vim-patch:8.1.0512: 'helplang' default is inconsistent for C and C.UTF-8

Problem:    'helplang' default is inconsistent for C and C.UTF-8.
Solution:   Don't accept a value unless it starts with two letters.
https://github.com/vim/vim/commit/389ab7122bec99c11ad4ce6d87cc6f38a21e4e40

1 files changed, 9 insertions, 4 deletions

diff --git a/src/nvim/ex_cmds2.c b/src/nvim/ex_cmds2.c
index 90fb7b8bc3..a148f51527 100644
--- a/src/nvim/ex_cmds2.c
+++ b/src/nvim/ex_cmds2.c
@@ -3506,7 +3506,12 @@ static char *get_locale_val(int what)
 }
 #endif
 
-
+// Return true when "lang" starts with a valid language name.
+// Rejects NULL, empty string, "C", "C.UTF-8" and others.
+static bool is_valid_mess_lang(char *lang)
+{
+  return lang != NULL && ASCII_ISALPHA(lang[0]) && ASCII_ISALPHA(lang[1]);
+}
 
 /// Obtain the current messages language.  Used to set the default for
 /// 'helplang'.  May return NULL or an empty string.
@@ -3526,14 +3531,14 @@ char *get_mess_lang(void)
 #  endif
 # else
   p = os_getenv("LC_ALL");
-  if (p == NULL) {
+  if (!is_valid_mess_lang(p)) {
     p = os_getenv("LC_MESSAGES");
-    if (p == NULL) {
+    if (!is_valid_mess_lang(p)) {
       p = os_getenv("LANG");
     }
   }
 # endif
-  return p;
+  return is_valid_mess_lang(p) ? p : NULL;
 }
 
 // Complicated #if; matches with where get_mess_env() is used below.