test: skip_fragile(), TEST_SKIP_FRAGILE

Let build systems define TEST_SKIP_FRAGILE to skip tests that are known to be
resource-intensive (unreliable on slow systems).

References https://github.com/neovim/neovim/pull/5488#issuecomment-265622113

test/functional/ex_cmds/ctrl_c_spec.lua

@@ -11,6 +11,12 @@ describe("CTRL-C (mapped)", function()
   end)
 
   it("interrupts :global", function()
+    if helpers.skip_fragile(pending,
+      (os.getenv("TRAVIS") and os.getenv("CLANG_SANITIZER") == "ASAN_UBSAN"))
+    then
+      return
+    end
+
     source([[
       set nomore nohlsearch undolevels=-1
       nnoremap <C-C> <NOP>

test/functional/helpers.lua

@@ -520,12 +520,12 @@ local function create_callindex(func)
 end
 
 -- Helper to skip tests. Returns true in Windows systems.
--- pending_func is pending() from busted
-local function pending_win32(pending_func)
+-- pending_fn is pending() from busted
+local function pending_win32(pending_fn)
   clear()
   if uname() == 'Windows' then
-    if pending_func ~= nil then
-      pending_func('FIXME: Windows', function() end)
+    if pending_fn ~= nil then
+      pending_fn('FIXME: Windows', function() end)
     end
     return true
   else
@@ -533,6 +533,22 @@ local function pending_win32(pending_func)
   end
 end
 
+-- Calls pending() and returns `true` if the system is too slow to
+-- run fragile or expensive tests. Else returns `false`.
+local function skip_fragile(pending_fn, cond)
+  if pending_fn == nil or type(pending_fn) ~= type(function()end) then
+    error("invalid pending_fn")
+  end
+  if cond then
+    pending_fn("skipped (test is fragile on this system)", function() end)
+    return true
+  elseif os.getenv("TEST_SKIP_FRAGILE") then
+    pending_fn("skipped (TEST_SKIP_FRAGILE)", function() end)
+    return true
+  end
+  return false
+end
+
 local funcs = create_callindex(nvim_call)
 local meths = create_callindex(nvim)
 local uimeths = create_callindex(ui)
@@ -601,6 +617,7 @@ return function(after_each)
     curwinmeths = curwinmeths,
     curtabmeths = curtabmeths,
     pending_win32 = pending_win32,
+    skip_fragile = skip_fragile,
     tmpname = tmpname,
     NIL = mpack.NIL,
   }

test/functional/ui/mouse_spec.lua

@@ -153,9 +153,10 @@ describe('Mouse input', function()
     end)
 
     it('in tabline to the left moves tab left', function()
-      if os.getenv("TRAVIS") and (helpers.os_name() == "osx"
-          or os.getenv("CLANG_SANITIZER") == "ASAN_UBSAN") then
-        pending("[Fails on Travis macOS, ASAN_UBSAN. #4874]", function() end)
+      if helpers.skip_fragile(pending,
+        os.getenv("TRAVIS") and (helpers.os_name() == "osx"
+          or os.getenv("CLANG_SANITIZER") == "ASAN_UBSAN"))  -- #4874
+      then
         return
       end
 
@@ -257,9 +258,10 @@ describe('Mouse input', function()
     end)
 
     it('out of tabline to the left moves tab left', function()
-      if os.getenv("TRAVIS") and (helpers.os_name() == "osx"
-          or os.getenv("CLANG_SANITIZER") == "ASAN_UBSAN") then
-        pending("[Fails on Travis macOS, ASAN_UBSAN. #4874]", function() end)
+      if helpers.skip_fragile(pending,
+        os.getenv("TRAVIS") and (helpers.os_name() == "osx"
+          or os.getenv("CLANG_SANITIZER") == "ASAN_UBSAN"))  -- #4874
+      then
         return
       end