libio: Synthesize ESPIPE error if lseek returns 0 after reading bytes

This is required so that fclose, when trying to seek to the right
position after filling the input buffer, does not fail with EINVAL.
This fclose code path only ignores ESPIPE errors.

Reported by Petr Pisar on
<https://bugzilla.redhat.com/show_bug.cgi?id=2358265>.

Fixes commit be6818be31 ("Make fclose
seek input file to right offset (bug 12724)").

Reviewed-by: Frédéric Bérat <fberat@redhat.com>

libio/fileops.c

@@ -928,6 +928,16 @@ do_ftell (FILE *fp)
   if (result == EOF)
     return result;
 
+  if (result == 0 && offset < 0)
+    {
+      /* This happens for some character devices that always report
+	 file offset 0 even after some data has been read (instead of
+	 failing with ESPIPE).  The fclose path ignores this
+	 error.  */
+      __set_errno (ESPIPE);
+      return EOF;
+    }
+
   result += offset;
 
   if (result < 0)