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, 384 insertions, 0 deletions

diff --git a/libio/tst-ftell-active-handler.c b/libio/tst-ftell-active-handler.c
new file mode 100644
index 0000000000..175e9042ae
--- /dev/null
+++ b/libio/tst-ftell-active-handler.c
@@ -0,0 +1,384 @@
+/* Verify that ftell returns the correct value at various points before and
+   after the handler on which it is called becomes active.
+   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 <fcntl.h>
+#include <locale.h>
+#include <wchar.h>
+
+static int do_test (void);
+
+#define TEST_FUNCTION do_test ()
+#include "../test-skeleton.c"
+
+#define get_handles_fdopen(filename, fd, fp, fd_mode, mode) \
+({									      \
+  int ret = 0;								      \
+  (fd) = open ((filename), (fd_mode), 0);				      \
+  if ((fd) == -1)							      \
+    {									      \
+      printf ("open failed: %m\n");					      \
+      ret = 1;								      \
+    }									      \
+  else									      \
+    {									      \
+      (fp) = fdopen ((fd), (mode));					      \
+      if ((fp) == NULL)							      \
+        {								      \
+          printf ("fdopen failed: %m\n");				      \
+          close (fd);							      \
+          ret = 1;							      \
+        }								      \
+    }									      \
+  ret;									      \
+})
+
+#define get_handles_fopen(filename, fd, fp, mode) \
+({									      \
+  int ret = 0;								      \
+  (fp) = fopen ((filename), (mode));					      \
+  if ((fp) == NULL)							      \
+    {									      \
+      printf ("fopen failed: %m\n");					      \
+      ret = 1;								      \
+    }									      \
+  else									      \
+    {									      \
+      (fd) = fileno (fp);						      \
+      if ((fd) == -1)							      \
+        {								      \
+	  printf ("fileno failed: %m\n");				      \
+	  ret = 1;							      \
+	}								      \
+    }									      \
+  ret;									      \
+})
+
+/* 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.  */
+static const void *data;
+static const char *char_data = "abcdef";
+static const wchar_t *wide_data = L"abcdef";
+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;
+
+/* Test that the value of ftell is not cached when the stream handle is not
+   active.  */
+static int
+do_ftell_test (const char *filename)
+{
+  int ret = 0;
+  struct test
+    {
+      const char *mode;
+      int fd_mode;
+      size_t old_off;
+      size_t new_off;
+    } test_modes[] = {
+	  /* In w, w+ and r+ modes, the file position should be at the
+	     beginning of the file.  After the write, the offset should be
+	     updated to data_len.  */
+	  {"w", O_WRONLY, 0, data_len},
+	  {"w+", O_RDWR, 0, data_len},
+	  {"r+", O_RDWR, 0, data_len},
+	  /* For 'a' and 'a+' modes, the initial file position should be the
+	     current end of file. After the write, the offset has data_len
+	     added to the old value.  */
+	  {"a", O_WRONLY, data_len, 2 * data_len},
+	  {"a+", O_RDWR, 2 * data_len, 3 * data_len},
+    };
+  for (int j = 0; j < 2; j++)
+    {
+      for (int i = 0; i < sizeof (test_modes) / sizeof (struct test); i++)
+	{
+	  FILE *fp;
+	  int fd;
+	  printf ("\tftell: %s (file, \"%s\"): ", j == 0 ? "fdopen" : "fopen",
+		  test_modes[i].mode);
+
+	  if (j == 0)
+	    ret = get_handles_fdopen (filename, fd, fp, test_modes[i].fd_mode,
+				      test_modes[i].mode);
+	  else
+	    ret = get_handles_fopen (filename, fd, fp, test_modes[i].mode);
+
+	  if (ret != 0)
+	    return ret;
+
+	  long off = ftell (fp);
+	  if (off != test_modes[i].old_off)
+	    {
+	      printf ("Incorrect old offset.  Expected %zu but got %ld, ",
+		      test_modes[i].old_off, off);
+	      ret |= 1;
+	    }
+	  else
+	    printf ("old offset = %ld, ", off);
+
+	  /* The effect of this write on the offset should be seen in the ftell
+	     call that follows it.  */
+	  int ret = write (fd, data, data_len);
+	  off = ftell (fp);
+
+	  if (off != test_modes[i].new_off)
+	    {
+	      printf ("Incorrect new offset.  Expected %zu but got %ld\n",
+		      test_modes[i].old_off, off);
+	      ret |= 1;
+	    }
+	  else
+	    printf ("new offset = %ld\n", off);
+
+	  fclose (fp);
+	}
+    }
+
+  return ret;
+}
+
+/* This test opens the file for writing, moves the file offset of the
+   underlying file, writes out data and then checks if ftell trips on it.  */
+static int
+do_write_test (const char *filename)
+{
+  FILE *fp = NULL;
+  int fd;
+  int ret = 0;
+  struct test
+    {
+      const char *mode;
+      int fd_mode;
