logs: Fix ReadEndOfFileMedium test flakes

If the first char of a full chunk happens to be '\0' it assumes the file is less than max_size and just returns chunk which is incomplete.

Bug: chromium:1099000
Change-Id: Iaebbc1d908c9c6386cbacf151f3bb8c19000fb51
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2271245Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Ian Barkley-Yeung <iby@chromium.org>
Commit-Queue: Joon Ahn <joonbug@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783812}

chrome/browser/chromeos/system_logs/debug_daemon_log_source.cc

@@ -83,16 +83,19 @@ bool ReadEndOfFile(const base::FilePath& path,
   chunk[0] = '\0';
   last_chunk[0] = '\0';
 
+  size_t total_bytes_read = 0;
   size_t bytes_read = 0;
 
   // Since most logs are not seekable, read until the end keeping tracking of
   // last two chunks.
   while ((bytes_read = fread(chunk.get(), 1, max_size, fp.get())) == max_size) {
+    total_bytes_read += bytes_read;
     last_chunk.swap(chunk);
     chunk[0] = '\0';
   }
+  total_bytes_read += bytes_read;
 
-  if (last_chunk[0] == '\0') {
+  if (total_bytes_read < max_size) {
     // File is smaller than max_size
     contents->assign(chunk.get(), bytes_read);
   } else if (bytes_read == 0) {

chrome/browser/chromeos/system_logs/debug_daemon_log_source_unittest.cc

@@ -36,7 +36,7 @@ TEST_F(DebugDaemonLogSourceTest, ReadEndOfFileSmall) {
   const char kTestData[] = "0123456789";  // Length of 10
   std::string read_data;
 
-  base::FilePath file_path = temp_dir_.GetPath().Append("test.txt");
+  base::FilePath file_path = temp_dir_.GetPath().Append("test_small.txt");
 
   WriteFile(file_path, kTestData, strlen(kTestData));
 
@@ -61,6 +61,36 @@ TEST_F(DebugDaemonLogSourceTest, ReadEndOfFileSmall) {
   EXPECT_EQ("56789", read_data);
 }
 
+TEST_F(DebugDaemonLogSourceTest, ReadEndOfFileWithZeros) {
+  const size_t test_size = 10;
+  std::string test_data("abcd\0\0\0\0hi", test_size);
+  std::string read_data;
+
+  base::FilePath file_path = temp_dir_.GetPath().Append("test_zero.txt");
+
+  WriteFile(file_path, test_data.data(), test_size);
+
+  read_data.clear();
+  EXPECT_TRUE(ReadEndOfFile(file_path, &read_data, 15));
+  EXPECT_EQ(test_data, read_data);
+
+  read_data.clear();
+  EXPECT_TRUE(ReadEndOfFile(file_path, &read_data, 10));
+  EXPECT_EQ(test_data, read_data);
+
+  read_data.clear();
+  EXPECT_TRUE(ReadEndOfFile(file_path, &read_data, 2));
+  EXPECT_EQ(test_data.substr(test_size - 2, 2), read_data);
+
+  read_data.clear();
+  EXPECT_TRUE(ReadEndOfFile(file_path, &read_data, 3));
+  EXPECT_EQ(test_data.substr(test_size - 3, 3), read_data);
+
+  read_data.clear();
+  EXPECT_TRUE(ReadEndOfFile(file_path, &read_data, 5));
+  EXPECT_EQ(test_data.substr(test_size - 5, 5), read_data);
+}
+
 TEST_F(DebugDaemonLogSourceTest, ReadEndOfFileMedium) {
   std::string test_data = base::RandBytesAsString(10000);  // 10KB data
   std::string read_data;