util: add file_locker class

src/common/util.cpp

@@ -195,6 +195,73 @@ namespace tools
     catch (...) {}
   }
 
+  file_locker::file_locker(const std::string &filename)
+  {
+#ifdef WIN32
+    m_fd = INVALID_HANDLE_VALUE;
+    std::wstring filename_wide;
+    try
+    {
+      filename_wide = string_tools::utf8_to_utf16(filename);
+    }
+    catch (const std::exception &e)
+    {
+      MERROR("Failed to convert path \"" << filename << "\" to UTF-16: " << e.what());
+      return;
+    }
+    m_fd = CreateFileW(filename_wide.c_str(), GENERIC_READ, 0, NULL, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);
+    if (m_fd != INVALID_HANDLE_VALUE)
+    {
+      OVERLAPPED ov;
+      memset(&ov, 0, sizeof(ov));
+      if (!LockFileEx(m_fd, LOCKFILE_FAIL_IMMEDIATELY | LOCKFILE_EXCLUSIVE_LOCK, 0, 1, 0, &ov))
+      {
+        MERROR("Failed to lock " << filename << ": " << std::error_code(GetLastError(), std::system_category()));
+        CloseHandle(m_fd);
+        m_fd = INVALID_HANDLE_VALUE;
+      }
+    }
+    else
+    {
+      MERROR("Failed to open " << filename << ": " << std::error_code(GetLastError(), std::system_category()));
+    }
+#else
+    m_fd = open(filename, O_RDONLY | O_CREAT, 0666);
+    if (m_fd != -1)
+    {
+      if (flock(m_fd, LOCK_EX | LOCK_NB) == -1)
+      {
+        MERROR("Failed to lock " << filename << ": " << std::strerr(errno));
+        close(m_fd);
+        m_fd = -1;
+      }
+    }
+    else
+    {
+      MERROR("Failed to open " << filename << ": " << std::strerr(errno));
+    }
+#endif
+  }
+  file_locker::~file_locker()
+  {
+    if (locked())
+    {
+#ifdef WIN32
+      CloseHandle(m_fd);
+#else
+      close(m_fd);
+#endif
+    }
+  }
+  bool file_locker::locked() const
+  {
+#ifdef WIN32
+    return m_fd != INVALID_HANDLE_VALUE;
+#else
+    return m_fd != -1;
+#endif
+  }
+
 #ifdef WIN32
   std::string get_windows_version_display_string()
   {

src/common/util.h

@@ -91,6 +91,20 @@ namespace tools
     const std::string& filename() const noexcept { return m_filename; }
   };
 
+  class file_locker
+  {
+  public:
+    file_locker(const std::string &filename);
+    ~file_locker();
+    bool locked() const;
+  private:
+#ifdef WIN32
+    HANDLE m_fd;
+#else
+    int m_fd;
+#endif
+  };
+
   /*! \brief Returns the default data directory.
    *
    * \details Windows < Vista: C:\\Documents and Settings\\Username\\Application Data\\CRYPTONOTE_NAME