feat: NVIM_APPNAME supports relative paths #25233

Problem:
NVIM_APPNAME does not allow path separators in the name, so relative
paths can't be used:

    NVIM_APPNAME="neovim-configs/first-config" nvim
    NVIM_APPNAME="neovim-configs/second-config" nvim

Solution:
Let NVIM_APPNAME be a relative path. Absolute paths are not supported.

fix #23056
fix #24966

7 files changed, 60 insertions, 21 deletions

diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index 05a2d35f9a..ff0ccfb08f 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -169,6 +169,8 @@ The following new APIs and features were added.
 • |vim.lsp.util.locations_to_items()| sets the `user_data` of each item to the
   original LSP `Location` or `LocationLink`.
 
+• |$NVIM_APPNAME| can be set to a relative path instead of only a name.
+
 ==============================================================================
 CHANGED FEATURES                                                 *news-changed*
 
diff --git a/runtime/doc/starting.txt b/runtime/doc/starting.txt
index 17d5b4668d..d38712b067 100644
--- a/runtime/doc/starting.txt
+++ b/runtime/doc/starting.txt
@@ -1366,6 +1366,9 @@ The $XDG_CONFIG_HOME, $XDG_DATA_HOME, $XDG_RUNTIME_DIR, $XDG_STATE_HOME,
 $XDG_CACHE_HOME, $XDG_CONFIG_DIRS and $XDG_DATA_DIRS environment variables
 are used if defined, else default values (listed below) are used.
 
+Throughout the help pages these defaults are used as placeholders, e.g.
+"~/.config" is understood to mean "$XDG_CONFIG_HOME or ~/.config".
+
 CONFIG DIRECTORY (DEFAULT) ~
                   *$XDG_CONFIG_HOME*            Nvim: stdpath("config")
     Unix:         ~/.config                   ~/.config/nvim
@@ -1392,10 +1395,12 @@ CACHE DIRECTORY (DEFAULT) ~
     Windows:      ~/AppData/Local/Temp        ~/AppData/Local/Temp/nvim-data
 
 LOG FILE (DEFAULT) ~
-                  `$NVIM_LOG_FILE`              Nvim: stdpath("log")
+                  `$NVIM_LOG_FILE`              Nvim: stdpath("log")/log
     Unix:         ~/.local/state/nvim         ~/.local/state/nvim/log
     Windows:      ~/AppData/Local/nvim-data   ~/AppData/Local/nvim-data/log
 
+Note that stdpath("log") is currently an alias for stdpath("state").
+
 ADDITIONAL CONFIGS DIRECTORY (DEFAULT) ~
                   *$XDG_CONFIG_DIRS*            Nvim: stdpath("config_dirs")
     Unix:         /etc/xdg/                   /etc/xdg/nvim
@@ -1407,18 +1412,14 @@ ADDITIONAL DATA DIRECTORY (DEFAULT) ~
                   /usr/share                  /usr/share/nvim
     Windows:      Not applicable              Not applicable
 
-Note: Throughout the help pages these defaults are used as placeholders, e.g.
-"~/.config" is understood to mean "$XDG_CONFIG_HOME or ~/.config".
-
-Note: The log file directory is controlled by `$XDG_STATE_HOME`.
-
 NVIM_APPNAME					*$NVIM_APPNAME*
 The standard directories can be further configured by the `$NVIM_APPNAME`
 environment variable. This variable controls the sub-directory that Nvim will
 read from (and auto-create) in each of the base directories. For example,
 setting `$NVIM_APPNAME` to "foo" before starting will cause Nvim to look for
 configuration files in `$XDG_CONFIG_HOME/foo` instead of
-`$XDG_CONFIG_HOME/nvim`.
+`$XDG_CONFIG_HOME/nvim`. `$NVIM_APPNAME` must be a name, such as "foo", or a
+relative path, such as "foo/bar".
 
 One use-case for $NVIM_APPNAME is to "isolate" Nvim applications.
 Alternatively, for true isolation, on Linux you can use cgroups namespaces: >
diff --git a/src/nvim/fileio.c b/src/nvim/fileio.c
index 7ff3e0ec6e..7609eb12b4 100644
--- a/src/nvim/fileio.c
+++ b/src/nvim/fileio.c
@@ -3279,12 +3279,18 @@ 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 };
 
   (void)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);
@@ -3298,7 +3304,6 @@ static void vim_mktempdir(void)
     // "/tmp/" exists, now try to create "/tmp/nvim.<user>/".
     add_pathsep(tmp);
 
