[1/5] Add internal <file_change_detection.h> header file

Message ID 691b5b8d18c29b5c31de804b8393a1b9718e1a1d.1579631655.git.fweimer@redhat.com
State New
Headers show
Series
  • Race condition in /etc/resolv.conf reloading (bug 25420)
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Commit Message

Florian Weimer Jan. 21, 2020, 6:41 p.m.
The code started out with bits form resolv/resolv_conf.c, but it
was enhanced to deal with directories and FIFOs in a more predictable
manner.  A test case is included as well.

This will be used to implement the /etc/resolv.conf change detection.

This currently lives in a header file only.  Once there are multiple
users, the implementations should be moved into C files.
---
 include/file_change_detection.h | 140 ++++++++++++++++++++++
 io/Makefile                     |   2 +-
 io/tst-file_change_detection.c  | 206 ++++++++++++++++++++++++++++++++
 3 files changed, 347 insertions(+), 1 deletion(-)
 create mode 100644 include/file_change_detection.h
 create mode 100644 io/tst-file_change_detection.c

-- 
2.24.1

Comments

Adhemerval Zanella Feb. 10, 2020, 7:47 p.m. | #1
On 21/01/2020 15:41, Florian Weimer wrote:
> The code started out with bits form resolv/resolv_conf.c, but it

> was enhanced to deal with directories and FIFOs in a more predictable

> manner.  A test case is included as well.

> 

> This will be used to implement the /etc/resolv.conf change detection.

> 

> This currently lives in a header file only.  Once there are multiple

> users, the implementations should be moved into C files.


LGTM, with some nits below.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>


> ---

>  include/file_change_detection.h | 140 ++++++++++++++++++++++

>  io/Makefile                     |   2 +-

>  io/tst-file_change_detection.c  | 206 ++++++++++++++++++++++++++++++++

>  3 files changed, 347 insertions(+), 1 deletion(-)

>  create mode 100644 include/file_change_detection.h

>  create mode 100644 io/tst-file_change_detection.c

> 

> diff --git a/include/file_change_detection.h b/include/file_change_detection.h

> new file mode 100644

> index 0000000000..aaed0a9b6d

> --- /dev/null

> +++ b/include/file_change_detection.h

> @@ -0,0 +1,140 @@

> +/* Detecting file changes using modification times.

> +   Copyright (C) 2017-2020 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

> +   <https://www.gnu.org/licenses/>.  */

> +

> +#include <errno.h>

> +#include <stdbool.h>

> +#include <stddef.h>

> +#include <stdio.h>

> +#include <sys/stat.h>

> +#include <sys/types.h>

> +

> +/* Items for identifying a particular file version.  Excerpt from

> +   struct stat64.  */

> +struct file_change_detection

> +{

> +  /* Special values: 0 if file does not exist.  -1 to force mismatch

> +     with the next comparison.  */

> +  off64_t size;

> +

> +  ino64_t ino;

> +  struct timespec mtime;

> +  struct timespec ctime;

> +};

> +


Ok.

> +/* Returns true if *LEFT and *RIGHT describe the same version of the

> +   same file.  */

> +static bool __attribute__ ((unused))

> +file_is_unchanged (const struct file_change_detection *left,

> +                   const struct file_change_detection *right)

> +{

> +  if (left->size < 0 || right->size < 0)

> +    /* Negative sizes are used as markers and never match.  */

> +    return false;

> +  else if (left->size == 0 && right->size == 0)

> +    /* Both files are empty or do not exist, so they have the same

> +       content, no matter what the other fields indicate.  */

> +    return true;

> +  else

> +    return left->size == right->size

> +      && left->ino == right->ino

> +      && left->mtime.tv_sec == right->mtime.tv_sec

> +      && left->mtime.tv_nsec == right->mtime.tv_nsec

> +      && left->ctime.tv_sec == right->ctime.tv_sec

> +      && left->ctime.tv_nsec == right->ctime.tv_nsec;

> +}


Ok.

