feat(startup): validate --listen address

Problem:
`nvim --listen` does not error on EADDRINUSE. #30123

Solution:
Now that `$NVIM_LISTEN_ADDRESS` is deprecated and input *only* (instead
of the old, ambiguous situation where it was both an input *and* an
output), we can be fail fast instead of trying to "recover". This
reverts the "recovery" behavior of
704ba4151e, but that was basically
a workaround for the fragility of `$NVIM_LISTEN_ADDRESS`.

src/nvim/api/ui.c

@@ -93,15 +93,15 @@ void remote_ui_free_all_mem(void)
 }
 #endif
 
-/// Wait until ui has connected on stdio channel if only_stdio
-/// is true, otherwise any channel.
+/// Wait until UI has connected.
+///
+/// @param only_stdio UI is expected to connect on stdio.
 void remote_ui_wait_for_attach(bool only_stdio)
 {
   if (only_stdio) {
     Channel *channel = find_channel(CHAN_STDIO);
     if (!channel) {
-      // this function should only be called in --embed mode, stdio channel
-      // can be assumed.
+      // `only_stdio` implies --embed mode, thus stdio channel can be assumed.
       abort();
     }
 

src/nvim/main.c

@@ -332,12 +332,6 @@ int main(int argc, char **argv)
 #endif
   bool use_builtin_ui = (has_term && !headless_mode && !embedded_mode && !silent_mode);
 
-  // don't bind the server yet, if we are using builtin ui.
-  // This will be done when nvim server has been forked from the ui process
-  if (!use_builtin_ui) {
-    server_init(params.listen_addr);
-  }
-
   if (params.remote) {
     remote_request(&params, params.remote, params.server_addr, argc, argv,
                    use_builtin_ui);
@@ -355,11 +349,19 @@ int main(int argc, char **argv)
     ui_client_channel_id = rv;
   }
 
+  // NORETURN: Start builtin UI client.
   if (ui_client_channel_id) {
     time_finish();
     ui_client_run(remote_ui);  // NORETURN
   }
   assert(!ui_client_channel_id && !use_builtin_ui);
+  // Nvim server...
+
+  int listen_rv = server_init(params.listen_addr);
+  if (listen_rv != 0) {
+    mainerr("Failed to --listen", listen_rv < 0
+            ? os_strerror(listen_rv) : (listen_rv == 1 ? "empty address" : NULL));
+  }
 
   TIME_MSG("expanding arguments");
 

src/nvim/msgpack_rpc/server.c

@@ -28,27 +28,32 @@ static garray_T watchers = GA_EMPTY_INIT_VALUE;
 #endif
 
 /// Initializes the module
-bool server_init(const char *listen_addr)
+///
+/// @returns 0: success, 1: validation error, 2: already listening, -errno: failed to bind/listen.
+int server_init(const char *listen_addr)
 {
+  bool must_free = false;
   ga_init(&watchers, sizeof(SocketWatcher *), 1);
 
   // $NVIM_LISTEN_ADDRESS (deprecated)
-  if (!listen_addr && os_env_exists(ENV_LISTEN)) {
+  if ((!listen_addr || listen_addr[0] == '\0') && os_env_exists(ENV_LISTEN)) {
     listen_addr = os_getenv(ENV_LISTEN);
   }
 
-  int rv = listen_addr ? server_start(listen_addr) : 1;
-  if (0 != rv) {
+  if (!listen_addr || listen_addr[0] == '\0') {
     listen_addr = server_address_new(NULL);
-    if (!listen_addr) {
-      return false;
-    }
-    rv = server_start(listen_addr);
-    xfree((char *)listen_addr);
+    must_free = true;
   }
 
+  if (!listen_addr) {
+    abort();  // Cannot happen.
+  }
+
+  int rv = server_start(listen_addr);
+
   if (os_env_exists(ENV_LISTEN)) {
-    // Unset $NVIM_LISTEN_ADDRESS, it's a liability hereafter.
+    // Unset $NVIM_LISTEN_ADDRESS, it's a liability hereafter. It is "input only", it must not be
+    // leaked to child jobs or :terminal.
     os_unsetenv(ENV_LISTEN);
   }
 
@@ -57,7 +62,11 @@ bool server_init(const char *listen_addr)
     ELOG("test log message");
   }
 
-  return rv == 0;
+  if (must_free) {
+    xfree((char *)listen_addr);
+  }
+
+  return rv;
 }
 
 /// Teardown a single server