fix(startup): ignore broken $XDG_RUNTIME_DIR #30285

Problem:
$XDG_RUNTIME_DIR may be broken on WSL, which prevents starting (and even
building) Nvim. #30282

Solution:
- When startup fails, mention the servername in the error message.
- If an autogenerated server address fails, log an error and continue
  with an empty `v:servername`. It's only fatal if a user provides a bad
  `--listen` or `$NVIM_LISTEN_ADDRESS` address.

Before:

    $ nvim --headless --listen ./hello.sock
    nvim: Failed to --listen: "address already in use"
    $ NVIM_LISTEN_ADDRESS='./hello.sock' ./build/bin/nvim --headless
    nvim: Failed to --listen: "address already in use"

After:

    $ nvim --headless --listen ./hello.sock
    nvim: Failed to --listen: address already in use: "./hello.sock"
    $ NVIM_LISTEN_ADDRESS='./hello.sock' ./build/bin/nvim --headless
    nvim: Failed $NVIM_LISTEN_ADDRESS: address already in use: "./hello.sock"

3 files changed, 134 insertions, 75 deletions

diff --git a/src/nvim/main.c b/src/nvim/main.c
index a45ee81c81..b28667346b 100644
--- a/src/nvim/main.c
+++ b/src/nvim/main.c
@@ -266,7 +266,7 @@ int main(int argc, char **argv)
 
   if (argc > 1 && STRICMP(argv[1], "-ll") == 0) {
     if (argc == 2) {
-      print_mainerr(err_arg_missing, argv[1]);
+      print_mainerr(err_arg_missing, argv[1], NULL);
       exit(1);
     }
     nlua_run_script(argv, argc, 3);
@@ -357,10 +357,8 @@ int main(int argc, char **argv)
   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));
+  if (!server_init(params.listen_addr)) {
+    mainerr(IObuff, NULL, NULL);
   }
 
   TIME_MSG("expanding arguments");
@@ -1053,7 +1051,7 @@ static void command_line_scan(mparm_T *parmp)
     // "+" or "+{number}" or "+/{pat}" or "+{command}" argument.
     if (argv[0][0] == '+' && !had_minmin) {
       if (parmp->n_commands >= MAX_ARG_CMDS) {
-        mainerr(err_extra_cmd, NULL);
+        mainerr(err_extra_cmd, NULL, NULL);
       }
       argv_idx = -1;  // skip to next argument
       if (argv[0][1] == NUL) {
@@ -1074,7 +1072,7 @@ static void command_line_scan(mparm_T *parmp)
           parmp->no_swap_file = true;
         } else {
           if (parmp->edit_type > EDIT_STDIN) {
-            mainerr(err_too_many_args, argv[0]);
+            mainerr(err_too_many_args, argv[0], NULL);
           }
           parmp->had_stdin_file = true;
           parmp->edit_type = EDIT_STDIN;
@@ -1137,7 +1135,7 @@ static void command_line_scan(mparm_T *parmp)
           nlua_disable_preload = true;
         } else {
           if (argv[0][argv_idx]) {
-            mainerr(err_opt_unknown, argv[0]);
+            mainerr(err_opt_unknown, argv[0], NULL);
           }
           had_minmin = true;
         }
@@ -1211,7 +1209,7 @@ static void command_line_scan(mparm_T *parmp)
         break;
       case 'q':    // "-q" QuickFix mode
         if (parmp->edit_type != EDIT_NONE) {
-          mainerr(err_too_many_args, argv[0]);
+          mainerr(err_too_many_args, argv[0], NULL);
         }
         parmp->edit_type = EDIT_QF;
         if (argv[0][argv_idx]) {  // "-q{errorfile}"
@@ -1240,7 +1238,7 @@ static void command_line_scan(mparm_T *parmp)
         break;
       case 't':    // "-t {tag}" or "-t{tag}" jump to tag
         if (parmp->edit_type != EDIT_NONE) {
-          mainerr(err_too_many_args, argv[0]);
+          mainerr(err_too_many_args, argv[0], NULL);
         }
         parmp->edit_type = EDIT_TAG;
         if (argv[0][argv_idx]) {  // "-t{tag}"
@@ -1274,7 +1272,7 @@ static void command_line_scan(mparm_T *parmp)
       case 'c':    // "-c{command}" or "-c {command}" exec command
         if (argv[0][argv_idx] != NUL) {
           if (parmp->n_commands >= MAX_ARG_CMDS) {
-            mainerr(err_extra_cmd, NULL);
+            mainerr(err_extra_cmd, NULL, NULL);
           }
           parmp->commands[parmp->n_commands++] = argv[0] + argv_idx;
           argv_idx = -1;
@@ -1291,19 +1289,19 @@ static void command_line_scan(mparm_T *parmp)
         break;
 
       default:
-        mainerr(err_opt_unknown, argv[0]);
+        mainerr(err_opt_unknown, argv[0], NULL);
       }
 
