Fix offset computation for append+ mode on switching from read (BZ #16724)

The offset computation in write mode uses the fact that _IO_read_end
is kept in sync with the external file offset.  This however is not
true when O_APPEND is in effect since switching to write mode ought to
send the external file offset to the end of file without making the
necessary adjustment to _IO_read_end.

Hence in append mode, offset computation when writing should only
consider the effect of unflushed writes, i.e. from _IO_write_base to
_IO_write_ptr.

The wiki has a detailed document that describes the rationale for
offsets returned by ftell in various conditions:

https://sourceware.org/glibc/wiki/File%20offsets%20in%20a%20stdio%20stream%20and%20ftell

4 files changed, 193 insertions, 5 deletions

diff --git a/libio/Makefile b/libio/Makefile
index cca03454fc..b324ccc519 100644
--- a/libio/Makefile
+++ b/libio/Makefile
@@ -60,7 +60,8 @@ tests = tst_swprintf tst_wprintf tst_swscanf tst_wscanf tst_getwc tst_putwc   \
 	tst-wmemstream1 tst-wmemstream2 \
 	bug-memstream1 bug-wmemstream1 \
 	tst-setvbuf1 tst-popen1 tst-fgetwc bug-wsetpos tst-fseek \
-	tst-fwrite-error tst-ftell-partial-wide tst-ftell-active-handler
+	tst-fwrite-error tst-ftell-partial-wide tst-ftell-active-handler \
+	tst-ftell-append
 ifeq (yes,$(build-shared))
 # Add test-fopenloc only if shared library is enabled since it depends on
 # shared localedata objects.
@@ -160,6 +161,7 @@ tst-fgetwc-ENV = LOCPATH=$(common-objpfx)localedata
 tst-fseek-ENV = LOCPATH=$(common-objpfx)localedata
 tst-ftell-partial-wide-ENV = LOCPATH=$(common-objpfx)localedata
 tst-ftell-active-handler-ENV = LOCPATH=$(common-objpfx)localedata
+tst-ftell-append-ENV = LOCPATH=$(common-objpfx)localedata
 
 generated += tst-fopenloc.mtrace tst-fopenloc.check
 
diff --git a/libio/fileops.c b/libio/fileops.c
index cf68dbfe52..204cfeaa35 100644
--- a/libio/fileops.c
+++ b/libio/fileops.c
@@ -91,7 +91,9 @@ extern struct __gconv_trans_data __libio_translit attribute_hidden;
 
    The position in the buffer that corresponds to the position
    in external file system is normally _IO_read_end, except in putback
-   mode, when it is _IO_save_end.
+   mode, when it is _IO_save_end and also when the file is in append mode,
+   since switching from read to write mode automatically sends the position in
+   the external file system to the end of file.
    If the field _fb._offset is >= 0, it gives the offset in
    the file as a whole corresponding to eGptr(). (?)
 
