test: run Lua harness with nvim -l

Problem:
The Lua test harness still ran through standalone -ll mode, so tests
depended on the low-level Lua path instead of the regular Nvim Lua
environment. That also meant os.exit() coverage had to carry an ASAN
workaround because Lua's raw process exit skipped Nvim teardown and let
LeakSanitizer interfere with the observed exit code.

Solution:
Run the harness and related fixtures with nvim -l. Patch os.exit() in
the main Lua state to exit through getout(), so scripts observe normal
Nvim shutdown while standalone -ll remains available for generator-style
scripts. As a consequence, the startup test can assert os.exit() without
disabling leak detection.

AI-assisted: Codex

test/functional/testnvim.lua

@@ -107,12 +107,14 @@ if prepend_argv then
 end
 
 local session --- @type test.Session?
+local sessions = {} --- @type table<test.Session, true>
+local sigpipe_handler --- @type uv.uv_signal_t?
 local loop_running --- @type boolean?
 local last_error --- @type string?
 local method_error --- @type string?
 
 if not is_os('win') then
-  local sigpipe_handler = assert(uv.new_signal())
+  sigpipe_handler = assert(uv.new_signal())
   uv.signal_start(sigpipe_handler, 'sigpipe', function()
     print('warning: got SIGPIPE signal. Likely related to a crash in nvim')
   end)
@@ -346,6 +348,7 @@ function M.expect_exit(fn_or_timeout, ...)
       end, fn_or_timeout, ...)
     )
   end
+  M.check_close()
 end
 
 --- Executes a Vimscript function via Lua.
@@ -447,16 +450,32 @@ function M.check_close(noblock)
     return
   end
 
-  session:close(nil, noblock)
+  local s = session
+  s:close(nil, noblock)
+  sessions[s] = nil
   session = nil
 end
 
+local function close_extra_sessions(test_id, noblock)
+  for s in pairs(sessions) do
+    if s ~= session and (test_id == nil or (s.data and s.data.test_id == test_id)) then
+      if not s.closed then
+        s:close(nil, noblock)
+      end
+      sessions[s] = nil
+    end
+  end
+end
+
 -- Creates a new Session connected by domain socket (named pipe) or TCP.
 function M.connect(file_or_address)
   local addr, port = string.match(file_or_address, '(.*):(%d+)')
   local stream = (addr and port) and SocketStream.connect(addr, port)
     or SocketStream.open(file_or_address)
-  return Session.new(stream)
+  local s = Session.new(stream)
+  s.data = { test_id = _G._nvim_test_id }
+  sessions[s] = true
+  return s
 end
 
 --- Starts a new, global Nvim session and clears the current one.
@@ -499,8 +518,10 @@ local n_processes = 0
 function M.new_session(keep, ...)
   local test_id = _G._nvim_test_id
   if not keep and session ~= nil then
+    local s = session
     -- Don't block for the previous session's exit if it's from a different test.
-    session:close(nil, session.data and session.data.test_id ~= test_id)
+    s:close(nil, s.data and s.data.test_id ~= test_id)
+    sessions[s] = nil
     session = nil
   end
 
@@ -524,6 +545,7 @@ function M.new_session(keep, ...)
   n_processes = n_processes + 1
 
   local new_session = Session.new(proc)
+  sessions[new_session] = true
   -- Make it possible to check whether two sessions are from the same test.
   new_session.data = { test_id = test_id }
   return new_session
@@ -531,6 +553,10 @@ end
 
 harness.on_suite_end(function()
   M.check_close(true)
+  -- Some tests create extra sessions without replacing the global one.
+  -- Close any that individual tests did not clean up before the runner exits.
+  close_extra_sessions(nil, true)
+
   local timed_out = false
   local timer = assert(vim.uv.new_timer())
   timer:start(10000, 0, function()
@@ -540,6 +566,14 @@ harness.on_suite_end(function()
     uv.run('once')
   end
   timer:close()
+  if sigpipe_handler and not sigpipe_handler:is_closing() then
+    sigpipe_handler:close()
+  end
+  -- Drain close callbacks queued by explicit Session/ProcStream cleanup before
+  -- Nvim checks the runner loop for active handles.
+  for _ = 1, 10 do
+    uv.run('nowait')
+  end
   if timed_out then
     print(('warning: %d dangling Nvim processes'):format(n_processes))
     io.stdout:flush()
@@ -1049,6 +1083,7 @@ return function()
 
   if after_each then
     after_each(function()
+      close_extra_sessions(_G._nvim_test_id, true)
       if not vim.endswith(_G._nvim_test_id, 'x') then
         -- Use a different test ID for skipped tests as well as Nvim instances spawned
         -- between this after_each() and the next before_each() (e.g. in setup()).