io/FileDescriptor: add Duplicate() returning UniqueFileDescriptor

src/io/FileDescriptor.cxx

@@ -28,6 +28,7 @@
  */
 
 #include "FileDescriptor.hxx"
+#include "UniqueFileDescriptor.hxx"
 #include "system/Error.hxx"
 
 #include <cassert>
@@ -234,6 +235,12 @@ FileDescriptor::DisableCloseOnExec() noexcept
 	fcntl(fd, F_SETFD, old_flags & ~FD_CLOEXEC);
 }
 
+UniqueFileDescriptor
+FileDescriptor::Duplicate() const noexcept
+{
+	return UniqueFileDescriptor{::dup(Get())};
+}
+
 bool
 FileDescriptor::CheckDuplicate(FileDescriptor new_fd) noexcept
 {

src/io/FileDescriptor.hxx

@@ -44,6 +44,8 @@
 #include <wchar.h>
 #endif
 
+class UniqueFileDescriptor;
+
 /**
  * An OO wrapper for a UNIX file descriptor.
  *
@@ -176,6 +178,14 @@ public:
 	 */
 	void DisableCloseOnExec() noexcept;
 
+	/**
+	 * Duplicate this file descriptor.
+	 *
+	 * @return the new file descriptor or UniqueFileDescriptor{}
+	 * on error
+	 */
+	UniqueFileDescriptor Duplicate() const noexcept;
+
 	/**
 	 * Duplicate the file descriptor onto the given file descriptor.
 	 */