@@ -966,6 +968,14 @@ do_ftell (_IO_FILE *fp)
       /* Adjust for unflushed data.  */
       if (!was_writing)
 	offset -= fp->_IO_read_end - fp->_IO_read_ptr;
+      /* We don't trust _IO_read_end to represent the current file offset when
+	 writing in append mode because the value would have to be shifted to
+	 the end of the file during a flush.  Use the write base instead, along
+	 with the new offset we got above when we did a seek to the end of the
+	 file.  */
+      else if (append_mode)
+	offset += fp->_IO_write_ptr - fp->_IO_write_base;
+      /* For all other modes, _IO_read_end represents the file offset.  */
       else
 	offset += fp->_IO_write_ptr - fp->_IO_read_end;
     }
diff --git a/libio/tst-ftell-append.c b/libio/tst-ftell-append.c
new file mode 100644
index 0000000000..604dc0342c
--- /dev/null
+++ b/libio/tst-ftell-append.c
@@ -0,0 +1,169 @@
+/* Verify that ftell returns the correct value after a read and a write on a
+   file opened in a+ mode.
+   Copyright (C) 2014 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <errno.h>
+#include <unistd.h>
+#include <locale.h>
+#include <wchar.h>
+
+/* data points to either char_data or wide_data, depending on whether we're
+   testing regular file mode or wide mode respectively.  Similarly,
+   fputs_func points to either fputs or fputws.  data_len keeps track of the
+   length of the current data and file_len maintains the current file
+   length.  */
+#define BUF_LEN 4
+static void *buf;
+static char char_buf[BUF_LEN];
+static wchar_t wide_buf[BUF_LEN];
+static const void *data;
+static const char *char_data = "abcdefghijklmnopqrstuvwxyz";
+static const wchar_t *wide_data = L"abcdefghijklmnopqrstuvwxyz";
+static size_t data_len;
+static size_t file_len;
+
+typedef int (*fputs_func_t) (const void *data, FILE *fp);
+fputs_func_t fputs_func;
+
+typedef void *(*fgets_func_t) (void *s, int size, FILE *stream);
+fgets_func_t fgets_func;
+
+static int do_test (void);
+
+#define TEST_FUNCTION do_test ()
+#include "../test-skeleton.c"
+
+static FILE *
+init_file (const char *filename)
+{
+  FILE *fp = fopen (filename, "w");
+  if (fp == NULL)
+    {
+      printf ("fopen: %m\n");
+      return NULL;
+    }
+
+  int written = fputs_func (data, fp);
+
+  if (written == EOF)
+    {
+      printf ("fputs failed to write data\n");
+      fclose (fp);
+      return NULL;
+    }
+
+  file_len = data_len;
+
+  fclose (fp);
+
+  fp = fopen (filename, "a+");
+  if (fp == NULL)
+    {
+      printf ("fopen(a+): %m\n");
+      return NULL;
+    }
+
+  return fp;
+}
+
+static int
+do_one_test (const char *filename)
+{
+  FILE *fp = init_file (filename);
+
+  if (fp == NULL)
+    return 1;
+
+  void *ret = fgets_func (buf, BUF_LEN, fp);
+
+  if (ret == NULL)
+    {
+      printf ("read failed: %m\n");
+      fclose (fp);
+      return 1;
+    }
+
+  int written = fputs_func (data, fp);
+
+  if (written == EOF)
+    {
+      printf ("fputs failed to write data\n");
+      fclose (fp);
+      return 1;
+    }
+
+  file_len += data_len;
+
+  long off = ftell (fp);
+
+  if (off != file_len)
+    {
+      printf ("Incorrect offset %ld, expected %zu\n", off, file_len);
+      fclose (fp);
+      return 1;
+    }
+  else
+    printf ("Correct offset %ld after write.\n", off);
+
+  return 0;
+}
+
+/* Run the tests for regular files and wide mode files.  */
+static int
+do_test (void)
+{
+  int ret = 0;
+  char *filename;
+  int fd = create_temp_file ("tst-ftell-append-tmp.", &filename);
+
+  if (fd == -1)
+    {
+      printf ("create_temp_file: %m\n");
+      return 1;
+    }
+
+  close (fd);
+
+  /* Tests for regular files.  */
+  puts ("Regular mode:");
+  fputs_func = (fputs_func_t) fputs;
+  fgets_func = (fgets_func_t) fgets;
+  data = char_data;
+  buf = char_buf;
+  data_len = strlen (char_data);
+  ret |= do_one_test (filename);
+
+  /* Tests for wide files.  */
+  puts ("Wide mode:");
+  if (setlocale (LC_ALL, "en_US.UTF-8") == NULL)
+    {
+      printf ("Cannot set en_US.UTF-8 locale.\n");
+      return 1;
+    }
+  fputs_func = (fputs_func_t) fputws;
+  fgets_func = (fgets_func_t) fgetws;
+  data = wide_data;
+  buf = wide_buf;
+  data_len = wcslen (wide_data);
+  ret |= do_one_test (filename);
+
+  return ret;
+}
diff --git a/libio/wfileops.c b/libio/wfileops.c
index 3199861b4d..f123add354 100644
--- a/libio/wfileops.c
+++ b/libio/wfileops.c
@@ -713,9 +713,16 @@ 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).  */
-	  offset -= fp->_IO_read_end - fp->_IO_write_ptr;
+	  /* We don't trust _IO_read_end to represent the current file offset
+	     when writing in append mode because the value would have to be
+	     shifted to the end of the file during a flush.  Use the write base
+	     instead, along with the new offset we got above when we did a seek
+	     to the end of the file.  */
+	  if (append_mode)
+	    offset += fp->_IO_write_ptr - fp->_IO_write_base;
+	  /* For all other modes, _IO_read_end represents the file offset.  */
+	  else
+	    offset += fp->_IO_write_ptr - fp->_IO_read_end;
 	}
     }