> +

> +/* Extract file change information to *FILE from the stat buffer

> +   *ST.  */

> +static void __attribute__ ((unused))

> +file_change_detection_for_stat (struct file_change_detection *file,

> +                                const struct stat64 *st)

> +{

> +  if (S_ISDIR (st->st_mode))

> +    /* Treat as empty file.  */

> +    file->size = 0;

> +  else if (!S_ISREG (st->st_mode))

> +    /* Non-regular files cannot be cached.  */

> +    file->size = -1;

> +  else

> +    {

> +      file->size = st->st_size;

> +      file->ino = st->st_ino;

> +      file->mtime = st->st_mtim;

> +      file->ctime = st->st_ctim;

> +    }

> +}


Ok.

> +

> +/* Writes file change information for PATH to *FILE.  Returns true on

> +   success.  For benign errors, *FILE is cleared, and true is

> +   returned.  For errors indicating resource outages and the like,

> +   false is returned.  */

> +static bool __attribute__ ((unused))

> +file_change_detection_for_path (struct file_change_detection *file,

> +                                const char *path)

> +{

> +  struct stat64 st;

> +  if (stat64 (path, &st) != 0)

> +    switch (errno)

> +      {

> +      case EACCES:

> +      case EISDIR:

> +      case ELOOP:

> +      case ENOENT:

> +      case ENOTDIR:

> +      case EPERM:

> +        /* Ignore errors due to file system contents.  Instead, treat

> +           the file as empty.  */

> +        file->size = 0;

> +        return true;

> +      default:

> +        /* Other errors are fatal.  */

> +        return false;

> +      }

> +  else /* stat64 was successfull.  */

> +    {

> +      file_change_detection_for_stat (file, &st);

> +      return true;

> +    }

> +}


Ok.

> +

> +/* Writes file change information for the stream FP to *FILE.  Returns

> +   ture on success, false on failure.  If FP is NULL, treat the file

> +   as non-existing.  */

> +static bool __attribute__ ((unused))

> +file_change_detection_for_fp (struct file_change_detection *file,

> +                              FILE *fp)

> +{

> +  if (fp == NULL)

> +    {

> +      /* The file does not exist.  */

> +      file->size = 0;

> +      return true;

> +    }

> +  else

> +    {

> +      struct stat64 st;

> +      if (fstat64 (__fileno (fp), &st) != 0)

> +        /* If we already have a file descriptor, all errors are fatal.  */

> +        return false;

> +      else

> +        {

> +          file_change_detection_for_stat (file, &st);

> +          return true;

> +        }

> +    }

> +}


Ok.

> diff --git a/io/Makefile b/io/Makefile

> index d9a1da4566..437a7732f0 100644

> --- a/io/Makefile

> +++ b/io/Makefile

> @@ -74,7 +74,7 @@ tests		:= test-utime test-stat test-stat2 test-lfs tst-getcwd \

>  		   tst-posix_fallocate tst-posix_fallocate64 \

>  		   tst-fts tst-fts-lfs tst-open-tmpfile \

>  		   tst-copy_file_range tst-getcwd-abspath tst-lockf \

> -		   tst-ftw-lnk

> +		   tst-ftw-lnk tst-file_change_detection

>  

>  # Likewise for statx, but we do not need static linking here.

>  tests-internal += tst-statx


Ok.

> diff --git a/io/tst-file_change_detection.c b/io/tst-file_change_detection.c

> new file mode 100644

> index 0000000000..035dd39c4d

> --- /dev/null

> +++ b/io/tst-file_change_detection.c

> @@ -0,0 +1,206 @@

> +/* Test for <file_change_detection.c>.

> +   Copyright (C) 2020 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

> +   <https://www.gnu.org/licenses/>.  */

> +

> +/* The header uses the internal __fileno symbol, which is not

> +   available outside of libc (even to internal tests).  */

> +#define __fileno(fp) fileno (fp)


Ok.

