fix(startup): server fails if $NVIM_APPNAME is relative dir #30310

Problem:
If $NVIM_APPNAME is a relative dir path, Nvim fails to start its
primary/default server, and `v:servername` is empty.
Root cause is d34c64e342dfba9248d1055e702d02620a1b31a8, but this wasn't
noticed until 96128a5076b7 started reporting the error more loudly.

Solution:
- `server_address_new`: replace slashes "/" in the appname before using
  it as a servername.
- `vim_mktempdir`: always prefer the system-wide top-level "nvim.user/"
  directory. That isn't intended to be specific to NVIM_APPNAME; rather,
  each *subdirectory* ("nvim.user/xxx") is owned by each Nvim instance.
  Nvim "apps" can be identified by the server socket(s) stored in those
  per-Nvim subdirs.

fix #30256

5 files changed, 26 insertions, 20 deletions

diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index de1d784577..aed6fdae14 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -7641,7 +7641,7 @@ static void get_xdg_var_list(const XDGVarType xdg, typval_T *rettv)
     return;
   }
   const void *iter = NULL;
-  const char *appname = get_appname();
+  const char *appname = get_appname(false);
   do {
     size_t dir_len;
     const char *dir;
diff --git a/src/nvim/fileio.c b/src/nvim/fileio.c
index f6a25a27a6..31a1c0e293 100644
--- a/src/nvim/fileio.c
+++ b/src/nvim/fileio.c
@@ -3264,18 +3264,12 @@ static void vim_mktempdir(void)
   char tmp[TEMP_FILE_PATH_MAXLEN];
   char path[TEMP_FILE_PATH_MAXLEN];
   char user[40] = { 0 };
-  char appname[40] = { 0 };
 
   os_get_username(user, sizeof(user));
   // Usernames may contain slashes! #19240
   memchrsub(user, '/', '_', sizeof(user));
   memchrsub(user, '\\', '_', sizeof(user));
 
-  // Appname may be a relative path, replace slashes to make it name-like.
-  xstrlcpy(appname, get_appname(), sizeof(appname));
-  memchrsub(appname, '/', '%', sizeof(appname));
-  memchrsub(appname, '\\', '%', sizeof(appname));
-
   // Make sure the umask doesn't remove the executable bit.
   // "repl" has been reported to use "0177".
   mode_t umask_save = umask(0077);
@@ -3283,14 +3277,15 @@ static void vim_mktempdir(void)
     // Expand environment variables, leave room for "/tmp/nvim.<user>/XXXXXX/999999999".
     expand_env((char *)temp_dirs[i], tmp, TEMP_FILE_PATH_MAXLEN - 64);
     if (!os_isdir(tmp)) {
+      if (strequal("$TMPDIR", temp_dirs[i])) {
+        WLOG("$TMPDIR tempdir not a directory (or does not exist): %s", tmp);
+      }
       continue;
     }
 
     // "/tmp/" exists, now try to create "/tmp/nvim.<user>/".
     add_pathsep(tmp);
-
-    xstrlcat(tmp, appname, sizeof(tmp));
-    xstrlcat(tmp, ".", sizeof(tmp));
+    xstrlcat(tmp, "nvim.", sizeof(tmp));
     xstrlcat(tmp, user, sizeof(tmp));
     os_mkdir(tmp, 0700);  // Always create, to avoid a race.
     bool owned = os_file_owned(tmp);
diff --git a/src/nvim/msgpack_rpc/server.c b/src/nvim/msgpack_rpc/server.c
index 9cfe46454d..dc2b85154f 100644
--- a/src/nvim/msgpack_rpc/server.c
+++ b/src/nvim/msgpack_rpc/server.c
@@ -121,14 +121,15 @@ char *server_address_new(const char *name)
 {
   static uint32_t count = 0;
   char fmt[ADDRESS_MAX_SIZE];
-  const char *appname = get_appname();
 #ifdef MSWIN
+  (void)get_appname(true);
   int r = snprintf(fmt, sizeof(fmt), "\\\\.\\pipe\\%s.%" PRIu64 ".%" PRIu32,
-                   name ? name : appname, os_get_pid(), count++);
+                   name ? name : NameBuff, os_get_pid(), count++);
 #else
   char *dir = stdpaths_get_xdg_var(kXDGRuntimeDir);
+  (void)get_appname(true);
   int r = snprintf(fmt, sizeof(fmt), "%s/%s.%" PRIu64 ".%" PRIu32,
-                   dir, name ? name : appname, os_get_pid(), count++);
+                   dir, name ? name : NameBuff, os_get_pid(), count++);
   xfree(dir);
 #endif
   if ((size_t)r >= sizeof(fmt)) {
diff --git a/src/nvim/os/stdpaths.c b/src/nvim/os/stdpaths.c
index e4435bcaa8..e9a74d197f 100644
--- a/src/nvim/os/stdpaths.c
+++ b/src/nvim/os/stdpaths.c
@@ -63,22 +63,32 @@ static const char *const xdg_defaults[] = {
 #endif
 };
 
