logging: move to XDG_CACHE_HOME (#13739)

while there is some controversy, stdpath('cache') looks like  a better fit for logs than stdpath('data'): you can remove logs without preventing nvim to work which fits the XDG_CACHE_HOME definition of `user specific non-essential data files`.

src/nvim/README.md

@@ -38,7 +38,7 @@ alternate file (e.g. stderr) use `LOG_CALLSTACK_TO_FILE(FILE*)`. Requires
 Many log messages have a shared prefix, such as "UI" or "RPC". Use the shell to
 filter the log, e.g. at DEBUG level you might want to exclude UI messages:
 
-    tail -F ~/.local/share/nvim/log | cat -v | stdbuf -o0 grep -v UI | stdbuf -o0 tee -a log
+    tail -F ~/.cache/nvim/log | cat -v | stdbuf -o0 grep -v UI | stdbuf -o0 tee -a log
 
 Build with ASAN
 ---------------

src/nvim/log.c

@@ -51,7 +51,7 @@ static bool log_try_create(char *fname)
 
 /// Initializes path to log file. Sets $NVIM_LOG_FILE if empty.
 ///
-/// Tries $NVIM_LOG_FILE, or falls back to $XDG_DATA_HOME/nvim/log. Path to log
+/// Tries $NVIM_LOG_FILE, or falls back to $XDG_CACHE_HOME/nvim/log. Path to log
 /// file is cached, so only the first call has effect, unless first call was not
 /// successful. Failed initialization indicates either a bug in expand_env()
 /// or both $NVIM_LOG_FILE and $HOME environment variables are undefined.
@@ -70,7 +70,7 @@ static bool log_path_init(void)
       || os_isdir((char_u *)log_file_path)
       || !log_try_create(log_file_path)) {
     // Invalid $NVIM_LOG_FILE or failed to expand; fall back to default.
-    char *defaultpath = stdpaths_user_data_subpath("log", 0, true);
+    char *defaultpath = stdpaths_user_cache_subpath("log");
     size_t len = xstrlcpy(log_file_path, defaultpath, size);
     xfree(defaultpath);
     // Fall back to .nvimlog

src/nvim/os/stdpaths.c

@@ -108,6 +108,17 @@ char *get_xdg_home(const XDGVarType idx)
   return dir;
 }
 
+/// Return subpath of $XDG_CACHE_HOME
+///
+/// @param[in]  fname  New component of the path.
+///
+/// @return [allocated] `$XDG_CACHE_HOME/nvim/{fname}`
+char *stdpaths_user_cache_subpath(const char *fname)
+  FUNC_ATTR_WARN_UNUSED_RESULT FUNC_ATTR_NONNULL_ALL FUNC_ATTR_NONNULL_RET
+{
+  return concat_fnames_realloc(get_xdg_home(kXDGCacheHome), fname, true);
+}
+
 /// Return subpath of $XDG_CONFIG_HOME
 ///
 /// @param[in]  fname  New component of the path.