> +

> +#include <file_change_detection.h>

> +

> +#include <array_length.h>

> +#include <stdlib.h>

> +#include <support/check.h>

> +#include <support/support.h>

> +#include <support/temp_file.h>

> +#include <support/test-driver.h>

> +#include <support/xstdio.h>

> +#include <support/xunistd.h>

> +#include <unistd.h>

> +

> +static void

> +all_same (struct file_change_detection *array, size_t length)

> +{

> +  for (size_t i = 0; i < length; ++i)

> +    for (size_t j = 0; j < length; ++j)

> +      {

> +        if (test_verbose > 0)

> +          printf ("info: comparing %zu and %zu\n", i, j);

> +        TEST_VERIFY (file_is_unchanged (array + i, array + j));

> +      }

> +}

> +


Wouldn't it the following be slight better?

  for (size_t i = 0; i < length - 1; ++i)
    for (size_t j = i + 1 < length, j++)
       
Or do you really to check all possible permutations?

> +static void

> +all_different (struct file_change_detection *array, size_t length)

> +{

> +  for (size_t i = 0; i < length; ++i)

> +    for (size_t j = 0; j < length; ++j)

> +      {

> +        if (i == j)

> +          continue;

> +        if (test_verbose > 0)

> +          printf ("info: comparing %zu and %zu\n", i, j);

> +        TEST_VERIFY (!file_is_unchanged (array + i, array + j));

> +      }

> +}


Same as before.

> +

> +static int

> +do_test (void)

> +{

> +  /* Use a temporary directory with various paths.  */

> +  char *tempdir = support_create_temp_directory ("tst-file_change_detection-");

> +

> +  char *path_dangling = xasprintf ("%s/dangling", tempdir);

> +  char *path_does_not_exist = xasprintf ("%s/does-not-exist", tempdir);

> +  char *path_empty1 = xasprintf ("%s/empty1", tempdir);

> +  char *path_empty2 = xasprintf ("%s/empty2", tempdir);

> +  char *path_fifo = xasprintf ("%s/fifo", tempdir);

> +  char *path_file1 = xasprintf ("%s/file1", tempdir);

> +  char *path_file2 = xasprintf ("%s/file2", tempdir);

> +  char *path_loop = xasprintf ("%s/loop", tempdir);

> +  char *path_to_empty1 = xasprintf ("%s/to-empty1", tempdir);

> +  char *path_to_file1 = xasprintf ("%s/to-file1", tempdir);

> +

> +  add_temp_file (path_dangling);

> +  add_temp_file (path_empty1);

> +  add_temp_file (path_empty2);

> +  add_temp_file (path_fifo);

> +  add_temp_file (path_file1);

> +  add_temp_file (path_file2);

> +  add_temp_file (path_loop);

> +  add_temp_file (path_to_empty1);

> +  add_temp_file (path_to_file1);

> +

> +  xsymlink ("target-does-not-exist", path_dangling);

> +  support_write_file_string (path_empty1, "");

> +  support_write_file_string (path_empty2, "");

> +  TEST_COMPARE (mknod (path_fifo, 0777 | S_IFIFO, 0), 0);

> +  support_write_file_string (path_file1, "line\n");

> +  support_write_file_string (path_file2, "line\n");

> +  xsymlink ("loop", path_loop);

> +  xsymlink ("empty1", path_to_empty1);

> +  xsymlink ("file1", path_to_file1);


Ok.

> +

> +  FILE *fp_file1 = xfopen (path_file1, "r");

> +  FILE *fp_file2 = xfopen (path_file2, "r");

> +  FILE *fp_empty1 = xfopen (path_empty1, "r");

> +  FILE *fp_empty2 = xfopen (path_empty2, "r");

> +

> +  /* Test for the same (empty) files.  */

> +  {

> +    struct file_change_detection fcd[10];

> +    int i = 0;

> +    /* Two empty files always have the same contents.  */

> +    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_empty1));

> +    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_empty2));