       // Handle option arguments with argument.
       if (want_argument) {
         // Check for garbage immediately after the option letter.
         if (argv[0][argv_idx] != NUL) {
-          mainerr(err_opt_garbage, argv[0]);
+          mainerr(err_opt_garbage, argv[0], NULL);
         }
 
         argc--;
         if (argc < 1 && c != 'S') {  // -S has an optional argument
-          mainerr(err_arg_missing, argv[0]);
+          mainerr(err_arg_missing, argv[0], NULL);
         }
         argv++;
         argv_idx = -1;
@@ -1312,7 +1310,7 @@ static void command_line_scan(mparm_T *parmp)
         case 'c':    // "-c {command}" execute command
         case 'S':    // "-S {file}" execute Vim script
           if (parmp->n_commands >= MAX_ARG_CMDS) {
-            mainerr(err_extra_cmd, NULL);
+            mainerr(err_extra_cmd, NULL, NULL);
           }
           if (c == 'S') {
             char *a;
@@ -1343,7 +1341,7 @@ static void command_line_scan(mparm_T *parmp)
           if (strequal(argv[-1], "--cmd")) {
             // "--cmd {command}" execute command
             if (parmp->n_pre_commands >= MAX_ARG_CMDS) {
-              mainerr(err_extra_cmd, NULL);
+              mainerr(err_extra_cmd, NULL, NULL);
             }
             parmp->pre_commands[parmp->n_pre_commands++] = argv[0];
           } else if (strequal(argv[-1], "--listen")) {
@@ -1425,7 +1423,7 @@ scripterror:
 
       // Check for only one type of editing.
       if (parmp->edit_type > EDIT_STDIN) {
-        mainerr(err_too_many_args, argv[0]);
+        mainerr(err_too_many_args, argv[0], NULL);
       }
       parmp->edit_type = EDIT_FILE;
 
@@ -1472,7 +1470,7 @@ scripterror:
   }
 
   if (embedded_mode && (silent_mode || parmp->luaf)) {
-    mainerr(_("--embed conflicts with -es/-Es/-l"), NULL);
+    mainerr(_("--embed conflicts with -es/-Es/-l"), NULL, NULL);
   }
 
   // If there is a "+123" or "-c" command, set v:swapcommand to the first one.
@@ -2135,28 +2133,30 @@ static int execute_env(char *env)
   return OK;
 }
 
-/// Prints the following then exits:
-/// - An error message `errstr`
-/// - A string `str` if not null
+/// Prints a message of the form "{msg1}: {msg2}: {msg3}", then exits with code 1.
 ///
-/// @param errstr  string containing an error message
-/// @param str     string to append to the primary error message, or NULL
-static void mainerr(const char *errstr, const char *str)
+/// @param msg1  error message
+/// @param msg2  extra message, or NULL
+/// @param msg3  extra message, or NULL
+static void mainerr(const char *msg1, const char *msg2, const char *msg3)
   FUNC_ATTR_NORETURN
 {
-  print_mainerr(errstr, str);
+  print_mainerr(msg1, msg2, msg3);
   os_exit(1);
 }
 
-static void print_mainerr(const char *errstr, const char *str)
+static void print_mainerr(const char *msg1, const char *msg2, const char *msg3)
 {
   char *prgname = path_tail(argv0);
 
   signal_stop();              // kill us with CTRL-C here, if you like
 
-  fprintf(stderr, "%s: %s", prgname, _(errstr));
-  if (str != NULL) {
-    fprintf(stderr, ": \"%s\"", str);
+  fprintf(stderr, "%s: %s", prgname, _(msg1));
+  if (msg2 != NULL) {
+    fprintf(stderr, ": \"%s\"", msg2);
+  }
+  if (msg3 != NULL) {
+    fprintf(stderr, ": \"%s\"", msg3);
   }
   fprintf(stderr, _("\nMore info with \""));
   fprintf(stderr, "%s -h\"\n", prgname);
diff --git a/src/nvim/msgpack_rpc/server.c b/src/nvim/msgpack_rpc/server.c
index ae34829181..9cfe46454d 100644
--- a/src/nvim/msgpack_rpc/server.c
+++ b/src/nvim/msgpack_rpc/server.c
@@ -11,12 +11,14 @@
 #include "nvim/event/socket.h"
 #include "nvim/garray.h"
 #include "nvim/garray_defs.h"
+#include "nvim/globals.h"
 #include "nvim/log.h"
 #include "nvim/main.h"
 #include "nvim/memory.h"
 #include "nvim/msgpack_rpc/server.h"
 #include "nvim/os/os.h"
 #include "nvim/os/stdpaths_defs.h"
+#include "nvim/types_defs.h"
 
