Fix subproc listening when setting filter to non-t (Bug#36591)

* src/process.c (Fset_process_filter): Call add_process_read_fd
according to the state of process filter before it's updated.  This
restores the correct functioning as it was before 2016-02-16 "Allow
setting the filter masks later".  Inline the set_process_filter_masks
call instead of fixing it that function, because it is also called
from connect_network_socket, and we don't want to change the behavior
of that function so close to release.
* test/src/process-tests.el (set-process-filter-t): New test.

1 files changed, 29 insertions, 0 deletions

diff --git a/test/src/process-tests.el b/test/src/process-tests.el
index 7cccc5a02cb..7a6762a9226 100644
--- a/test/src/process-tests.el
+++ b/test/src/process-tests.el
@@ -144,6 +144,35 @@
     (should (equal "hello stderr!\n"
                    (mapconcat #'identity (nreverse stderr-output) "")))))
 
+(ert-deftest set-process-filter-t ()
+  "Test setting process filter to t and back." ;; Bug#36591
+  (with-temp-buffer
+    (let* ((print-level nil)
+           (print-length nil)
+           (proc (start-process
+                  "test proc" (current-buffer)
+                  (concat invocation-directory invocation-name)
+                  "-Q" "--batch" "--eval"
+                  (prin1-to-string
+                   '(let (s)
+                      (while (setq s (read-from-minibuffer "$ "))
+                        (princ s)
+                        (princ "\n")))))))
+      (set-process-query-on-exit-flag proc nil)
+      (send-string proc "one\n")
+      (should
+       (accept-process-output proc 1))  ; Read "one".
+      (should (equal (buffer-string) "$ one\n$ "))
+      (set-process-filter proc t)       ; Stop reading from proc.
+      (send-string proc "two\n")
+      (should-not
+       (accept-process-output proc 1))  ; Can't read "two" yet.
+      (should (equal (buffer-string) "$ one\n$ "))
+      (set-process-filter proc nil)     ; Resume reading from proc.
+      (should
+       (accept-process-output proc 1))  ; Read "two" from proc.
+      (should (equal (buffer-string) "$ one\n$ two\n$ ")))))
+
 (ert-deftest start-process-should-not-modify-arguments ()
   "`start-process' must not modify its arguments in-place."
   ;; See bug#21831.