system/FileDescriptor: add CheckDuplicate()

src/system/FileDescriptor.cxx

@@ -188,6 +188,16 @@ FileDescriptor::DisableCloseOnExec() noexcept
 	fcntl(fd, F_SETFD, old_flags & ~FD_CLOEXEC);
 }
 
+bool
+FileDescriptor::CheckDuplicate(int new_fd) noexcept
+{
+	if (fd == new_fd) {
+		DisableCloseOnExec();
+		return true;
+	} else
+		return Duplicate(new_fd);
+}
+
 #endif
 
 #ifdef USE_EVENTFD

src/system/FileDescriptor.hxx

@@ -146,6 +146,14 @@ public:
 	bool Duplicate(int new_fd) const noexcept {
 		return ::dup2(Get(), new_fd) == 0;
 	}
+
+	/**
+	 * Similar to Duplicate(), but if destination and source file
+	 * descriptor are equal, clear the close-on-exec flag.  Use
+	 * this method to inject file descriptors into a new child
+	 * process, to be used by a newly executed program.
+	 */
+	bool CheckDuplicate(int new_fd) noexcept;
 #endif
 
 #ifdef USE_EVENTFD