empty
+  }
+}
+
+#endif  // nsReadLine_h__