+    } test_modes[] = {
+	  {"w", O_WRONLY},
+	  {"w+", O_RDWR},
+	  {"r+", O_RDWR}
+    };
+
+  for (int j = 0; j < 2; j++)
+    {
+      for (int i = 0; i < sizeof (test_modes) / sizeof (struct test); i++)
+	{
+	  printf ("\twrite: %s (file, \"%s\"): ", j == 0 ? "fopen" : "fdopen",
+		  test_modes[i].mode);
+
+	  if (j == 0)
+	    ret = get_handles_fopen (filename, fd, fp, test_modes[i].mode);
+	  else
+	    ret = get_handles_fdopen (filename, fd, fp, test_modes[i].fd_mode,
+				      test_modes[i].mode);
+
+	  if (ret != 0)
+	    return ret;
+
+	  /* Move offset to just before the end of the file.  */
+	  off_t ret = lseek (fd, file_len - 1, SEEK_SET);
+	  if (ret == -1)
+	    {
+	      printf ("lseek failed: %m\n");
+	      ret |= 1;
+	    }
+
+	  /* Write some data.  */
+	  size_t written = fputs_func (data, fp);
+
+	  if (written == EOF)
+	    {
+	      printf ("fputs[1] failed to write data\n");
+	      ret |= 1;
+	    }
+
+	  /* Verify that the offset points to the end of the file.  The length
+	     of the file would be the original length + the length of data
+	     written to it - the amount by which we moved the offset using
+	     lseek.  */
+	  long offset = ftell (fp);
+	  file_len = file_len - 1 + data_len;
+
+	  if (offset != file_len)
+	    {
+	      printf ("Incorrect offset.  Expected %zu, but got %ld\n",
+		      file_len, offset);
+
+	      ret |= 1;
+	    }
+
+	  printf ("offset = %ld\n", offset);
+	  fclose (fp);
+        }
+    }
+
+  return ret;
+}
+
+/* This test opens a file in append mode, writes some data, and then verifies
+   that ftell does not trip over it.  */
+static int
+do_append_test (const char *filename)
+{
+  FILE *fp = NULL;
+  int ret = 0;
+  int fd;
+
+  struct test
+    {
+      const char *mode;
+      int fd_mode;
+    } test_modes[] = {
+	  {"a", O_WRONLY},
+	  {"a+", O_RDWR}
+    };
+
+  for (int j = 0; j < 2; j++)
+    {
+      for (int i = 0; i < sizeof (test_modes) / sizeof (struct test); i++)
+	{
+	  printf ("\tappend: %s (file, \"%s\"): ", j == 0 ? "fopen" : "fdopen",
+		  test_modes[i].mode);
+
+	  if (j == 0)
+	    ret = get_handles_fopen (filename, fd, fp, test_modes[i].mode);
+	  else
+	    ret = get_handles_fdopen (filename, fd, fp, test_modes[i].fd_mode,
+				      test_modes[i].mode);
+
+	  if (ret != 0)
+	    return ret;
+
+	  /* Write some data.  */
+	  size_t written = fputs_func (data, fp);
+
+	  if (written == EOF)
+	    {
+	      printf ("fputs[1] failed to write all data\n");
+	      ret |= 1;
+	    }
+
+	  /* Verify that the offset points to the end of the file.  The file
+	     len is maintained by adding data_len each time to reflect the data
+	     written to it.  */
+	  long offset = ftell (fp);
+	  file_len += data_len;
+
+	  if (offset != file_len)
+	    {
+	      printf ("Incorrect offset.  Expected %zu, but got %ld\n",
+		      file_len, offset);
+
+	      ret |= 1;
+	    }
+
+	  printf ("offset = %ld\n", offset);
+	  fclose (fp);
+	}
+    }
+
+  return ret;
+}
+
+static int
+do_one_test (const char *filename)
+{
+  int ret = 0;
+
+  ret |= do_ftell_test (filename);
+  ret |= do_write_test (filename);
+  ret |= do_append_test (filename);
+
+  return ret;
+}
+
+/* Run a set of tests for ftell for regular files and wide mode files.  */
+static int
+do_test (void)
+{
+  int ret = 0;
+  FILE *fp = NULL;
+  char *filename;
+  size_t written;
+  int fd = create_temp_file ("tst-active-handler-tmp.", &filename);
+
+  if (fd == -1)
+    {
+      printf ("create_temp_file: %m\n");
+      return 1;
+    }
+
+  fp = fdopen (fd, "w");
+  if (fp == NULL)
+    {
+      printf ("fdopen[0]: %m\n");
+      close (fd);
+      return 1;
+    }
+
+  data = char_data;
+  data_len = strlen (char_data);
+  file_len = strlen (char_data);
+  written = fputs (data, fp);
+
+  if (written == EOF)
+    {
+      printf ("fputs[1] failed to write data\n");
+      ret = 1;
+    }
+
+  fclose (fp);
+  if (ret)
+    return ret;
+
+  /* Tests for regular files.  */
+  puts ("Regular mode:");
+  fputs_func = (fputs_func_t) fputs;
+  data = char_data;
+  data_len = strlen (char_data);
+  ret |= do_one_test (filename);
+
+  /* Truncate the file before repeating the tests in wide mode.  */
+  fp = fopen (filename, "w");
+  if (fp == NULL)
+    {
+      printf ("fopen failed %m\n");
+      return 1;
+    }
+  fclose (fp);
+
+  /* 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;
+  data = wide_data;
+  data_len = wcslen (wide_data);
+  ret |= do_one_test (filename);
+
+  return ret;
+}