> +    /* So does a missing file (which is treated as empty).  */

> +    TEST_VERIFY (file_change_detection_for_path (&fcd[i++],

> +                                                 path_does_not_exist));

> +    /* And a symbolic link loop.  */

> +    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_loop));

> +    /* And a dangling symbolic link.  */

> +    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_dangling));

> +    /* And a directory.  */

> +    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], tempdir));

> +    /* And a symbolic link to an empty file.  */

> +    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_to_empty1));

> +    /* Likewise for access the file via a FILE *.  */

> +    TEST_VERIFY (file_change_detection_for_fp (&fcd[i++], fp_empty1));

> +    TEST_VERIFY (file_change_detection_for_fp (&fcd[i++], fp_empty2));

> +    /* And a NULL FILE * (missing file).  */

> +    TEST_VERIFY (file_change_detection_for_fp (&fcd[i++], NULL));

> +    TEST_COMPARE (i, array_length (fcd));

> +

> +    all_same (fcd, array_length (fcd));

> +  }


Ok.

> +

> +  /* Symbolic links are resolved.  */

> +  {

> +    struct file_change_detection fcd[3];

> +    int i = 0;

> +    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_file1));

> +    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_to_file1));

> +    TEST_VERIFY (file_change_detection_for_fp (&fcd[i++], fp_file1));

> +    TEST_COMPARE (i, array_length (fcd));

> +    all_same (fcd, array_length (fcd));

> +  }


Ok.

> +

> +  /* Test for different files.  */

> +  {

> +    struct file_change_detection fcd[5];

> +    int i = 0;

> +    /* The other files are not empty.  */

> +    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_empty1));

> +    /* These two files have the same contents, but have different file

> +       identity.  */

> +    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_file1));

> +    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_file2));

> +    /* FIFOs are always different, even with themselves.  */

> +    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_fifo));

> +    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_fifo));

> +    TEST_COMPARE (i, array_length (fcd));

> +    all_different (fcd, array_length (fcd));

> +

> +    /* Replacing the file with its symbolic link does not make a

> +       difference.  */

> +    TEST_VERIFY (file_change_detection_for_path (&fcd[1], path_to_file1));

> +    all_different (fcd, array_length (fcd));

> +  }


Ok.

> +

> +  /* Wait for a file change.  Depending on file system time stamp

> +     resolution, this subtest blocks for a while.  */

> +  for (int use_stdio = 0; use_stdio < 2; ++use_stdio)

> +    {

> +      struct file_change_detection initial;

> +      TEST_VERIFY (file_change_detection_for_path (&initial, path_file1));

> +      while (true)

> +        {

> +          support_write_file_string (path_file1, "line\n");

> +          struct file_change_detection current;

> +          if (use_stdio)

> +            TEST_VERIFY (file_change_detection_for_fp (&current, fp_file1));

> +          else

> +            TEST_VERIFY (file_change_detection_for_path (&current, path_file1));

> +          if (!file_is_unchanged (&initial, &current))

> +            break;

> +          /* Wait for a bit to reduce system load.  */

> +          usleep (100 * 1000);

> +        }

> +    }


Ok, although the usleep seems excessive large (the testing will most likely
timeout prior usleep return).

> +

> +  fclose (fp_empty1);

> +  fclose (fp_empty2);

> +  fclose (fp_file1);

> +  fclose (fp_file2);

> +

> +  free (path_dangling);

> +  free (path_does_not_exist);

> +  free (path_empty1);

> +  free (path_empty2);

> +  free (path_fifo);

> +  free (path_file1);

> +  free (path_file2);

> +  free (path_loop);

> +  free (path_to_empty1);

> +  free (path_to_file1);

> +

> +  free (tempdir);

> +

> +  return 0;

> +}

> +

> +#include <support/test-driver.c>

> 


Ok.
Florian Weimer Feb. 10, 2020, 7:57 p.m. | #2
* Adhemerval Zanella:

> On 21/01/2020 15:41, Florian Weimer wrote:

>> The code started out with bits form resolv/resolv_conf.c, but it

>> was enhanced to deal with directories and FIFOs in a more predictable

>> manner.  A test case is included as well.

