Fix offset caching for streams and use it for ftell (BZ #16680)

The ftell implementation was made conservative to ensure that
incorrectly cached offsets never affect it.  However, this causes
problems for append mode when a file stream is rewound.  Additionally,
the 'clever' trick of using stat to get position for append mode files
caused more problems than it solved and broke old behavior.  I have
described the various problems that it caused and then finally the
solution.

For a and a+ mode files, rewinding the stream should result in ftell
returning 0 as the offset, but the stat() trick caused it to
(incorrectly) always return the end of file.  Now I couldn't find
anything in POSIX that specifies the stream position after rewind()
for a file opened in 'a' mode, but for 'a+' mode it should be set to
0.  For 'a' mode too, it probably makes sense to keep it set to 0 in
the interest of retaining old behavior.

The initial file position for append mode files is implementation
defined, so the implementation could either retain the current file
position or move the position to the end of file.  The earlier ftell
implementation would move the offset to end of file for append-only
mode, but retain the old offset for a+ mode.  It would also cache the
offset (this detail is important).  My patch broke this and would set
the initial position to end of file for both append modes, thus
breaking old behavior.  I was ignorant enough to write an incorrect
test case for it too.

The Change:

I have now brought back the behavior of seeking to end of file for
append-only streams, but with a slight difference.  I don't cache the
offset though, since we would want ftell to query the current file
position through lseek while the stream is not active.  Since the
offset is moved to the end of file, we can rely on the file position
reported by lseek and we don't need to resort to the stat() nonsense.

Finally, the cache is always reliable, except when there are unflished
writes in an append mode stream (i.e. both a and a+).  In the latter
case, it is safe to just do an lseek to SEEK_END.  The value can be
safely cached too, since the file handle is already active at this
point.  Incidentally, this is the only state change we affect in the
file handle (apart from taking locks of course).

I have also updated the test case to correct my impression of the
initial file position for a+ streams to the initial behavior.  I have
verified that this does not break any existing tests in the testsuite
and also passes with the new tests.

1 files changed, 37 insertions, 65 deletions

diff --git a/libio/fileops.c b/libio/fileops.c
index 2e7bc8d..cf68dbf 100644
--- a/libio/fileops.c
+++ b/libio/fileops.c
@@ -232,13 +232,18 @@ _IO_file_open (fp, filename, posix_mode, prot, read_write, is32not64)
     return NULL;
   fp->_fileno = fdesc;
   _IO_mask_flags (fp, read_write,_IO_NO_READS+_IO_NO_WRITES+_IO_IS_APPENDING);
-  if ((read_write & _IO_IS_APPENDING) && (read_write & _IO_NO_READS))
-    if (_IO_SEEKOFF (fp, (_IO_off64_t)0, _IO_seek_end, _IOS_INPUT|_IOS_OUTPUT)
-	== _IO_pos_BAD && errno != ESPIPE)
-      {
-	close_not_cancel (fdesc);
-	return NULL;
-      }
+  /* For append mode, send the file offset to the end of the file.  Don't
+     update the offset cache though, since the file handle is not active.  */
+  if ((read_write & (_IO_IS_APPENDING | _IO_NO_READS))
+      == (_IO_IS_APPENDING | _IO_NO_READS))
+    {
+      _IO_off64_t new_pos = _IO_SYSSEEK (fp, 0, _IO_seek_end);
+      if (new_pos == _IO_pos_BAD && errno != ESPIPE)
+	{
+	  close_not_cancel (fdesc);
+	  return NULL;
+	}
+    }
   _IO_link_in ((struct _IO_FILE_plus *) fp);
   return fp;
 }
@@ -929,43 +934,13 @@ _IO_file_sync_mmap (_IO_FILE *fp)
   return 0;
 }
 
