vim-patch:8.0.0702: error in a timer can make Vim unusable #9826

Problem:    An error in a timer can make Vim unusable.
Solution:   Don't set the error flag or exception from a timer.  Stop a timer
            if it causes an error 3 out of 3 times.  Discard an exception
            caused inside a timer.
c577d813b7

closes #9826

runtime/doc/eval.txt

@@ -4007,14 +4007,14 @@ getchar([expr])						*getchar()*
 			not consumed.  Return zero if no character available.
 
 		Without [expr] and when [expr] is 0 a whole character or
-		special key is returned.  If it is an 8-bit character, the
+		special key is returned.  If it is a single character, the
 		result is a number.  Use nr2char() to convert it to a String.
 		Otherwise a String is returned with the encoded character.
-		For a special key it's a sequence of bytes starting with 0x80
-		(decimal: 128).  This is the same value as the string
-		"\<Key>", e.g., "\<Left>".  The returned value is also a
-		String when a modifier (shift, control, alt) was used that is
-		not included in the character.
+		For a special key it's a String with a sequence of bytes
+		starting with 0x80 (decimal: 128).  This is the same value as
+		the String "\<Key>", e.g., "\<Left>".  The returned value is
+		also a String when a modifier (shift, control, alt) was used
+		that is not included in the character.
 
 		When [expr] is 0 and Esc is typed, there will be a short delay
 		while Vim waits to see if this is the start of an escape
@@ -8096,6 +8096,10 @@ timer_start({time}, {callback} [, {options}])
 		   "repeat"	Number of times to repeat calling the
 				callback.  -1 means forever.  When not present
 				the callback will be called once.
+				If the timer causes an error three times in a
+				row the repeat is cancelled.  This avoids that
+				Vim becomes unusable because of all the error
+				messages.
 
 		Example: >
 			func MyHandler(timer)

src/nvim/eval.c

@@ -449,6 +449,7 @@ typedef struct {
   int timer_id;
   int repeat_count;
   int refcount;
+  int emsg_count;  ///< Errors in a repeating timer.
   long timeout;
   bool stopped;
   bool paused;
@@ -17160,6 +17161,7 @@ static void f_timer_start(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   timer->refcount = 1;
   timer->stopped = false;
   timer->paused = false;
+  timer->emsg_count = 0;
   timer->repeat_count = repeat;
   timer->timeout = timeout;
   timer->timer_id = last_timer_id++;
@@ -17202,6 +17204,9 @@ static void f_timer_stopall(typval_T *argvars, typval_T *unused, FunPtr fptr)
 static void timer_due_cb(TimeWatcher *tw, void *data)
 {
   timer_T *timer = (timer_T *)data;
+  int save_did_emsg = did_emsg;
+  int save_called_emsg = called_emsg;
+
   if (timer->stopped || timer->paused) {
     return;
   }
@@ -17216,8 +17221,24 @@ static void timer_due_cb(TimeWatcher *tw, void *data)
   argv[0].v_type = VAR_NUMBER;
   argv[0].vval.v_number = timer->timer_id;
   typval_T rettv = TV_INITIAL_VALUE;
+  called_emsg = false;
 
   callback_call(&timer->callback, 1, argv, &rettv);
+
+  // Handle error message
+  if (called_emsg && did_emsg) {
+    timer->emsg_count++;
+    if (current_exception != NULL) {
+      discard_current_exception();
+    }
+  }
+  did_emsg = save_did_emsg;
+  called_emsg = save_called_emsg;
+
+  if (timer->emsg_count >= 3) {
+    timer_stop(timer);
+  }
+
   tv_clear(&rettv);
 
   if (!timer->stopped && timer->timeout == 0) {

src/nvim/option.c

@@ -3383,7 +3383,7 @@ ambw_end:
   check_redraw(options[opt_idx].flags);
 
   return errmsg;
-}
+}  // NOLINT(readability/fn_size)
 
 /// Simple int comparison function for use with qsort()
 static int int_cmp(const void *a, const void *b)

src/nvim/testdir/test_timers.vim

@@ -200,6 +200,23 @@ func Test_input_in_timer()
   call assert_equal('hello', g:val)
 endfunc
 
+func FuncWithError(timer)
+  let g:call_count += 1
+  if g:call_count == 4
+    return
+  endif
+  doesnotexist
+endfunc
+
+func Test_timer_errors()
+  let g:call_count = 0
+  let timer = timer_start(10, 'FuncWithError', {'repeat': -1})
+  " Timer will be stopped after failing 3 out of 3 times.
+  call WaitFor('g:call_count == 3')
+  sleep 50m
+  call assert_equal(3, g:call_count)
+endfunc
+
 func FuncWithCaughtError(timer)
   let g:call_count += 1
   try