Separate ftell from fseek logic and avoid modifying FILE data (#16532)

ftell semantics are distinct from fseek(SEEK_CUR) especially when it
is called on a file handler that is not yet active.  Due to this
caveat, much care needs to be taken while modifying the handler data
and hence, this first iteration on separating out ftell focusses on
maintaining handler data integrity at all times while it figures out
the current stream offset.  The result is that it makes a syscall for
every offset request.

There is scope for optimizing this by caching offsets when we know
that the handler is active.  A simple way to find out is when the
buffers have data.  It is not so simple to find this out when the
buffer is empty without adding some kind of flag.

1 files changed, 127 insertions, 102 deletions

diff --git a/libio/wfileops.c b/libio/wfileops.c
index 9cebe77126..7f4c92399f 100644
--- a/libio/wfileops.c
+++ b/libio/wfileops.c
@@ -596,29 +596,24 @@ done:
   return 0;
 }
 
-_IO_off64_t
-_IO_wfile_seekoff (fp, offset, dir, mode)
-     _IO_FILE *fp;
-     _IO_off64_t offset;
-     int dir;
-     int mode;
+/* ftell{,o} implementation for wide mode.  Don't modify any state of the file
+   pointer while we try to get the current state of the stream.  */
+static _IO_off64_t
+do_ftell_wide (_IO_FILE *fp)
 {
-  _IO_off64_t result;
-  _IO_off64_t delta, new_offset;
-  long int count;
-  /* POSIX.1 8.2.3.7 says that after a call the fflush() the file
-     offset of the underlying file must be exact.  */
-  int must_be_exact = ((fp->_wide_data->_IO_read_base
-			== fp->_wide_data->_IO_read_end)
-		       && (fp->_wide_data->_IO_write_base
-			   == fp->_wide_data->_IO_write_ptr));
+  _IO_off64_t result, offset = 0;
 
-  bool was_writing = ((fp->_wide_data->_IO_write_ptr
-		       > fp->_wide_data->_IO_write_base)
-		      || _IO_in_put_mode (fp));
-
-  if (mode == 0)
+  /* No point looking for offsets in the buffer if it hasn't even been
+     allocated.  */
+  if (fp->_wide_data->_IO_buf_base != NULL)
     {
+      const wchar_t *wide_read_base;
+      const wchar_t *wide_read_ptr;
+      const wchar_t *wide_read_end;
+      bool was_writing = ((fp->_wide_data->_IO_write_ptr
+			   > fp->_wide_data->_IO_write_base)
+			  || _IO_in_put_mode (fp));
+
       /* XXX For wide stream with backup store it is not very
 	 reasonable to determine the offset.  The pushed-back
 	 character might require a state change and we need not be
@@ -633,14 +628,120 @@ _IO_wfile_seekoff (fp, offset, dir, mode)
 	      return -1;
 	    }
 
-	  /* There is no more data in the backup buffer.  We can
-	     switch back.  */
-	  _IO_switch_to_main_wget_area (fp);
+	  /* Nothing in the backup store, so note the backed up pointers
+	     without changing the state.  */
+	  wide_read_base = fp->_wide_data->_IO_save_base;
+	  wide_read_ptr = wide_read_base;
+	  wide_read_end = fp->_wide_data->_IO_save_end;
+	}
+      else
+	{
+	  wide_read_base = fp->_wide_data->_IO_read_base;
+	  wide_read_ptr = fp->_wide_data->_IO_read_ptr;
+	  wide_read_end = fp->_wide_data->_IO_read_end;
 	}
 
