feat(server): instance "name", store pipes in stdpath(state)

Problem:
- Unix sockets are created in random /tmp dirs.
  - /tmp is messy, unclear when OSes actually clear it.
  - The generated paths are very ugly. This adds friction to reasoning
    about which paths belong to which Nvim instances.
- No way to provide a human-friendly way to identify Nvim instances in
  logs or server addresses.

Solution:
- Store unix sockets in stdpath('state')
- Allow --listen "name" and serverstart("name") to given a name (which
  is appended to a generated path).

TODO:
- is stdpath(state) the right place?

src/nvim/msgpack_rpc/server.c

@@ -42,7 +42,7 @@ bool server_init(const char *listen_addr)
 
   int rv = listen_addr ? server_start(listen_addr) : 1;
   if (0 != rv) {
-    listen_addr = server_address_new();
+    listen_addr = server_address_new(NULL);
     if (!listen_addr) {
       return false;
     }
@@ -55,7 +55,7 @@ bool server_init(const char *listen_addr)
     os_unsetenv(ENV_LISTEN);
   }
 
-  // TODO: this is for logging_spec. Can remove this after nvim_log #7062 is merged.
+  // 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");
   }
@@ -87,23 +87,26 @@ void server_teardown(void)
 
 /// Generates unique address for local server.
 ///
-/// In Windows this is a named pipe in the format
-///     \\.\pipe\nvim-<PID>-<COUNTER>.
-///
-/// For other systems it is a path returned by vim_tempname().
-///
-/// This function is NOT thread safe
-char *server_address_new(void)
+/// Named pipe format:
+/// - Windows: "\\.\pipe\<name>.<pid>.<counter>"
+/// - Other: "~/.local/state/nvim/<name>.<pid>.<counter>"
+char *server_address_new(const char *name)
 {
-#ifdef WIN32
   static uint32_t count = 0;
-  char template[ADDRESS_MAX_SIZE];
-  snprintf(template, ADDRESS_MAX_SIZE,
-           "\\\\.\\pipe\\nvim-%" PRIu64 "-%" PRIu32, os_get_pid(), count++);
-  return xstrdup(template);
+  char fmt[ADDRESS_MAX_SIZE];
+#ifdef WIN32
+  int r = snprintf(fmt, sizeof(fmt), "\\\\.\\pipe\\%s.%" PRIu64 ".%" PRIu32,
+                   name ? name : "nvim", os_get_pid(), count++);
 #else
-  return (char *)vim_tempname();
+  char *dir = get_xdg_home(kXDGStateHome);
+  int r = snprintf(fmt, sizeof(fmt), "%s/%s.%" PRIu64 ".%" PRIu32,
+                   dir, name ? name : "nvim", os_get_pid(), count++);
+  xfree(dir);
 #endif
+  if ((size_t)r >= sizeof(fmt)) {
+    ELOG("truncated server address");
+  }
+  return xstrdup(fmt);
 }
 
 /// Check if this instance owns a pipe address.
@@ -118,35 +121,35 @@ bool server_owns_pipe_address(const char *path)
   return false;
 }
 
-/// Starts listening for API calls.
+/// Starts listening for RPC calls.
 ///
-/// The socket type is determined by parsing `endpoint`: If it's a valid IPv4
-/// or IPv6 address in 'ip:[port]' format, then it will be a TCP socket.
-/// Otherwise it will be a Unix socket or named pipe (Windows).
+/// Socket type is decided by the format of `addr`:
+/// - TCP socket if it looks like an IPv4/6 address ("ip:[port]").
+///   - If [port] is omitted, a random one is assigned.
+/// - Unix socket (or named pipe on Windows) otherwise.
+///   - If the name doesn't contain slashes it is appended to a generated path. #8519
 ///
-/// If no port is given, a random one will be assigned.
-///
-/// @param endpoint Address of the server. Either a 'ip:[port]' string or an
-///                 arbitrary identifier (trimmed to 256 bytes) for the Unix
-///                 socket or named pipe.
-/// @returns 0: success, 1: validation error, 2: already listening,
-///          -errno: failed to bind or listen.
-int server_start(const char *endpoint)
+/// @param addr Server address: a "ip:[port]" string or arbitrary name or filepath (max 256 bytes)
+///             for the Unix socket or named pipe.
+/// @returns 0: success, 1: validation error, 2: already listening, -errno: failed to bind/listen.
+int server_start(const char *addr)
 {
-  if (endpoint == NULL || endpoint[0] == '\0') {
-    WLOG("Empty or NULL endpoint");
+  if (addr == NULL || addr[0] == '\0') {
+    WLOG("Empty or NULL address");
     return 1;
   }
 
+  bool isname = !strstr(addr, ":") && !strstr(addr, "/") && !strstr(addr, "\\");
+  char *addr_gen = isname ? server_address_new(addr) : NULL;
   SocketWatcher *watcher = xmalloc(sizeof(SocketWatcher));
-
-  int result = socket_watcher_init(&main_loop, watcher, endpoint);
+  int result = socket_watcher_init(&main_loop, watcher, isname ? addr_gen : addr);
+  xfree(addr_gen);
   if (result < 0) {
     xfree(watcher);
     return result;
   }
 
-  // Check if a watcher for the endpoint already exists
+  // Check if a watcher for the address already exists.
   for (int i = 0; i < watchers.ga_len; i++) {
     if (!strcmp(watcher->addr, ((SocketWatcher **)watchers.ga_data)[i]->addr)) {
       ELOG("Already listening on %s", watcher->addr);