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

1 files changed, 4 insertions, 3 deletions

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)) {