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diff --git a/comm/mailnews/base/test/gtest/TestLineReader.cpp b/comm/mailnews/base/test/gtest/TestLineReader.cpp
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+++ b/comm/mailnews/base/test/gtest/TestLineReader.cpp
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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/. */
+
+#include "gtest/gtest.h"
+#include "nsString.h"
+#include "LineReader.h"
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/Span.h"
+
+// Invocation:
+// $ ./mach gtest "TestLineReader.*"
+
+TEST(TestLineReader, Basic)
+{
+  struct {
+    nsTArray<nsCString> chunks;
+    int expectedLines;
+  } testCases[] = {
+      // Split lines as expected?
+      {
+          {"Line one\r\nLine two\r\n"_ns},
+          2,
+      },
+
+      // Plain LFs should be accepted.
+      {
+          {"Line one\nLine two\n"_ns},
+          2,
+      },
+
+      // Empty lines appear as expected?
+      {
+          {"\r\n\r\n\r\n"_ns},
+          3,
+      },
+
+      // Empty case handled?
+      {
+          {""_ns},
+          0,
+      },
+
+      // Split in mid-CRLF.
+      {
+          {
+              "EOL split across\r"_ns,
+              "\nchunks.\r\n"_ns,
+          },
+          2,
+      },
+
+      // Chunks join up correctly?
+      {
+          {"This single line is "_ns, "fed in as "_ns,
+           "multiple chunks\r\n"_ns},
+          1,
+      },
+      // Handle an empty chunk OK?
+      {
+          {"foo"_ns, ""_ns,
+           "bar\n"_ns
+           "wibble\n"_ns},
+          2,
+      },
+      // Handle lines without EOL?
+      {
+          {"This line has no EOL and relies on Flush()."_ns},
+          1,
+      },
+  };
+
+  for (size_t i = 0; i < mozilla::ArrayLength(testCases); ++i) {
+    auto const& t = testCases[i];
+
+    // Join chunks into one string - we expect the output to be
+    // this, byte-for-byte.
+    nsCString expectedText;
+    for (auto chunk : t.chunks) {
+      expectedText.Append(chunk);
+    }
+
+    // Callback to collect the lines and count them.
+    int gotLineCount = 0;
+    nsCString gotText;
+    auto callback = [&](mozilla::Span<const char> line) {
+      ++gotLineCount;
+      gotText.Append(line);
+      return true;
+    };
+    // Parse the chunks.
+    LineReader chopper;
+    for (auto chunk : t.chunks) {
+      chopper.Feed(chunk, callback);
+    }
+    chopper.Flush(callback);
+
+    ASSERT_EQ(t.expectedLines, gotLineCount);
+    ASSERT_EQ(expectedText, gotText);
+  }
+}
+
+// Check that processing is aborted when callback returns false.
+TEST(TestLineReader, Stop)
+{
+  struct {
+    nsTArray<nsCString> chunks;
+    int expectedLines;
+  } testCases[] = {
+      // Stop at 2 lines.
+      {
+          {"Line one\nSTOP\nThis line is never seen.\n"_ns},
+          2,
+      },
+
+      // Line split up over multiple chunks.
+      {
+          {"one\r\nST"_ns, "OP\r\nblah blah\r\n"_ns},
+          2,
+      },
+
+      // Empty string -> no lines.
+      {
+          {""_ns},
+          0,
+      },
+
+      // No EOL, relies on Flush().
+      {
+          {
+              "STOP"_ns,
+          },
+          1,
+      },
+  };
+
+  for (size_t i = 0; i < mozilla::ArrayLength(testCases); ++i) {
+    auto const& t = testCases[i];
+
+    // Callback to collect the lines and count them, stopping when
+    // we find a line containing "STOP".
+    int gotLineCount = 0;
+    auto callback = [&](mozilla::Span<const char> line) -> bool {
+      ++gotLineCount;
+      return nsCString(line).Find("STOP"_ns) == kNotFound;
+    };
+    // Parse.
+    LineReader chopper;
+    for (auto chunk : t.chunks) {
+      chopper.Feed(chunk, callback);
+    }
+    chopper.Flush(callback);
+    ASSERT_EQ(t.expectedLines, gotLineCount);
+  }
+}
+
+// Test the SplitLines() fn.
+TEST(TestLineReader, SplitLines)
+{
+  struct {
+    nsCString input;
+    nsTArray<nsCString> expect;  // The lines we expect to see.
+    nsCString expectLeftover;    // The unconsumed data we expect at the end.
+  } testCases[] = {
+      // Empty string -> no lines.
+      {""_ns, {}, ""_ns},
+      // Incomplete line
+      {"foo"_ns, {}, "foo"_ns},
+      // Blank lines split as expected?
+      {"\r\n\r\n\r\n"_ns, {"\r\n"_ns, "\r\n"_ns, "\r\n"_ns}, ""_ns},
+      // A couple of normal-looking lines.
+      {"one\r\ntwo\r\n"_ns, {"one\r\n"_ns, "two\r\n"_ns}, ""_ns},
+      // Handles bare LFs?
+      {"one\ntwo\n"_ns, {"one\n"_ns, "two\n"_ns}, ""_ns},
+      // Ignores bare CRs?
+      {"one\rtwo\r"_ns, {}, "one\rtwo\r"_ns},
+
+      // Early-out works?
+      {"one\r\nSTOP\r\n3\r\n4\r\n"_ns,
+       {"one\r\n"_ns, "STOP\r\n"_ns},
+       "3\r\n4\r\n"_ns},
+  };
+
+  for (auto const& t : testCases) {
+    nsTArray<nsCString> got;
+    auto fn = [&](mozilla::Span<const char> line) -> bool {
+      got.AppendElement(line);
+      // Finish early if line contains "STOP".
+      return nsCString(line).Find("STOP"_ns) == kNotFound;
+    };
+    mozilla::Span<const char> leftover = SplitLines(t.input, fn);
+
+    ASSERT_EQ(t.expect.Length(), got.Length());
+    for (size_t i = 0; i < t.expect.Length(); ++i) {
+      ASSERT_EQ(t.expect[i], got[i]);
+    }
+    ASSERT_EQ(t.expectLeftover, nsCString(leftover));
+  }
+}