When renaming a file, ask only if EEXIST or ENOSYS

* src/fileio.c (Frename_file): Avoid calling Ffile_directory_p
more than once on FILE.  Use renameat_noreplace, so that we can
ask the user (and unlink and retry) only if this fails with errno
== EEXIST or ENOSYS.  This avoids the need to ask the user for
permission to do an operation that will fail anyway.  Simplify
computation of ok_if_already_exists for subsidiary functions.
* src/filelock.c (rename_lock_file): Prefer renameat_noreplace
if it works, as this avoids the need to link and unlink.
* src/lisp.h (renameat_noreplace): New decl.
* src/sysdep.c [HAVE_LINUX_FS_H]: Include linux/fs.h and sys/syscall.h.
(renameat_noreplace): New function.

1 files changed, 20 insertions, 0 deletions

diff --git a/src/sysdep.c b/src/sysdep.c
index db99f53299c..22446b25d16 100644
--- a/src/sysdep.c
+++ b/src/sysdep.c
@@ -37,6 +37,11 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 #include "sysselect.h"
 #include "blockinput.h"
 
+#ifdef HAVE_LINUX_FS_H
+# include <linux/fs.h>
+# include <sys/syscall.h>
+#endif
+
 #if defined DARWIN_OS || defined __FreeBSD__
 # include <sys/sysctl.h>
 #endif
@@ -2678,6 +2683,21 @@ set_file_times (int fd, const char *filename,
   timespec[1] = mtime;
   return fdutimens (fd, filename, timespec);
 }
+
+/* Rename directory SRCFD's entry SRC to directory DSTFD's entry DST.
+   This is like renameat except that it fails if DST already exists,
+   or if this operation is not supported atomically.  Return 0 if
+   successful, -1 (setting errno) otherwise.  */
+int
+renameat_noreplace (int srcfd, char const *src, int dstfd, char const *dst)
+{
+#ifdef SYS_renameat2
+  return syscall (SYS_renameat2, srcfd, src, dstfd, dst, RENAME_NOREPLACE);
+#else
+  errno = ENOSYS;
+  return -1;
+#endif
+}
 
 /* Like strsignal, except async-signal-safe, and this function typically
    returns a string in the C locale rather than the current locale.  */