[wpiutil] Split DataLog background writer into different class (#6590)

DataLog is now a base class, with DataLogBackgroundWriter being the
background thread version and DataLogWriter being a non-threaded version.

Also split the C header into a separate file to make it more wpiformat friendly.

wpiutil/src/main/native/include/wpi/DataLogWriter.h

@@ -0,0 +1,90 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#pragma once
+
+#include <memory>
+#include <string_view>
+#include <system_error>
+
+#include "wpi/DataLog.h"
+
+namespace wpi {
+class raw_ostream;
+class Logger;
+}  // namespace wpi
+
+namespace wpi::log {
+
+/**
+ * A data log writer that flushes the data log to a file when Flush() is called.
+ *
+ * The lifetime of this object must be longer than any data log entry objects
+ * that refer to it.
+ */
+class DataLogWriter final : public DataLog {
+ public:
+  /**
+   * Constructs with a filename.
+   *
+   * @param filename filename to use
+   * @param ec error code if failed to open file (output)
+   * @param extraHeader extra header data
+   */
+  explicit DataLogWriter(std::string_view filename, std::error_code& ec,
+                         std::string_view extraHeader = "");
+
+  /**
+   * Construct with a filename.
+   *
+   * @param msglog message logger
+   * @param filename filename to use
+   * @param ec error code if failed to open file (output)
+   * @param extraHeader extra header data
+   */
+  DataLogWriter(wpi::Logger& msglog, std::string_view filename,
+                std::error_code& ec, std::string_view extraHeader = "");
+
+  /**
+   * Constructs with an output stream.
+   *
+   * @param os output stream
+   * @param extraHeader extra header data
+   */
+  explicit DataLogWriter(std::unique_ptr<wpi::raw_ostream> os,
+                         std::string_view extraHeader = "");
+
+  /**
+   * Constructs with an output stream.
+   *
+   * @param msglog message logger
+   * @param os output stream
+   * @param extraHeader extra header data
+   */
+  DataLogWriter(wpi::Logger& msglog, std::unique_ptr<wpi::raw_ostream> os,
+                std::string_view extraHeader = "");
+
+  ~DataLogWriter() final;
+  DataLogWriter(const DataLogWriter&) = delete;
+  DataLogWriter& operator=(const DataLogWriter&) = delete;
+  DataLogWriter(DataLogWriter&&) = delete;
+  DataLogWriter& operator=(const DataLogWriter&&) = delete;
+
+  /**
+   * Flushes the log data to disk.
+   */
+  void Flush() final;
+
+  /**
+   * Stops appending all records to the log, and closes the log file.
+   */
+  void Stop() final;
+
+ private:
+  bool BufferFull() final;
+
+  std::unique_ptr<wpi::raw_ostream> m_os;
+};
+
+}  // namespace wpi::log