 #define MAX_CONNECTIONS 32
 #define ENV_LISTEN "NVIM_LISTEN_ADDRESS"  // deprecated
@@ -27,20 +29,24 @@ static garray_T watchers = GA_EMPTY_INIT_VALUE;
 # include "msgpack_rpc/server.c.generated.h"
 #endif
 
-/// Initializes the module
+/// Initializes resources, handles `--listen`, starts the primary server at v:servername.
 ///
-/// @returns 0: success, 1: validation error, 2: already listening, -errno: failed to bind/listen.
-int server_init(const char *listen_addr)
+/// @returns true on success, false on fatal error (message stored in IObuff)
+bool server_init(const char *listen_addr)
 {
+  bool ok = true;
   bool must_free = false;
+  TriState user_arg = kTrue;  // User-provided --listen arg.
   ga_init(&watchers, sizeof(SocketWatcher *), 1);
 
   // $NVIM_LISTEN_ADDRESS (deprecated)
   if ((!listen_addr || listen_addr[0] == '\0') && os_env_exists(ENV_LISTEN)) {
+    user_arg = kFalse;  // User-provided env var.
     listen_addr = os_getenv(ENV_LISTEN);
   }
 
   if (!listen_addr || listen_addr[0] == '\0') {
+    user_arg = kNone;  // Autogenerated server address.
     listen_addr = server_address_new(NULL);
     must_free = true;
   }
@@ -51,12 +57,6 @@ int server_init(const char *listen_addr)
 
   int rv = server_start(listen_addr);
 
-  if (os_env_exists(ENV_LISTEN)) {
-    // 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);
-  }
-
   // TODO(justinmk): this is for logging_spec. Can remove this after nvim_log #7062 is merged.
   if (os_env_exists("__NVIM_TEST_LOG")) {
     ELOG("test log message");
@@ -66,7 +66,28 @@ int server_init(const char *listen_addr)
     xfree((char *)listen_addr);
   }
 
-  return rv;
+  if (rv == 0 || user_arg == kNone) {
+    // The autogenerated servername can fail if the user has a broken $XDG_RUNTIME_DIR. #30282
+    // But that is not fatal (startup will continue, logged in $NVIM_LOGFILE, empty v:servername).
+    goto end;
+  }
+
+  (void)snprintf(IObuff, IOSIZE,
+                 user_arg ==
+                 kTrue ? "Failed to --listen: %s: \"%s\""
+                       : "Failed $NVIM_LISTEN_ADDRESS: %s: \"%s\"",
+                 rv < 0 ? os_strerror(rv) : (rv == 1 ? "empty address" : "?"),
+                 listen_addr ? listen_addr : "(empty)");
+  ok = false;
+
+end:
+  if (os_env_exists(ENV_LISTEN)) {
+    // 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);
+  }
+
+  return ok;
 }
 
 /// Teardown a single server
diff --git a/test/functional/vimscript/server_spec.lua b/test/functional/vimscript/server_spec.lua
index f3c72b7da8..85a179b3d5 100644
--- a/test/functional/vimscript/server_spec.lua
+++ b/test/functional/vimscript/server_spec.lua
@@ -41,6 +41,21 @@ describe('server', function()
     end
   end)
 
+  it('broken $XDG_RUNTIME_DIR is not fatal #30282', function()
+    clear {
+      args_rm = { '--listen' },
+      env = { NVIM_LOG_FILE = testlog, XDG_RUNTIME_DIR = '/non-existent-dir/subdir//' },
+    }
+
+    if is_os('win') then
+      -- Windows pipes have a special namespace and thus aren't decided by $XDG_RUNTIME_DIR.
+      matches('nvim', api.nvim_get_vvar('servername'))
+    else
+      eq('', api.nvim_get_vvar('servername'))
+      t.assert_log('Failed to start server%: no such file or directory', testlog, 100)
+    end
+  end)
+
   it('serverstart(), serverstop() does not set $NVIM', function()
     clear()
     local s = eval('serverstart()')
@@ -175,56 +190,79 @@ describe('server', function()
 end)
 
 describe('startup --listen', function()
-  it('validates', function()
-    clear { env = { NVIM_LOG_FILE = testlog } }
+  -- Tests Nvim output when failing to start, with and without "--headless".
+  -- TODO(justinmk): clear() should have a way to get stdout if Nvim fails to start.
+  local function _test(args, env, expected)
+    local function run(cmd)
+      return n.exec_lua(function(cmd_, env_)
+        return vim
+          .system(cmd_, {
+            text = true,
+            env = vim.tbl_extend(
+              'force',
+              -- Avoid noise in the logs; we expect failures for these tests.
+              { NVIM_LOG_FILE = testlog },
+              env_ or {}
+            ),
+          })
+          :wait()
+      end, cmd, env) --[[@as vim.SystemCompleted]]
+    end
+
+    local cmd = vim.list_extend({ n.nvim_prog, '+qall!', '--headless' }, args)
+    local r = run(cmd)
+    eq(1, r.code)
+    matches(expected, (r.stderr .. r.stdout):gsub('\\n', ' '))
 
