[v4] libio: Add test case for fflush

diff --git a/libio/Makefile b/libio/Makefile
index 4370152964..1948811d96 100644
--- a/libio/Makefile
+++ b/libio/Makefile
@@ -100,6 +100,8 @@  tests = \
   tst-fclose-unopened \
   tst-fclose-unopened2 \
   tst-fdopen-seek-failure \
+  tst-fflush \
+  tst-fflush-NULL \
   tst-fgetc-after-eof \
   tst-fgetwc \
   tst-fgetws \
@@ -146,6 +148,9 @@  tests = \
   tst_wscanf \
   # tests
 
+# tst-fflush-NULL as XFAIL until read stream bug is fixed (BZ#32369)
+test-xfail-tst-fflush-NULL = yes
+
 $(objpfx)tst-popen-fork: $(shared-thread-library)
 
 tests-internal = tst-vtables tst-vtables-interposed
diff --git a/libio/tst-fflush-NULL.c b/libio/tst-fflush-NULL.c
new file mode 100644
index 0000000000..4c8fe7cb87
--- /dev/null
+++ b/libio/tst-fflush-NULL.c
@@ -0,0 +1,24 @@ 
+/* Test that fflush (FILE) and fflush (NULL) are semantically equivalent.
+   This is the `fflush (NULL)` part.
+
+   Copyright (C) 2024 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/>.  */
+
+#define FILE_FLUSH_TYPE 0
+#define S_FLUSH_TYPE "NULL"
+
+#include "tst-fflush-skeleton.c"
diff --git a/libio/tst-fflush-skeleton.c b/libio/tst-fflush-skeleton.c
new file mode 100644
index 0000000000..ac61660c29
--- /dev/null
+++ b/libio/tst-fflush-skeleton.c
@@ -0,0 +1,166 @@ 
+/* Test that fflush (FILE) and fflush (NULL) are semantically equivalent.
+
+   Copyright (C) 2024 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/>.  */
+
+/* A success on this test doesn't imply the effectiveness of fflush as
+   we can't ensure that the file wasn't already in the expected state
+   before the call of the function. It only ensures that, if the test
+   fails, fflush is broken.  */
+
+#include <fcntl.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include <sys/stat.h>
+#include <sys/mman.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>
+
+#define CONTENT_SZ_MAX 32
+#define TEST_FILE_COUNT 10
+
+struct file_tracking
+{
+  FILE *file;
+  char *name;
+  int fd;
+  char *mfile;
+} files[TEST_FILE_COUNT];
+
+static void
+file_init (int file)
+{
+  int fd = -1;
+
+  files[file] = (struct file_tracking) { .fd = -1, };
+
+  if (file >= TEST_FILE_COUNT)
+    return;
+
+  xclose (create_temp_file ("tst-fflush", &files[file].name));
+
+  fd = xopen (files[file].name, O_RDONLY, 0);
+  files[file].mfile = xmmap (NULL, CONTENT_SZ_MAX, PROT_READ, MAP_SHARED, fd);
+  xclose (fd);
+}
+
+static void
+file_cleanup (int file)
+{
+  free (files[file].name);
+  xmunmap (files[file].mfile, CONTENT_SZ_MAX);
+  files[file] = (struct file_tracking) { .fd = -1, };
+}
+
+static void
+file_changed (int to_check, const char *mode)
+{
+  struct stat stats = { };
+  char expected[CONTENT_SZ_MAX] = { };
+
+  verbose_printf ("Check that %s (%d) exactly contains the data we put in\n",
+		  files[to_check].name, to_check);
+
+  /* File should contain "N:M" where both N and M are one digit exactly.  */
+  snprintf (expected, sizeof (expected), "%d:%d", FILE_FLUSH_TYPE, to_check);
+  TEST_COMPARE_BLOB (files[to_check].mfile, sizeof (expected),
+		     expected, sizeof (expected));
+
+  TEST_VERIFY (fstat (files[to_check].fd, &stats) >= 0);
+  TEST_VERIFY (stats.st_size == 3);
+  /* In read mode we expect to be at position 1, in write mode at position 3 */
+  TEST_COMPARE (lseek (files[to_check].fd, 0, SEEK_CUR),
+		mode[0] == 'r' ? 1 : 3);
+
+  if (support_record_failure_is_failed ())
+    FAIL_EXIT1 ("exiting due to previous failure");
+
+  /* Not reached if the data doesn't match.  */
+}
+
+static void
+file_flush (const char *mode)
+{
+  for (int i = 0; i < TEST_FILE_COUNT; i++)
+    {
+      files[i].file = xfopen (files[i].name, mode);
+      files[i].fd = fileno (files[i].file);
+    }
+
+  /* Print a unique identifier in each file, that is not too long nor contain
+     new line to not trigger _IO_OVERFLOW/_IO_SYNC.  */
+  for (int i = 0; i < TEST_FILE_COUNT; i++)
+    {
+      if (mode[0] == 'r')
+	{
+	  fgetc (files[i].file);
+	}
+      else
+	{
+	  fprintf (files[i].file, "%d:%d", FILE_FLUSH_TYPE, i);
+	}
+    }
+
+  if (!FILE_FLUSH_TYPE)
+    {
+      TEST_VERIFY (fflush (NULL) == 0);
+    }
+  else
+    {
+      for (int i = 0; i < TEST_FILE_COUNT; i++)
+	TEST_VERIFY (fflush (files[i].file) == 0);
+    }
+
+  for (int i = 0; i < TEST_FILE_COUNT; i++)
+    {
+      verbose_printf ("Check that file %s has been modified after fflush\n",
+		      files[i].name);
+      file_changed (i, mode);
+    }
+
+  for (int i = 0; i < TEST_FILE_COUNT; i++)
+    xfclose (files[i].file);
+}
+
+static int
+do_test (void)
+{
+  for (int i = 0; i < TEST_FILE_COUNT; i++)
+    file_init (i);
+
+  verbose_printf ("Checking fflush(" S_FLUSH_TYPE "), WRITE mode\n");
+  file_flush ("w");
+
+  verbose_printf ("Checking fflush(" S_FLUSH_TYPE "), READWRITE mode\n");
+  file_flush ("r+");
+
+  for (int i = 0; i < TEST_FILE_COUNT; i++)
+    file_cleanup (i);
+
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/libio/tst-fflush.c b/libio/tst-fflush.c
new file mode 100644
index 0000000000..d3a4b78f43
--- /dev/null
+++ b/libio/tst-fflush.c
@@ -0,0 +1,24 @@ 
+/* Test that fflush (FILE) and fflush (NULL) are semantically equivalent.
+   This is the `fflush (FILE)` part.
+
+   Copyright (C) 2024 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/>.  */
+
+#define FILE_FLUSH_TYPE 1
+#define S_FLUSH_TYPE "FILE"
+
+#include "tst-fflush-skeleton.c"