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commit 4a68e8ded4fef2b00fbfc6baf1e79e46389871ca
Author: Siddhesh Poyarekar <siddhesh@redhat.com>
Date:   Thu Feb 20 15:55:16 2014 +0530

    Use cached offset in ftell when reliable
    
    The cached offset is reliable to use in ftell when the stream handle
    is active.  We can consider a stream as being active when there is
    unbuffered data.  However, even in this case, we can use the cached
    offset only when the stream is not being written to in a+ mode,
    because this case may have unbuffered data and a stale offset; the
    previous read could have sent it off somewhere other than the end of
    the file.
    
    There were a couple of adjustments necessary to get this to work.
    Firstly, fdopen now ceases to use _IO_attach_fd because it sets the
    offset cache to the current file position.  This is not correct
    because there could be changes to the file descriptor before the
    stream handle is activated, which would not get reflected.
    
    A similar offset caching action is done in _IO_fwide, claiming that
    wide streams have 'problems' with the file offsets.  There don't seem
    to be any obvious problems with not having the offset cache available,
    other than that it will have to be queried in a subsequent
    read/write/seek.  I have removed this as well.
    
    The testsuite passes successfully with these changes on x86_64.

diff --git a/libio/fileops.c b/libio/fileops.c
index a177302..c44a5da 100644
--- a/libio/fileops.c
+++ b/libio/fileops.c
@@ -952,12 +952,8 @@ get_file_offset (_IO_FILE *fp)
 static _IO_off64_t
 do_ftell (_IO_FILE *fp)
 {
-  _IO_off64_t result;
-
-  result = get_file_offset (fp);
-
-  if (result == EOF)
-    return result;
+  _IO_off64_t result = 0;
+  bool use_cached_offset = false;
 
   /* No point looking at unflushed data if we haven't allocated buffers
      yet.  */
@@ -971,8 +967,34 @@ do_ftell (_IO_FILE *fp)
 	result -= 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 buffered 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));
     }
 
+  if (use_cached_offset)
+    result += fp->_offset;
+  else
+    result += get_file_offset (fp);
+
+  if (result == EOF)
+    return result;
+
   if (result < 0)
     {
       __set_errno (EINVAL);
diff --git a/libio/iofdopen.c b/libio/iofdopen.c
index 066ff19..4525f0f 100644
--- a/libio/iofdopen.c
+++ b/libio/iofdopen.c
@@ -141,9 +141,6 @@ _IO_new_fdopen (fd, mode)
 #ifdef _IO_MTSAFE_IO
   new_f->fp.file._lock = &new_f->lock;
 #endif
-  /* Set up initially to use the `maybe_mmap' jump tables rather than using
-     __fopen_maybe_mmap to do it, because we need them in place before we
-     call _IO_file_attach or else it will allocate a buffer immediately.  */
   _IO_no_init (&new_f->fp.file, 0, 0, &new_f->wd,
 #ifdef _G_HAVE_MMAP
 	       (use_mmap && (read_write & _IO_NO_WRITES))
@@ -159,13 +156,10 @@ _IO_new_fdopen (fd, mode)
 #if  !_IO_UNIFIED_JUMPTABLES
   new_f->fp.vtable = NULL;
 #endif
-  if (_IO_file_attach ((_IO_FILE *) &new_f->fp, fd) == NULL)
-    {
-      _IO_setb (&new_f->fp.file, NULL, NULL, 0);
-      _IO_un_link (&new_f->fp);
-      free (new_f);
-      return NULL;
-    }
+  /* We only record the fd because _IO_file_init will have unset the offset.
+     We don't need to get the current offset in the file now since it could
+     change between now and when the handle is activated.  */
+  new_f->fp.file._fileno = fd;
   new_f->fp.file._flags &= ~_IO_DELETE_DONT_CLOSE;
 
   _IO_mask_flags (&new_f->fp.file, read_write,
diff --git a/libio/iofwide.c b/libio/iofwide.c
index 5cff632..64187e4 100644
--- a/libio/iofwide.c
+++ b/libio/iofwide.c
@@ -199,12 +199,6 @@ _IO_fwide (fp, mode)
 
       /* From now on use the wide character callback functions.  */
       ((struct _IO_FILE_plus *) fp)->vtable = fp->_wide_data->_wide_vtable;
-
-      /* One last twist: we get the current stream position.  The wide
-	 char streams have much more problems with not knowing the
-	 current position and so we should disable the optimization
-	 which allows the functions without knowing the position.  */
-      fp->_offset = _IO_SYSSEEK (fp, 0, _IO_seek_cur);
     }
 
   /* Set the mode now.  */
diff --git a/libio/wfileops.c b/libio/wfileops.c
index eda7828..651f5ce 100644
--- a/libio/wfileops.c
+++ b/libio/wfileops.c
@@ -600,6 +600,7 @@ static _IO_off64_t
 do_ftell_wide (_IO_FILE *fp)
 {
   _IO_off64_t result, offset = 0;
+  bool use_cached_offset = false;
 
   /* No point looking for offsets in the buffer if it hasn't even been
      allocated.  */
@@ -696,13 +697,36 @@ do_ftell_wide (_IO_FILE *fp)
 	      offset += outstop - out;
 	    }
 
-	  /* _IO_read_end coincides with fp._offset, so the actual file position
-	     is fp._offset - (_IO_read_end - new_write_ptr).  */
+	  /* _IO_read_end coincides with fp._offset, so the actual file
+	     position is fp._offset - (_IO_read_end - new_write_ptr).  */
 	  offset -= fp->_IO_read_end - fp->_IO_write_ptr;
+
 	}
+
+      /* 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
+	 buffered 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 = (offset != 0 && fp->_offset != _IO_pos_BAD
+			   && ((fp->_flags & (_IO_IS_APPENDING | _IO_NO_READS))
+			       == (_IO_IS_APPENDING | _IO_NO_READS)
+			       && was_writing));
     }
 
-  result = get_file_offset (fp);
+  if (use_cached_offset)
+    result = fp->_offset;
+  else
+    result = get_file_offset (fp);
 
   if (result == EOF)
     return result;