Avoid extra output in Vprin1_to_string_buffer (bug#78842)

print_error_message can throw after producing some output, so use
unwind-protect to ensure prin1-to-string-buffer is cleared.

* src/print.c (erase_prin1_to_string_buffer): New.
(Ferror_message_string): Use it to catch errors thrown in
'print_error_message'.
* test/src/print-tests.el (error-message-string-circular): Expand
test.

1 files changed, 3 insertions, 1 deletions

diff --git a/test/src/print-tests.el b/test/src/print-tests.el
index 036248fd091..ce8c095d496 100644
--- a/test/src/print-tests.el
+++ b/test/src/print-tests.el
@@ -356,7 +356,9 @@ otherwise, use a different charset."
 (print-tests--deftest error-message-string-circular ()
   (let ((err (list 'error)))
     (setcdr err err)
-    (should-error (error-message-string err) :type 'circular-list)))
+    (should-error (error-message-string err) :type 'circular-list)
+    ;; check that prin1-to-string-buffer is cleared (bug#78842)
+    (should (equal "37.0" (prin1-to-string 37.0)))))
 
 (print-tests--deftest print-hash-table-test ()
   (should