Refactor SingleLogSource for better test coverage

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) {
+const char kDefaultSystemLogDirPath[] = "/var/log";
+
+// Converts a logs source type to the corresponding file path, relative to the
+// base system log directory path. In the future, if non-file source types are
+// added, this function should return an empty file path.
+base::FilePath GetLogFileSourceRelativeFilePath(
+    SingleLogSource::SupportedSource source) {
   switch (source) {
     case SingleLogSource::SupportedSource::kMessages:
-      return "/var/log/messages";
+      return base::FilePath("messages");
     case SingleLogSource::SupportedSource::kUiLatest:
-      return "/var/log/ui/ui.LATEST";
+      return base::FilePath("ui/ui.LATEST");
   }
   NOTREACHED();
-  return "";
+  return base::FilePath();
 }
 
 }  // namespace
 
 SingleLogSource::SingleLogSource(SupportedSource source)
-    : SystemLogsSource(GetLogFileSourceFilename(source)),
+    : SystemLogsSource(GetLogFileSourceRelativeFilePath(source).value()),
+      log_file_dir_path_(kDefaultSystemLogDirPath),
       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(base::MayBlock(), 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()),
+    file_.Initialize(base::FilePath(log_file_dir_path_).Append(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);
@@ skipping 16 matching lines @@
     return;
   }
 
   num_bytes_read_ += size_read;
 
   // Pass it back to the callback.
   result->emplace(source_name(), anonymizer_.Anonymize(result_string));
 }
 
 }  // namespace system_logs