fix(:restart): reuse --listen addr on Windows #38539

Problem:
On Windows, :restart cannot immediately reuse the canonical --listen
address because named pipe release is asynchronous.

Solution:
Start the new Nvim server on a temporary address; in the new Nvim,
retry serverstart() with the original ("canonical") address until it
succeeds.

1 files changed, 50 insertions, 7 deletions

diff --git a/src/nvim/ex_docmd.c b/src/nvim/ex_docmd.c
index 96a27bdf4b..79c53b2900 100644
--- a/src/nvim/ex_docmd.c
+++ b/src/nvim/ex_docmd.c
@@ -4977,8 +4977,10 @@ static void ex_restart(exarg_T *eap)
   char **argv = xcalloc((size_t)argc + 3, sizeof(char *));
   size_t i = 0;
   const char *listen_arg = NULL;
-#ifdef MSWIN  // FIXME: --listen doesn't work on Windows and needs to be dropped
-# define HANDLE_LISTEN_ADDR li = next_li; continue
+#ifdef MSWIN
+  // On Windows, don't pass --listen to new server (named pipe can't be reused immediately).
+  // Instead pass the address via RPC, and new server will rebind to it after startup.
+# define HANDLE_LISTEN_ADDR listen_arg = addr; li = next_li; continue
 #else
 # define HANDLE_LISTEN_ADDR listen_arg = addr
 #endif
@@ -5020,11 +5022,27 @@ static void ex_restart(exarg_T *eap)
   });
 #undef HANDLE_LISTEN_ADDR
 
-  bool server_stopped = false;
-  if (listen_arg != NULL) {
-    // Stop listening on the --listen address so that the new server can listen.
-    server_stopped = server_stop(listen_arg, true);
+#ifdef MSWIN
+  // On Windows, --listen is omitted from child argv because the named pipe can't be reused immediately.
+  // Recover the canonical address from the Lua module state (set by the previous rebind_old_addr_after_restart() call),
+  // and keep the current listener alive (new server reclaims it).
+  char *listen_arg_alloc = NULL;
+  if (listen_arg == NULL) {
+    Error lua_err = ERROR_INIT;
+    Object rv = NLUA_EXEC_STATIC("return require('vim._core.server').restart_canonical_addr",
+                                 (Array)ARRAY_DICT_INIT, kRetObject, NULL, &lua_err);
+    if (!ERROR_SET(&lua_err) && rv.type == kObjectTypeString && rv.data.string.size > 0) {
+      listen_arg_alloc = xstrdup(rv.data.string.data);
+      listen_arg = listen_arg_alloc;
+    }
+    api_free_object(rv);
+    api_clear_error(&lua_err);
   }
+  bool server_stopped = false;
+#else
+  // Stop listening on the --listen address so that the new server can listen.
+  bool server_stopped = listen_arg ? server_stop(listen_arg, true) : false;
+#endif
 
 #ifdef MSWIN
   bool restart_alloc_console_env = false;
@@ -5086,7 +5104,8 @@ static void ex_restart(exarg_T *eap)
     result_mem = NULL;
   }
 
-  // Get new server's listen address.
+  // Get the new server's initial listen address. On Windows this is the
+  // temporary bootstrap address that UIs should reconnect to first.
   MAXSIZE_TEMP_ARRAY(servername_args, 1);
   ADD_C(servername_args, CSTR_AS_OBJ("servername"));
   Object result = rpc_send_call(channel->id, "nvim_get_vvar", servername_args, &result_mem, &err);
@@ -5101,6 +5120,27 @@ static void ex_restart(exarg_T *eap)
   arena_mem_free(result_mem);
   result_mem = NULL;
 
+#ifdef MSWIN
+  if (listen_arg != NULL) {
+    // Tell the new server to reclaim the canonical --listen address once the old listener exits,
+    // then retire the bootstrap address after all UIs have reattached (or timeout).
+    MAXSIZE_TEMP_ARRAY(lua_args, 2);
+    ADD_C(lua_args,
+          CSTR_AS_OBJ("return require('vim._core.server').rebind_old_addr_after_restart(...)"));
+    MAXSIZE_TEMP_ARRAY(handoff_params, 3);
+    ADD_C(handoff_params, CSTR_AS_OBJ(listen_arg));
+    ADD_C(handoff_params, CSTR_AS_OBJ(listen_addr));
+    ADD_C(handoff_params, INTEGER_OBJ((Integer)ui_active()));
+    ADD_C(lua_args, ARRAY_OBJ(handoff_params));
+    rpc_send_call(channel->id, "nvim_exec_lua", lua_args, &result_mem, &err);
+    if (ERROR_SET(&err)) {
+      goto fail_2;
+    }
+    arena_mem_free(result_mem);
+    result_mem = NULL;
+  }
+#endif
+
   // Send restart event with new listen address to all UIs.
   ui_call_restart(cstr_as_string(listen_addr));
   ui_flush();
@@ -5152,6 +5192,9 @@ fail_1:
   if (server_stopped && server_start(listen_arg) != 0) {
     semsg("couldn't resume listening on %s", listen_arg);
   }
+#ifdef MSWIN
+  xfree(listen_arg_alloc);
+#endif
 }
 
 /// ":close": close current window, unless it is the last one