-/// Get the value of $NVIM_APPNAME or "nvim" if not set.
+/// Gets the value of $NVIM_APPNAME, or "nvim" if not set.
+///
+/// @param namelike Write "name-like" value (no path separators) in `NameBuff`.
 ///
 /// @return $NVIM_APPNAME value
-const char *get_appname(void)
+const char *get_appname(bool namelike)
 {
   const char *env_val = os_getenv("NVIM_APPNAME");
   if (env_val == NULL || *env_val == NUL) {
     env_val = "nvim";
   }
+
+  if (namelike) {
+    // Appname may be a relative path, replace slashes to make it name-like.
+    xstrlcpy(NameBuff, env_val, sizeof(NameBuff));
+    memchrsub(NameBuff, '/', '-', sizeof(NameBuff));
+    memchrsub(NameBuff, '\\', '-', sizeof(NameBuff));
+  }
+
   return env_val;
 }
 
 /// Ensure that APPNAME is valid. Must be a name or relative path.
 bool appname_is_valid(void)
 {
-  const char *appname = get_appname();
+  const char *appname = get_appname(false);
   if (path_is_absolute(appname)
       // TODO(justinmk): on Windows, path_is_absolute says "/" is NOT absolute. Should it?
       || strequal(appname, "/")
@@ -193,7 +203,7 @@ char *get_xdg_home(const XDGVarType idx)
   FUNC_ATTR_WARN_UNUSED_RESULT
 {
   char *dir = stdpaths_get_xdg_var(idx);
-  const char *appname = get_appname();
+  const char *appname = get_appname(false);
   size_t appname_len = strlen(appname);
   assert(appname_len < (IOSIZE - sizeof("-data")));
 
diff --git a/src/nvim/runtime.c b/src/nvim/runtime.c
index 3aa558f305..030bda4fa5 100644
--- a/src/nvim/runtime.c
+++ b/src/nvim/runtime.c
@@ -1559,7 +1559,7 @@ static inline char *add_env_sep_dirs(char *dest, const char *const val, const ch
     return dest;
   }
   const void *iter = NULL;
-  const char *appname = get_appname();
+  const char *appname = get_appname(false);
   const size_t appname_len = strlen(appname);
   do {
     size_t dir_len;
@@ -1626,7 +1626,7 @@ static inline char *add_dir(char *dest, const char *const dir, const size_t dir_
     if (!after_pathsep(dest - 1, dest)) {
       *dest++ = PATHSEP;
     }
-    const char *appname = get_appname();
+    const char *appname = get_appname(false);
     size_t appname_len = strlen(appname);
     assert(appname_len < (IOSIZE - sizeof("-data")));
     xmemcpyz(IObuff, appname, appname_len);
@@ -1697,7 +1697,7 @@ char *runtimepath_default(bool clean_arg)
   size_t config_len = 0;
   size_t vimruntime_len = 0;
   size_t libdir_len = 0;
-  const char *appname = get_appname();
+  const char *appname = get_appname(false);
   size_t appname_len = strlen(appname);
   if (data_home != NULL) {
     data_len = strlen(data_home);