* fileio.c: Use O_APPEND to append.

This corresponds better to the natural interpretation of "append",
and avoids the need to open the output file twice, or to invoke
lseek when APPEND is neither nil nor a number.
This relies on POSIX 1003.1-1988 or later, which is OK nowadays.
(Fwrite_region): Simplify.  Use O_APPEND instead of opening the
file possibly twice, and lseeking to its end; this avoids the
need to lseek on non-regular files.  Do not use O_EXCL and O_TRUNC
at the same time: the combination is never needed and apparently
it doesn't work with DOS_NT.

2 files changed, 28 insertions, 23 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index 57b36782697..c66963738bd 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,5 +1,16 @@
 2013-01-19  Paul Eggert  <eggert@cs.ucla.edu>
 
+        * fileio.c: Use O_APPEND to append.
+        This corresponds better to the natural interpretation of "append",
+        and avoids the need to open the output file twice, or to invoke
+        lseek when APPEND is neither nil nor a number.
+        This relies on POSIX 1003.1-1988 or later, which is OK nowadays.
+        (Fwrite_region): Simplify.  Use O_APPEND instead of opening the
+        file possibly twice, and lseeking to its end; this avoids the
+        need to lseek on non-regular files.  Do not use O_EXCL and O_TRUNC
+        at the same time: the combination is never needed and apparently
+        it doesn't work with DOS_NT.
+
         Fix size bug on DOS_NT introduced by CIFS workaround (Bug#13149).
         * fileio.c (Fwrite_region): Use O_BINARY in checking code, too.
 
diff --git a/src/fileio.c b/src/fileio.c
index a2413c8a52f..51f966787b9 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -4741,7 +4741,9 @@ This calls `write-region-annotate-functions' at the start, and
   (Lisp_Object start, Lisp_Object end, Lisp_Object filename, Lisp_Object append, Lisp_Object visit, Lisp_Object lockname, Lisp_Object mustbenew)
 {
   int desc;
-  off_t offset;
+  int open_flags;
+  int mode;
+  off_t offset IF_LINT (= 0);
   bool ok;
   int save_errno = 0;
   const char *fn;
@@ -4861,28 +4863,20 @@ This calls `write-region-annotate-functions' at the start, and
 #endif /* CLASH_DETECTION */
 
   encoded_filename = ENCODE_FILE (filename);
-
   fn = SSDATA (encoded_filename);
-  offset = 0;
-  desc = -1;
-  if (!NILP (append))
-    {
-      if (NUMBERP (append))
-        offset = file_offset (append);
-      desc = emacs_open (fn, O_WRONLY | O_BINARY, 0);
-    }
-
-  if (desc < 0 && (NILP (append) || errno == ENOENT))
+  open_flags = O_WRONLY | O_BINARY | O_CREAT;
+  open_flags |= EQ (mustbenew, Qexcl) ? O_EXCL : !NILP (append) ? 0 : O_TRUNC;
+  if (NUMBERP (append))
+    offset = file_offset (append);
+  else if (!NILP (append))
+    open_flags |= O_APPEND;
 #ifdef DOS_NT
-  desc = emacs_open (fn,
-                     O_WRONLY | O_CREAT | O_BINARY
-                     | (EQ (mustbenew, Qexcl) ? O_EXCL : O_TRUNC),
-                     S_IREAD | S_IWRITE);
-#else  /* not DOS_NT */
-  desc = emacs_open (fn, O_WRONLY | O_TRUNC | O_CREAT
-                     | (EQ (mustbenew, Qexcl) ? O_EXCL : 0),
-                     auto_saving ? auto_save_mode_bits : 0666);
-#endif /* not DOS_NT */
+  mode = S_IREAD | S_IWRITE;
+#else
+  mode = auto_saving ? auto_save_mode_bits : 0666;
+#endif
+
+  desc = emacs_open (fn, open_flags, mode);
 
   if (desc < 0)
     {
@@ -4897,9 +4891,9 @@ This calls `write-region-annotate-functions' at the start, and
 
   record_unwind_protect (close_file_unwind, make_number (desc));
 
-  if (!NILP (append))
+  if (NUMBERP (append))
     {
-      off_t ret = lseek (desc, offset, NUMBERP (append) ? SEEK_SET : SEEK_END);
+      off_t ret = lseek (desc, offset, SEEK_SET);
       if (ret < 0)
         {
 #ifdef CLASH_DETECTION