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diff --git a/comm/mailnews/base/src/LineReader.jsm b/comm/mailnews/base/src/LineReader.jsm
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+++ b/comm/mailnews/base/src/LineReader.jsm
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+/* 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/. */
+
+const EXPORTED_SYMBOLS = ["LineReader"];
+
+/**
+ * For a single request, mail servers may return several multi-line responses. A
+ * definition of multi-line responses can be found at rfc3977#section-3.1.1.
+ *
+ * This class helps dealing with multi-line responses by:
+ * - Break up a response to lines
+ * - Join incomplete line from a previous response with the current response
+ * - Remove stuffed dot (.. at the beginning of a line)
+ * - Detect the end of the response (\r\n.\r\n)
+ */
+class LineReader {
+  processingMultiLineResponse = false;
+  _data = "";
+
+  /**
+   * Read a multi-line response, emit each line through a callback.
+   *
+   * @param {string} data - A multi-line response received from the server.
+   * @param {Function} lineCallback - A line will be passed to the callback each
+   *   time.
+   * @param {Function} doneCallback - A function to be called when data is ended.
+   */
+  read(data, lineCallback, doneCallback) {
+    this._data += data;
+    if (this._data == ".\r\n" || this._data.endsWith("\r\n.\r\n")) {
+      this.processingMultiLineResponse = false;
+      this._data = this._data.slice(0, -3);
+    } else {
+      this.processingMultiLineResponse = true;
+    }
+    if (this._running) {
+      // This function can be called multiple times, but this._data should only
+      // be consumed once.
+      return;
+    }
+
+    let i = 0;
+    this._running = true;
+    while (this._data) {
+      let index = this._data.indexOf("\r\n");
+      if (index == -1) {
+        // Not enough data, save it for the next round.
+        break;
+      }
+      let line = this._data.slice(0, index + 2);
+      if (line.startsWith("..")) {
+        // Remove stuffed dot.
+        line = line.slice(1);
+      }
+      lineCallback(line);
+      this._data = this._data.slice(index + 2);
+      if (++i % 100 == 0) {
+        // Prevent blocking main process for too long.
+        Services.tm.spinEventLoopUntilEmpty();
+      }
+    }
+    this._running = false;
+    if (!this.processingMultiLineResponse && !this._data) {
+      doneCallback();
+    }
+  }
+}