>> 

>> This will be used to implement the /etc/resolv.conf change detection.

>> 

>> This currently lives in a header file only.  Once there are multiple

>> users, the implementations should be moved into C files.

>

> LGTM, with some nits below.

>

> Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>


Thanks.
>> +static void

>> +all_same (struct file_change_detection *array, size_t length)

>> +{

>> +  for (size_t i = 0; i < length; ++i)

>> +    for (size_t j = 0; j < length; ++j)

>> +      {

>> +        if (test_verbose > 0)

>> +          printf ("info: comparing %zu and %zu\n", i, j);

>> +        TEST_VERIFY (file_is_unchanged (array + i, array + j));

>> +      }

>> +}

>> +

>

> Wouldn't it the following be slight better?

>

>   for (size_t i = 0; i < length - 1; ++i)

>     for (size_t j = i + 1 < length, j++)

>        

> Or do you really to check all possible permutations?


Iterating over all pairs checks that the operation is commutative.

>> +  /* Wait for a file change.  Depending on file system time stamp

>> +     resolution, this subtest blocks for a while.  */

>> +  for (int use_stdio = 0; use_stdio < 2; ++use_stdio)

>> +    {

>> +      struct file_change_detection initial;

>> +      TEST_VERIFY (file_change_detection_for_path (&initial, path_file1));

>> +      while (true)

>> +        {

>> +          support_write_file_string (path_file1, "line\n");

>> +          struct file_change_detection current;

>> +          if (use_stdio)

>> +            TEST_VERIFY (file_change_detection_for_fp (&current, fp_file1));

>> +          else

>> +            TEST_VERIFY (file_change_detection_for_path (&current, path_file1));

>> +          if (!file_is_unchanged (&initial, &current))

>> +            break;

>> +          /* Wait for a bit to reduce system load.  */

>> +          usleep (100 * 1000);

>> +        }

>> +    }

>

> Ok, although the usleep seems excessive large (the testing will most likely

> timeout prior usleep return).


Hmm, I thought that this would wait 100 milliseconds?  So the loop
should exit in a second or two with low-resolution timestamps in the
file system.
Adhemerval Zanella Feb. 10, 2020, 8:57 p.m. | #3
On 10/02/2020 16:57, Florian Weimer wrote:
> 

>>> +  /* Wait for a file change.  Depending on file system time stamp

>>> +     resolution, this subtest blocks for a while.  */

>>> +  for (int use_stdio = 0; use_stdio < 2; ++use_stdio)

>>> +    {

>>> +      struct file_change_detection initial;

>>> +      TEST_VERIFY (file_change_detection_for_path (&initial, path_file1));

>>> +      while (true)

>>> +        {

>>> +          support_write_file_string (path_file1, "line\n");

>>> +          struct file_change_detection current;

>>> +          if (use_stdio)

>>> +            TEST_VERIFY (file_change_detection_for_fp (&current, fp_file1));

>>> +          else

>>> +            TEST_VERIFY (file_change_detection_for_path (&current, path_file1));

>>> +          if (!file_is_unchanged (&initial, &current))

>>> +            break;

>>> +          /* Wait for a bit to reduce system load.  */

>>> +          usleep (100 * 1000);

>>> +        }

>>> +    }

>>

>> Ok, although the usleep seems excessive large (the testing will most likely

>> timeout prior usleep return).

> 

> Hmm, I thought that this would wait 100 milliseconds?  So the loop

> should exit in a second or two with low-resolution timestamps in the

> file system.

> 


Right, it should be ok then.

