Add SingleLogSource to system_logs sources

chrome/browser/chromeos/system_logs/single_log_source.cc

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "chrome/browser/chromeos/system_logs/single_log_source.h"
+
+#include "base/bind.h"
+#include "base/files/file_path.h"
+#include "base/task_scheduler/post_task.h"
+#include "content/public/browser/browser_thread.h"
+
+namespace system_logs {
+
+namespace {
+
+// Converts a logs source type to the corresponding filename. In the future, if
+// non-file source types are added, this function should return an empty string.
+std::string GetLogFileSourceFilename(SingleLogSource::SupportedSource source) {
+  switch (source) {
+    case SingleLogSource::SupportedSource::kMessages:
+      return "/var/log/messages";
+    case SingleLogSource::SupportedSource::kUiLatest:
+      return "/var/log/ui/ui.LATEST";
+  }
+}
+
+}  // namespace
+
+SingleLogSource::SingleLogSource(SupportedSource source)
+    : SystemLogsSource(GetLogFileSourceFilename(source)),
+      num_bytes_read_(0),
+      weak_ptr_factory_(this) {}
+
+SingleLogSource::~SingleLogSource() {}
+
+void SingleLogSource::Fetch(const SysLogsSourceCallback& callback) {
+  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
+  DCHECK(!callback.is_null());
+
+  SystemLogsResponse* response = new SystemLogsResponse;
+  base::PostTaskWithTraitsAndReply(
+      FROM_HERE,
+      base::TaskTraits().MayBlock().WithPriority(
+          base::TaskPriority::BACKGROUND),
+      base::Bind(&SingleLogSource::ReadFile, weak_ptr_factory_.GetWeakPtr(),
+                 response),
+      base::Bind(callback, base::Owned(response)));
+}
+
+void SingleLogSource::ReadFile(SystemLogsResponse* result) {
+  // Attempt to open the file if it was not previously opened.
+  if (!file_.IsValid()) {
+    file_.Initialize(base::FilePath(source_name()),
+                     base::File::FLAG_OPEN | base::File::FLAG_READ);
+    if (!file_.IsValid())
+      return;
+  }
+
+  // Check for file size reset.
+  const size_t length = file_.GetLength();
+  if (length < num_bytes_read_) {
+    num_bytes_read_ = 0;
+    file_.Seek(base::File::FROM_BEGIN, 0);
+  }
+
+  // Read from file until end.
+  const size_t size_to_read = length - num_bytes_read_;
+  std::string result_string(size_to_read, ' ');

[afakhry, 2017/05/10 04:28:03]

Nit: I think it would be better if we default construct it, then call
resize(size_to_read); This fills the new characters with the default constructed
CharT() instead of spaces, which is equivalent to

std::vector<char> read_result(size_to_read);

[ http://en.cppreference.com/w/cpp/string/basic_string/resize ]

Could potentially be better for compiler optimizations.

[Simon Que, 2017/05/10 14:53:15]

Done.

+  const size_t size_read =
+      file_.ReadAtCurrentPos(&result_string[0], size_to_read);
+  result_string.resize(size_read);
+
+  // The reader may only read complete lines.
+  if (!result_string.empty() && result_string.back() == '\n') {
+    num_bytes_read_ += size_read;
+  } else {
+    // If an incomplete line was read, reset the file read offset to before the
+    // most recent read.
+    file_.Seek(base::File::FROM_CURRENT, -size_read);
+    result_string.clear();

[afakhry, 2017/05/10 04:28:03]

Nit: You might consider doing the following here:

// Without clearing the string.
result->emplace(source_name(), "");
return;

There's no need to invoke the anonymizer on an empty string. ;)

[Simon Que, 2017/05/10 14:53:15]

Done.

+  }
+
+  // Pass it back to the callback.
+  result->emplace(source_name(), anonymizer_.Anonymize(result_string));
+}
+
+}  // namespace system_logs