-/* Get the current file offset using a system call.  This is the safest method
-   to get the current file offset, since we are sure that we get the current
-   state of the file.  Before the stream handle is activated (by using fread,
-   fwrite, etc.), an application may alter the state of the file descriptor
-   underlying it by calling read/write/lseek on it.  Using a cached offset at
-   this point will result in returning the incorrect value.  Same is the case
-   when one switches from reading in a+ mode to writing, where the buffer has
-   not been flushed - the cached offset would reflect the reading position
-   while the actual write position would be at the end of the file.
-
-   do_ftell and do_ftell_wide may resort to using the cached offset in some
-   special cases instead of calling get_file_offset, but those cases should be
-   thoroughly described.  */
-_IO_off64_t
-get_file_offset (_IO_FILE *fp)
-{
-  if ((fp->_flags & _IO_IS_APPENDING) == _IO_IS_APPENDING)
-    {
-      struct stat64 st;
-      bool ret = (_IO_SYSSTAT (fp, &st) == 0 && S_ISREG (st.st_mode));
-      if (ret)
-	return st.st_size;
-      else
-	return EOF;
-    }
-  else
-    return _IO_SYSSEEK (fp, 0, _IO_seek_cur);
-}
-
-
-/* ftell{,o} implementation.  Don't modify any state of the file pointer while
-   we try to get the current state of the stream.  */
+/* ftell{,o} implementation.  The only time we modify the state of the stream
+   is when we have unflushed writes.  In that case we seek to the end and
+   record that offset in the stream object.  */
 static _IO_off64_t
 do_ftell (_IO_FILE *fp)
 {
-  _IO_off64_t result = 0;
-  bool use_cached_offset = false;
+  _IO_off64_t result, offset = 0;
 
   /* No point looking at unflushed data if we haven't allocated buffers
      yet.  */
@@ -974,39 +949,37 @@ do_ftell (_IO_FILE *fp)
       bool was_writing = (fp->_IO_write_ptr > fp->_IO_write_base
 			  || _IO_in_put_mode (fp));
 
+      bool append_mode = (fp->_flags & _IO_IS_APPENDING) == _IO_IS_APPENDING;
+
+      /* When we have unflushed writes in append mode, seek to the end of the
+	 file and record that offset.  This is the only time we change the file
+	 stream state and it is safe since the file handle is active.  */
+      if (was_writing && append_mode)
+	{
+	  result = _IO_SYSSEEK (fp, 0, _IO_seek_end);
+	  if (result == _IO_pos_BAD)
+	    return EOF;
+	  else
+	    fp->_offset = result;
+	}
+
       /* Adjust for unflushed data.  */
       if (!was_writing)
-	result -= fp->_IO_read_end - fp->_IO_read_ptr;
+	offset -= fp->_IO_read_end - fp->_IO_read_ptr;
       else
-	result += fp->_IO_write_ptr - fp->_IO_read_end;
-
-      /* It is safe to use the cached offset when available if there is
-	 unbuffered data (indicating that the file handle is active) and the
-	 handle is not for a file open in a+ mode.  The latter condition is
-	 because there could be a scenario where there is a switch from read
-	 mode to write mode using an fseek to an arbitrary position.  In this
-	 case, there would be unbuffered data due to be appended to the end of
-	 the file, but the offset may not necessarily be the end of the
-	 file.  It is fine to use the cached offset when the a+ stream is in
-	 read mode though, since the offset is maintained correctly in that
-	 case.  Note that this is not a comprehensive set of cases when the
-	 offset is reliable.  The offset may be reliable even in some cases
-	 where there is no unflushed input and the handle is active, but it's
-	 just that we don't have a way to identify that condition reliably.  */
-      use_cached_offset = (result != 0 && fp->_offset != _IO_pos_BAD
-			   && ((fp->_flags & (_IO_IS_APPENDING | _IO_NO_READS))
-			       == (_IO_IS_APPENDING | _IO_NO_READS)
-			       && was_writing));
+	offset += fp->_IO_write_ptr - fp->_IO_read_end;
     }
 
-  if (use_cached_offset)
-    result += fp->_offset;
+  if (fp->_offset != _IO_pos_BAD)
+    result = fp->_offset;
   else
-    result += get_file_offset (fp);
+    result = _IO_SYSSEEK (fp, 0, _IO_seek_cur);
 
   if (result == EOF)
     return result;
 
+  result += offset;
+
   if (result < 0)
     {
       __set_errno (EINVAL);
@@ -1016,7 +989,6 @@ do_ftell (_IO_FILE *fp)
   return result;
 }
 
-
 _IO_off64_t
 _IO_new_file_seekoff (fp, offset, dir, mode)
      _IO_FILE *fp;