Patch

diff --git a/include/file_change_detection.h b/include/file_change_detection.h
new file mode 100644
index 0000000000..aaed0a9b6d
--- /dev/null
+++ b/include/file_change_detection.h
@@ -0,0 +1,140 @@ 
+/* Detecting file changes using modification times.
+   Copyright (C) 2017-2020 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
+   <https://www.gnu.org/licenses/>.  */
+
+#include <errno.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+
+/* Items for identifying a particular file version.  Excerpt from
+   struct stat64.  */
+struct file_change_detection
+{
+  /* Special values: 0 if file does not exist.  -1 to force mismatch
+     with the next comparison.  */
+  off64_t size;
+
+  ino64_t ino;
+  struct timespec mtime;
+  struct timespec ctime;
+};
+
+/* Returns true if *LEFT and *RIGHT describe the same version of the
+   same file.  */
+static bool __attribute__ ((unused))
+file_is_unchanged (const struct file_change_detection *left,
+                   const struct file_change_detection *right)
+{
+  if (left->size < 0 || right->size < 0)
+    /* Negative sizes are used as markers and never match.  */
+    return false;
+  else if (left->size == 0 && right->size == 0)
+    /* Both files are empty or do not exist, so they have the same
+       content, no matter what the other fields indicate.  */
+    return true;
+  else
+    return left->size == right->size
+      && left->ino == right->ino
+      && left->mtime.tv_sec == right->mtime.tv_sec
+      && left->mtime.tv_nsec == right->mtime.tv_nsec
+      && left->ctime.tv_sec == right->ctime.tv_sec
+      && left->ctime.tv_nsec == right->ctime.tv_nsec;
+}
+
+/* Extract file change information to *FILE from the stat buffer
+   *ST.  */
+static void __attribute__ ((unused))
+file_change_detection_for_stat (struct file_change_detection *file,
+                                const struct stat64 *st)
+{
+  if (S_ISDIR (st->st_mode))
+    /* Treat as empty file.  */
+    file->size = 0;
+  else if (!S_ISREG (st->st_mode))
+    /* Non-regular files cannot be cached.  */
+    file->size = -1;
+  else
+    {
+      file->size = st->st_size;
+      file->ino = st->st_ino;
+      file->mtime = st->st_mtim;
+      file->ctime = st->st_ctim;
+    }
+}
+
+/* Writes file change information for PATH to *FILE.  Returns true on
+   success.  For benign errors, *FILE is cleared, and true is
+   returned.  For errors indicating resource outages and the like,
+   false is returned.  */
+static bool __attribute__ ((unused))
+file_change_detection_for_path (struct file_change_detection *file,
+                                const char *path)
+{
+  struct stat64 st;
+  if (stat64 (path, &st) != 0)
+    switch (errno)
+      {
+      case EACCES:
+      case EISDIR:
+      case ELOOP:
+      case ENOENT:
+      case ENOTDIR:
+      case EPERM:
+        /* Ignore errors due to file system contents.  Instead, treat
+           the file as empty.  */
+        file->size = 0;
+        return true;
+      default:
+        /* Other errors are fatal.  */
+        return false;
+      }
+  else /* stat64 was successfull.  */
+    {
+      file_change_detection_for_stat (file, &st);
+      return true;
+    }
+}
+
+/* Writes file change information for the stream FP to *FILE.  Returns
+   ture on success, false on failure.  If FP is NULL, treat the file
+   as non-existing.  */
+static bool __attribute__ ((unused))
+file_change_detection_for_fp (struct file_change_detection *file,
+                              FILE *fp)
+{
+  if (fp == NULL)
+    {
+      /* The file does not exist.  */
+      file->size = 0;
+      return true;
+    }
+  else
+    {
+      struct stat64 st;
+      if (fstat64 (__fileno (fp), &st) != 0)
+        /* If we already have a file descriptor, all errors are fatal.  */
+        return false;
+      else
+        {
+          file_change_detection_for_stat (file, &st);
+          return true;
+        }
+    }
+}
diff --git a/io/Makefile b/io/Makefile
index d9a1da4566..437a7732f0 100644
--- a/io/Makefile
+++ b/io/Makefile
@@ -74,7 +74,7 @@  tests		:= test-utime test-stat test-stat2 test-lfs tst-getcwd \
 		   tst-posix_fallocate tst-posix_fallocate64 \
 		   tst-fts tst-fts-lfs tst-open-tmpfile \
 		   tst-copy_file_range tst-getcwd-abspath tst-lockf \
-		   tst-ftw-lnk
+		   tst-ftw-lnk tst-file_change_detection
 
