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

1 files changed, 169 insertions, 0 deletions

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;
+}