-      dir = _IO_seek_cur, offset = 0; /* Don't move any pointers. */
+      struct _IO_codecvt *cv = fp->_codecvt;
+      int clen = (*cv->__codecvt_do_encoding) (cv);
+
+      if (!was_writing)
+	{
+	  if (clen > 0)
+	    {
+	      offset -= (wide_read_end - wide_read_ptr) * clen;
+	      offset -= fp->_IO_read_end - fp->_IO_read_ptr;
+	    }
+	  else
+	    {
+	      int nread;
+
+	      size_t delta = wide_read_ptr - wide_read_base;
+	      __mbstate_t state = fp->_wide_data->_IO_last_state;
+	      nread = (*cv->__codecvt_do_length) (cv, &state,
+						  fp->_IO_read_base,
+						  fp->_IO_read_end, delta);
+	      offset -= fp->_IO_read_end - fp->_IO_read_base - nread;
+	    }
+	}
+      else
+	{
+	  if (clen > 0)
+	    offset += (fp->_wide_data->_IO_write_ptr
+		       - fp->_wide_data->_IO_write_base) * clen;
+	  else
+	    {
+	      size_t delta = (fp->_wide_data->_IO_write_ptr
+			      - fp->_wide_data->_IO_write_base);
+
+	      /* Allocate enough space for the conversion.  */
+	      size_t outsize = delta * sizeof (wchar_t);
+	      char *out = malloc (outsize);
+	      char *outstop = out;
+	      const wchar_t *in = fp->_wide_data->_IO_write_base;
+
+	      enum __codecvt_result status;
+
+	      __mbstate_t state = fp->_wide_data->_IO_last_state;
+	      status = (*cv->__codecvt_do_out) (cv, &state,
+						in, in + delta, &in,
+						out, out + outsize, &outstop);
+
+	      /* We don't check for __codecvt_partial because it can be
+		 returned on one of two conditions: either the output
+		 buffer is full or the input sequence is incomplete.  We
+		 take care to allocate enough buffer and our input
+		 sequences must be complete since they are accepted as
+		 wchar_t; if not, then that is an error.  */
+	      if (__glibc_unlikely (status != __codecvt_ok))
+		return WEOF;
+
+	      offset += outstop - out;
+	    }
+
+	  /* _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;
+	}
     }
 
+  result = get_file_offset (fp);
+
+  if (result == EOF)
+    return result;
+
+  result += offset;
+
+  return result;
+}
+
+_IO_off64_t
+_IO_wfile_seekoff (fp, offset, dir, mode)
+     _IO_FILE *fp;
+     _IO_off64_t offset;
+     int dir;
+     int mode;
+{
+  _IO_off64_t result;
+  _IO_off64_t delta, new_offset;
+  long int count;
+
+  /* Short-circuit into a separate function.  We don't want to mix any
+     functionality and we don't want to touch anything inside the FILE
+     object. */
+  if (mode == 0)
+    return do_ftell_wide (fp);
+
+  /* POSIX.1 8.2.3.7 says that after a call the fflush() the file
+     offset of the underlying file must be exact.  */
+  int must_be_exact = ((fp->_wide_data->_IO_read_base
+			== fp->_wide_data->_IO_read_end)
+		       && (fp->_wide_data->_IO_write_base
+			   == fp->_wide_data->_IO_write_ptr));
+
+  bool was_writing = ((fp->_wide_data->_IO_write_ptr
+		       > fp->_wide_data->_IO_write_base)
+		      || _IO_in_put_mode (fp));
+
   /* Flush unwritten characters.
      (This may do an unneeded write if we seek within the buffer.
      But to be able to switch to reading, we would need to set
@@ -648,7 +749,7 @@ _IO_wfile_seekoff (fp, offset, dir, mode)
      which assumes file_ptr() is eGptr.  Anyway, since we probably
      end up flushing when we close(), it doesn't make much difference.)
      FIXME: simulate mem-mapped files. */
-  else if (was_writing && _IO_switch_to_wget_mode (fp))
+  if (was_writing && _IO_switch_to_wget_mode (fp))
     return WEOF;
 
   if (fp->_wide_data->_IO_buf_base == NULL)
@@ -693,7 +794,6 @@ _IO_wfile_seekoff (fp, offset, dir, mode)
 	    {
 	      int nread;
 
