*: Fix some problems found during review

Still missing: problems in window.c, it should be possible to construct a test
for them.

test/functional/eval/interrupt_spec.lua

@@ -0,0 +1,61 @@
+local helpers = require('test.functional.helpers')(after_each)
+
+local command = helpers.command
+local meths = helpers.meths
+local clear = helpers.clear
+local sleep = helpers.sleep
+local wait = helpers.wait
+local feed = helpers.feed
+local eq = helpers.eq
+
+local dur
+local min_dur = 8
+local len = 131072
+
+describe('List support code', function()
+  if not pending('does not actually allows interrupting with just got_int', function() end) then return end
+  -- The following tests are confirmed to work with os_breakcheck() just before
+  -- `if (got_int) {break;}` in tv_list_copy and list_join_inner() and not to
+  -- work without.
+  setup(function()
+    clear()
+    dur = 0
+    while true do
+      command(([[
+        let rt = reltime()
+        let bl = range(%u)
+        let dur = reltimestr(reltime(rt))
+      ]]):format(len))
+      dur = tonumber(meths.get_var('dur'))
+      if dur >= min_dur then
+        -- print(('Using len %u, dur %g'):format(len, dur))
+        break
+      else
+        len = len * 2
+      end
+    end
+  end)
+  it('allows interrupting copy', function()
+    feed(':let t_rt = reltime()<CR>:let t_bl = copy(bl)<CR>')
+    sleep(min_dur / 16 * 1000)
+    feed('<C-c>')
+    wait()
+    command('let t_dur = reltimestr(reltime(t_rt))')
+    local t_dur = tonumber(meths.get_var('t_dur'))
+    if t_dur >= dur / 8 then
+      eq(nil, ('Took too long to cancel: %g >= %g'):format(t_dur, dur / 8))
+    end
+  end)
+  it('allows interrupting join', function()
+    feed(':let t_rt = reltime()<CR>:let t_j = join(bl)<CR>')
+    sleep(min_dur / 16 * 1000)
+    feed('<C-c>')
+    wait()
+    command('let t_dur = reltimestr(reltime(t_rt))')
+    local t_dur = tonumber(meths.get_var('t_dur'))
+    print(('t_dur: %g'):format(t_dur))
+    if t_dur >= dur / 8 then
+      eq(nil, ('Took too long to cancel: %g >= %g'):format(t_dur, dur / 8))
+    end
+  end)
+end)