-    -- Tests args with and without "--headless".
-    local function _test(args, expected)
-      local function run(cmd)
-        return n.exec_lua(function(cmd_)
-          return vim
-            .system(cmd_, {
-              text = true,
-              env = {
-                -- Avoid noise in the logs; we expect failures for these tests.
-                NVIM_LOG_FILE = testlog,
-              },
-            })
-            :wait()
-        end, cmd) --[[@as vim.SystemCompleted]]
-      end
-
-      local cmd = vim.list_extend({ n.nvim_prog, '+qall!', '--headless' }, args)
-      local r = run(cmd)
-      eq(1, r.code)
-      matches(expected, (r.stderr .. r.stdout):gsub('\\n', ' '))
-
-      if is_os('win') then
-        return -- On Windows, output without --headless is garbage.
-      end
-      table.remove(cmd, 3) -- Remove '--headless'.
-      assert(not vim.tbl_contains(cmd, '--headless'))
-      r = run(cmd)
-      eq(1, r.code)
-      matches(expected, (r.stderr .. r.stdout):gsub('\\n', ' '))
+    if is_os('win') then
+      return -- On Windows, output without --headless is garbage.
     end
+    table.remove(cmd, 3) -- Remove '--headless'.
+    assert(not vim.tbl_contains(cmd, '--headless'))
+    r = run(cmd)
+    eq(1, r.code)
+    matches(expected, (r.stderr .. r.stdout):gsub('\\n', ' '))
+  end
+
+  it('validates', function()
+    clear { env = { NVIM_LOG_FILE = testlog } }
+    local in_use = n.eval('v:servername') ---@type string Address already used by another server.
 
     t.assert_nolog('Failed to start server', testlog, 100)
     t.assert_nolog('Host lookup failed', testlog, 100)
 
-    _test({ '--listen' }, 'nvim.*: Argument missing after: "%-%-listen"')
-    _test({ '--listen2' }, 'nvim.*: Garbage after option argument: "%-%-listen2"')
-    _test({ '--listen', n.eval('v:servername') }, 'nvim.*: Failed to %-%-listen: ".* already .*"')
-    _test({ '--listen', '/' }, 'nvim.*: Failed to %-%-listen: ".*"')
+    _test({ '--listen' }, nil, 'nvim.*: Argument missing after: "%-%-listen"')
+    _test({ '--listen2' }, nil, 'nvim.*: Garbage after option argument: "%-%-listen2"')
+    _test(
+      { '--listen', in_use },
+      nil,
+      ('nvim.*: Failed to %%-%%-listen: [^:]+ already [^:]+: "%s"'):format(vim.pesc(in_use))
+    )
+    _test({ '--listen', '/' }, nil, 'nvim.*: Failed to %-%-listen: [^:]+: "/"')
     _test(
       { '--listen', 'https://example.com' },
-      ('nvim.*: Failed to %%-%%-listen: "%s"'):format(
+      nil,
+      ('nvim.*: Failed to %%-%%-listen: %s: "https://example.com"'):format(
         is_os('mac') and 'unknown node or service' or 'service not available for socket type'
       )
     )
 
     t.assert_log('Failed to start server', testlog, 100)
     t.assert_log('Host lookup failed', testlog, 100)
+
+    _test(
+      {},
+      { NVIM_LISTEN_ADDRESS = in_use },
+      ('nvim.*: Failed $NVIM_LISTEN_ADDRESS: [^:]+ already [^:]+: "%s"'):format(vim.pesc(in_use))
+    )
+    _test({}, { NVIM_LISTEN_ADDRESS = '/' }, 'nvim.*: Failed $NVIM_LISTEN_ADDRESS: [^:]+: "/"')
+    _test(
+      {},
+      { NVIM_LISTEN_ADDRESS = 'https://example.com' },
+      ('nvim.*: Failed $NVIM_LISTEN_ADDRESS: %s: "https://example.com"'):format(
+        is_os('mac') and 'unknown node or service' or 'service not available for socket type'
+      )
+    )
   end)
 
   it('sets v:servername, overrides $NVIM_LISTEN_ADDRESS', function()