-    const char *appname = get_appname();
     xstrlcat(tmp, appname, sizeof(tmp));
     xstrlcat(tmp, ".", sizeof(tmp));
     xstrlcat(tmp, user, sizeof(tmp));
diff --git a/src/nvim/main.c b/src/nvim/main.c
index 4b3244de12..720387cb17 100644
--- a/src/nvim/main.c
+++ b/src/nvim/main.c
@@ -251,7 +251,7 @@ int main(int argc, char **argv)
   argv0 = argv[0];
 
   if (!appname_is_valid()) {
-    os_errmsg("$NVIM_APPNAME is not a valid file name.\n");
+    os_errmsg("$NVIM_APPNAME must be a name or relative path.\n");
     exit(1);
   }
 
diff --git a/src/nvim/os/stdpaths.c b/src/nvim/os/stdpaths.c
index 53ddda22fa..b129eb445a 100644
--- a/src/nvim/os/stdpaths.c
+++ b/src/nvim/os/stdpaths.c
@@ -69,15 +69,23 @@ const char *get_appname(void)
   return env_val;
 }
 
-/// Ensure that APPNAME is valid. In particular, it cannot contain directory separators.
+/// Ensure that APPNAME is valid. Must be a name or relative path.
 bool appname_is_valid(void)
 {
   const char *appname = get_appname();
-  const size_t appname_len = strlen(appname);
-  for (size_t i = 0; i < appname_len; i++) {
-    if (appname[i] == PATHSEP) {
-      return false;
-    }
+  if (path_is_absolute(appname)
+      // TODO(justinmk): on Windows, path_is_absolute says "/" is NOT absolute. Should it?
+      || strequal(appname, "/")
+      || strequal(appname, "\\")
+      || strequal(appname, ".")
+      || strequal(appname, "..")
+#ifdef BACKSLASH_IN_FILENAME
+      || strstr(appname, "\\..") != NULL
+      || strstr(appname, "..\\") != NULL
+#endif
+      || strstr(appname, "/..") != NULL
+      || strstr(appname, "../") != NULL) {
+    return false;
   }
   return true;
 }
diff --git a/test/functional/core/fileio_spec.lua b/test/functional/core/fileio_spec.lua
index 8de78234ce..f2b360f0d8 100644
--- a/test/functional/core/fileio_spec.lua
+++ b/test/functional/core/fileio_spec.lua
@@ -377,4 +377,14 @@ describe('tmpdir', function()
     rm_tmpdir()
     eq('E5431: tempdir disappeared (3 times)', meths.get_vvar('errmsg'))
   end)
+
+  it('$NVIM_APPNAME relative path', function()
+    clear({ env={
+      NVIM_APPNAME='a/b',
+      NVIM_LOG_FILE=testlog,
+      TMPDIR=os_tmpdir,
+    } })
+    matches([=[.*[/\\]a%%b%.[^/\\]+]=], funcs.tempname())
+  end)
+
 end)
diff --git a/test/functional/options/defaults_spec.lua b/test/functional/options/defaults_spec.lua
index 9b531bcafc..7858b626de 100644
--- a/test/functional/options/defaults_spec.lua
+++ b/test/functional/options/defaults_spec.lua
@@ -598,8 +598,7 @@ describe('stdpath()', function()
   end)
 
   it('reacts to $NVIM_APPNAME', function()
-    local appname = "NVIM_APPNAME_TEST____________________________________" ..
-      "______________________________________________________________________"
+    local appname = 'NVIM_APPNAME_TEST' .. ('_'):rep(106)
     clear({env={ NVIM_APPNAME=appname }})
     eq(appname, funcs.fnamemodify(funcs.stdpath('config'), ':t'))
     eq(appname, funcs.fnamemodify(funcs.stdpath('cache'), ':t'))
@@ -616,10 +615,24 @@ describe('stdpath()', function()
 
     -- Check that Nvim rejects invalid APPNAMEs
     -- Call jobstart() and jobwait() in the same RPC request to reduce flakiness.
-    eq(1, exec_lua([[
-      local child = vim.fn.jobstart({ vim.v.progpath }, { env = { NVIM_APPNAME = 'a/b\\c' } })
-      return vim.fn.jobwait({ child }, 3000)[1]
-    ]]))
+    local function test_appname(testAppname, expected_exitcode)
+      local lua_code = string.format([[
+        local child = vim.fn.jobstart({ vim.v.progpath, '--clean', '--headless', '+qall!' }, { env = { NVIM_APPNAME = %q } })
+        return vim.fn.jobwait({ child }, %d)[1]
+      ]], alter_slashes(testAppname), 3000)
+      eq(expected_exitcode, exec_lua(lua_code))
+    end
+    -- Invalid appnames:
+    test_appname('a/../b', 1)
+    test_appname('../a', 1)
+    test_appname('a/..', 1)
+    test_appname('..', 1)
+    test_appname('.', 1)
+    test_appname('/', 1)
+    test_appname(is_os('win') and 'C:/a/b' or '/a/b', 1)
+    -- Valid appnames:
+    test_appname('a/b', 0)
+    test_appname('a/b\\c', 0)
   end)
 
   describe('returns a String', function()