tst-fopen-threaded: Only check EOF for failing read

The fread race checker looks for EOF in every thread, which is incorrect
since threads calling fread successfully could lag behind and read the
EOF condition, resulting in multiple threads thinking that they
encountered an EOF.

Only look for EOF condition if fread fails to read a char.  Also drop
the clearerr() since it could mask the failure of another reader, thus
hiding a test failure.

Finally, also check for error in the stream for completeness.

Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed-by: Florian Weimer <fweimer@redhat.com>

sysdeps/pthread/tst-fopen-threaded.c

@@ -64,19 +64,27 @@ threadReadRoutine (void *argv)
   /* Wait for all threads to be ready to read.  */
   xpthread_barrier_wait (&barrier);
 
-  ret =
-    fread (&read_buffer, sizeof (char), sizeof (read_buffer), my_data->fd);
-  if (feof (my_data->fd) != 0)
+  ret = fread (&read_buffer, 1, sizeof (read_buffer), my_data->fd);
+  /* If no data is returned (we read only 1 byte, so there's no short read
+     situation here), look for EOF flag and record it in MY_DATA.  The EOF flag
+     is not cleared because that could result in a test failure being masked
+     when two threads fail to read and one of them clears error/EOF flags
+     before the second one has the chance to observe it.
+
+     Successful readers could still see the EOF if they fall behind the failing
+     read when calling feof(), which could result in a false test failure.  To
+     avoid this race, we only make the failing reader check for EOF or
+     error.  */
+  if (ret == 0)
     {
-      clearerr (my_data->fd);
-      my_data->eof = true;
+      if (feof (my_data->fd) != 0)
+	my_data->eof = true;
+      else
+	FAIL_EXIT1 ("fread failed (ferror: %d): %m", ferror (my_data->fd));
     }
   else
-    {
-      TEST_COMPARE (ret, 1);
-      /* Save the read value.  */
-      my_data->value = read_buffer;
-    }
+    /* Save the read value.  */
+    my_data->value = read_buffer;
   TEST_COMPARE (ferror (my_data->fd), 0);
   return NULL;
 }