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"

1 files changed, 31 insertions, 10 deletions

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