-	    flushed:
 	      delta = (fp->_wide_data->_IO_read_ptr
 		       - fp->_wide_data->_IO_read_base);
 	      fp->_wide_data->_IO_state = fp->_wide_data->_IO_last_state;
@@ -706,80 +806,9 @@ _IO_wfile_seekoff (fp, offset, dir, mode)
 	      offset -= fp->_IO_read_end - fp->_IO_read_base - nread;
 	    }
 	}
-      else
-	{
-	  char *new_write_ptr = fp->_IO_write_ptr;
-
-	  if (clen > 0)
-	    offset += (fp->_wide_data->_IO_write_ptr
-		       - fp->_wide_data->_IO_write_base) / clen;
-	  else
-	    {
-	      enum __codecvt_result status = __codecvt_ok;
-	      delta = (fp->_wide_data->_IO_write_ptr
-		       - fp->_wide_data->_IO_write_base);
-	      const wchar_t *write_base = fp->_wide_data->_IO_write_base;
-
-	      /* FIXME: This actually ends up in two iterations of conversion,
-		 one here and the next when the buffer actually gets flushed.
-		 It may be possible to optimize this in future so that
-		 wdo_write identifies already converted content and does not
-		 redo it.  In any case, this is much better than having to
-		 flush buffers for every ftell.  */
-	      do
-		{
-		  /* There is not enough space in the buffer to do the entire
-		     conversion, so there is no point trying to avoid the
-		     buffer flush.  Just do it and go back to how it was with
-		     the read mode.  */
-		  if (status == __codecvt_partial
-		      || (delta > 0 && new_write_ptr == fp->_IO_buf_end))
-		    {
-		      if (_IO_switch_to_wget_mode (fp))
-			return WEOF;
-		      goto flushed;
-		    }
-
-		  const wchar_t *new_wbase = fp->_wide_data->_IO_write_base;
-		  fp->_wide_data->_IO_state = fp->_wide_data->_IO_last_state;
-		  status = (*cv->__codecvt_do_out) (cv,
-						    &fp->_wide_data->_IO_state,
-						    write_base,
-						    write_base + delta,
-						    &new_wbase,
-						    new_write_ptr,
-						    fp->_IO_buf_end,
-						    &new_write_ptr);
-
-		  delta -= new_wbase - write_base;
-
-		  /* If there was an error, then return WEOF.
-		     TODO: set buffer state.  */
-		  if (__glibc_unlikely (status == __codecvt_error))
-		      return WEOF;
-		}
-	      while (delta > 0);
-	    }
-
-	  /* _IO_read_end coincides with fp._offset, so the actual file position
-	     is fp._offset - (_IO_read_end - new_write_ptr).  This is fine
-	     even if fp._offset is not set, since fp->_IO_read_end is then at
-	     _IO_buf_base and this adjustment is for unbuffered output.  */
-	  offset -= fp->_IO_read_end - new_write_ptr;
-	}
 
       if (fp->_offset == _IO_pos_BAD)
-	{
-	  if (mode != 0)
-	    goto dumb;
-	  else
-	    {
-	      result = _IO_SYSSEEK (fp, 0, dir);
-	      if (result == EOF)
-		return result;
-	      fp->_offset = result;
-	    }
-	}
+	goto dumb;
 
       /* Make offset absolute, assuming current pointer is file_ptr(). */
       offset += fp->_offset;
@@ -802,10 +831,6 @@ _IO_wfile_seekoff (fp, offset, dir, mode)
     }
   /* At this point, dir==_IO_seek_set. */
 
-  /* If we are only interested in the current position we've found it now.  */
-  if (mode == 0)
-    return offset;
-
   /* If destination is within current buffer, optimize: */
   if (fp->_offset != _IO_pos_BAD && fp->_IO_read_base != NULL
       && !_IO_in_backup (fp))