 # Likewise for statx, but we do not need static linking here.
 tests-internal += tst-statx
diff --git a/io/tst-file_change_detection.c b/io/tst-file_change_detection.c
new file mode 100644
index 0000000000..035dd39c4d
--- /dev/null
+++ b/io/tst-file_change_detection.c
@@ -0,0 +1,206 @@ 
+/* Test for <file_change_detection.c>.
+   Copyright (C) 2020 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
+   <https://www.gnu.org/licenses/>.  */
+
+/* The header uses the internal __fileno symbol, which is not
+   available outside of libc (even to internal tests).  */
+#define __fileno(fp) fileno (fp)
+
+#include <file_change_detection.h>
+
+#include <array_length.h>
+#include <stdlib.h>
+#include <support/check.h>
+#include <support/support.h>
+#include <support/temp_file.h>
+#include <support/test-driver.h>
+#include <support/xstdio.h>
+#include <support/xunistd.h>
+#include <unistd.h>
+
+static void
+all_same (struct file_change_detection *array, size_t length)
+{
+  for (size_t i = 0; i < length; ++i)
+    for (size_t j = 0; j < length; ++j)
+      {
+        if (test_verbose > 0)
+          printf ("info: comparing %zu and %zu\n", i, j);
+        TEST_VERIFY (file_is_unchanged (array + i, array + j));
+      }
+}
+
+static void
+all_different (struct file_change_detection *array, size_t length)
+{
+  for (size_t i = 0; i < length; ++i)
+    for (size_t j = 0; j < length; ++j)
+      {
+        if (i == j)
+          continue;
+        if (test_verbose > 0)
+          printf ("info: comparing %zu and %zu\n", i, j);
+        TEST_VERIFY (!file_is_unchanged (array + i, array + j));
+      }
+}
+
+static int
+do_test (void)
+{
+  /* Use a temporary directory with various paths.  */
+  char *tempdir = support_create_temp_directory ("tst-file_change_detection-");
+
+  char *path_dangling = xasprintf ("%s/dangling", tempdir);
+  char *path_does_not_exist = xasprintf ("%s/does-not-exist", tempdir);
+  char *path_empty1 = xasprintf ("%s/empty1", tempdir);
+  char *path_empty2 = xasprintf ("%s/empty2", tempdir);
+  char *path_fifo = xasprintf ("%s/fifo", tempdir);
+  char *path_file1 = xasprintf ("%s/file1", tempdir);
+  char *path_file2 = xasprintf ("%s/file2", tempdir);
+  char *path_loop = xasprintf ("%s/loop", tempdir);
+  char *path_to_empty1 = xasprintf ("%s/to-empty1", tempdir);
+  char *path_to_file1 = xasprintf ("%s/to-file1", tempdir);
+
+  add_temp_file (path_dangling);
+  add_temp_file (path_empty1);
+  add_temp_file (path_empty2);
+  add_temp_file (path_fifo);
+  add_temp_file (path_file1);
+  add_temp_file (path_file2);
+  add_temp_file (path_loop);
+  add_temp_file (path_to_empty1);
+  add_temp_file (path_to_file1);
+
+  xsymlink ("target-does-not-exist", path_dangling);
+  support_write_file_string (path_empty1, "");
+  support_write_file_string (path_empty2, "");
+  TEST_COMPARE (mknod (path_fifo, 0777 | S_IFIFO, 0), 0);
+  support_write_file_string (path_file1, "line\n");
+  support_write_file_string (path_file2, "line\n");
+  xsymlink ("loop", path_loop);
+  xsymlink ("empty1", path_to_empty1);
+  xsymlink ("file1", path_to_file1);
+
+  FILE *fp_file1 = xfopen (path_file1, "r");
+  FILE *fp_file2 = xfopen (path_file2, "r");
+  FILE *fp_empty1 = xfopen (path_empty1, "r");
+  FILE *fp_empty2 = xfopen (path_empty2, "r");
+
+  /* Test for the same (empty) files.  */
+  {
+    struct file_change_detection fcd[10];
+    int i = 0;
+    /* Two empty files always have the same contents.  */
+    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_empty1));
+    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_empty2));
