lib/fileutils: Add new dup_fd_cloexec function

This function duplicates and marks a file descriptor as close-on-exec.
Takes care of build and run-time support for the fcntl F_DUPFD_CLOEXEC
command, and other errors.

Signed-off-by: Guillem Jover <guillem@hadrons.org>

include/fileutils.h

@@ -25,6 +25,7 @@ static inline FILE *xfmkstemp(char **tmpname, char *dir)
 	return ret;
 }
 
+extern int dup_fd_cloexec(int oldfd, int lowfd);
 extern int get_fd_tabsize(void);
 
 extern int mkdir_p(const char *path, mode_t mode);

lib/fileutils.c

@@ -50,6 +50,36 @@ int xmkstemp(char **tmpname, char *dir)
 	return fd;
 }
 
+int dup_fd_cloexec(int oldfd, int lowfd)
+{
+	int fd, flags, errno_save;
+
+#ifdef F_DUPFD_CLOEXEC
+	fd = fcntl(oldfd, F_DUPFD_CLOEXEC, lowfd);
+	if (fd >= 0)
+		return fd;
+#endif
+
+	fd = dup(oldfd);
+	if (fd < 0)
+		return fd;
+
+	flags = fcntl(fd, F_GETFD);
+	if (flags < 0)
+		goto unwind;
+	if (fcntl(fd, F_SETFD, flags | FD_CLOEXEC) < 0)
+		goto unwind;
+
+	return fd;
+
+unwind:
+	errno_save = errno;
+	close(fd);
+	errno = errno_save;
+
+	return -1;
+}
+
 /*
  * portable getdtablesize()
  */