+    /* So does a missing file (which is treated as empty).  */
+    TEST_VERIFY (file_change_detection_for_path (&fcd[i++],
+                                                 path_does_not_exist));
+    /* And a symbolic link loop.  */
+    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_loop));
+    /* And a dangling symbolic link.  */
+    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_dangling));
+    /* And a directory.  */
+    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], tempdir));
+    /* And a symbolic link to an empty file.  */
+    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_to_empty1));
+    /* Likewise for access the file via a FILE *.  */
+    TEST_VERIFY (file_change_detection_for_fp (&fcd[i++], fp_empty1));
+    TEST_VERIFY (file_change_detection_for_fp (&fcd[i++], fp_empty2));
+    /* And a NULL FILE * (missing file).  */
+    TEST_VERIFY (file_change_detection_for_fp (&fcd[i++], NULL));
+    TEST_COMPARE (i, array_length (fcd));
+
+    all_same (fcd, array_length (fcd));
+  }
+
+  /* Symbolic links are resolved.  */
+  {
+    struct file_change_detection fcd[3];
+    int i = 0;
+    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_file1));
+    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_to_file1));
+    TEST_VERIFY (file_change_detection_for_fp (&fcd[i++], fp_file1));
+    TEST_COMPARE (i, array_length (fcd));
+    all_same (fcd, array_length (fcd));
+  }
+
+  /* Test for different files.  */
+  {
+    struct file_change_detection fcd[5];
+    int i = 0;
+    /* The other files are not empty.  */
+    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_empty1));
+    /* These two files have the same contents, but have different file
+       identity.  */
+    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_file1));
+    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_file2));
+    /* FIFOs are always different, even with themselves.  */
+    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_fifo));
+    TEST_VERIFY (file_change_detection_for_path (&fcd[i++], path_fifo));
+    TEST_COMPARE (i, array_length (fcd));
+    all_different (fcd, array_length (fcd));
+
+    /* Replacing the file with its symbolic link does not make a
+       difference.  */
+    TEST_VERIFY (file_change_detection_for_path (&fcd[1], path_to_file1));
+    all_different (fcd, array_length (fcd));
+  }
+
+  /* Wait for a file change.  Depending on file system time stamp
+     resolution, this subtest blocks for a while.  */
+  for (int use_stdio = 0; use_stdio < 2; ++use_stdio)
+    {
+      struct file_change_detection initial;
+      TEST_VERIFY (file_change_detection_for_path (&initial, path_file1));
+      while (true)
+        {
+          support_write_file_string (path_file1, "line\n");
+          struct file_change_detection current;
+          if (use_stdio)
+            TEST_VERIFY (file_change_detection_for_fp (&current, fp_file1));
+          else
+            TEST_VERIFY (file_change_detection_for_path (&current, path_file1));
+          if (!file_is_unchanged (&initial, &current))
+            break;
+          /* Wait for a bit to reduce system load.  */
+          usleep (100 * 1000);
+        }
+    }
+
+  fclose (fp_empty1);
+  fclose (fp_empty2);
+  fclose (fp_file1);
+  fclose (fp_file2);
+
+  free (path_dangling);
+  free (path_does_not_exist);
+  free (path_empty1);
+  free (path_empty2);
+  free (path_fifo);
+  free (path_file1);
+  free (path_file2);
+  free (path_loop);
+  free (path_to_empty1);
+  free (path_to_file1);
+
+  free (tempdir);
+
+  return 0;
+}
+
+#include <support/test-driver.c>