fix(coverity): use xstrndup() instead of vim_strsave() (#18363)

src/nvim/api/autocmd.c

@@ -460,7 +460,7 @@ Integer nvim_create_autocmd(uint64_t channel_id, Object event, Dict(create_autoc
       cb.data.luaref = api_new_luaref(callback->data.luaref);
     } else if (callback->type == kObjectTypeString) {
       cb.type = kCallbackFuncref;
-      cb.data.funcref = vim_strsave((char_u *)callback->data.string.data);
+      cb.data.funcref = (char_u *)string_to_cstr(callback->data.string);
     } else {
       api_set_error(err,
                     kErrorTypeException,
@@ -474,7 +474,7 @@ Integer nvim_create_autocmd(uint64_t channel_id, Object event, Dict(create_autoc
     Object *command = &opts->command;
     if (command->type == kObjectTypeString) {
       aucmd.type = CALLABLE_EX;
-      aucmd.callable.cmd = vim_strsave((char_u *)command->data.string.data);
+      aucmd.callable.cmd = (char_u *)string_to_cstr(command->data.string);
     } else {
       api_set_error(err,
                     kErrorTypeValidation,
@@ -814,7 +814,7 @@ void nvim_exec_autocmds(Object event, Dict(exec_autocmds) *opts, Error *err)
       goto cleanup;
     }
 
-    pattern = vim_strsave((char_u *)opts->pattern.data.string.data);
+    pattern = (char_u *)string_to_cstr(opts->pattern.data.string);
     set_pattern = true;
   }
 

src/nvim/api/private/helpers.c

@@ -488,6 +488,16 @@ String cstr_to_string(const char *str)
   };
 }
 
+/// Copies a String to an allocated, NUL-terminated C string.
+///
+/// @param str the String to copy
+/// @return the resulting C string
+char *string_to_cstr(String str)
+  FUNC_ATTR_NONNULL_RET FUNC_ATTR_WARN_UNUSED_RESULT
+{
+  return xstrndup(str.data, str.size);
+}
+
 /// Copies buffer to an allocated String.
 /// The resulting string is also NUL-terminated, to facilitate interoperating
 /// with code using C strings.
@@ -628,7 +638,7 @@ void modify_keymap(uint64_t channel_id, Buffer buffer, bool is_unmap, String mod
                      (char_u *)rhs.data, rhs.size, lua_funcref,
                      CPO_TO_CPO_FLAGS, &parsed_args);
   if (opts != NULL && opts->desc.type == kObjectTypeString) {
-    parsed_args.desc = xstrdup(opts->desc.data.string.data);
+    parsed_args.desc = string_to_cstr(opts->desc.data.string);
   } else {
     parsed_args.desc = NULL;
   }

src/nvim/api/ui.c

@@ -261,7 +261,7 @@ static void ui_set_option(UI *ui, bool init, String name, Object value, Error *e
       api_set_error(error, kErrorTypeValidation, "term_name must be a String");
       return;
     }
-    set_tty_option("term", xstrdup(value.data.string.data));
+    set_tty_option("term", string_to_cstr(value.data.string));
     return;
   }
 

src/nvim/api/vim.c

@@ -2522,7 +2522,7 @@ Dictionary nvim_parse_cmd(String str, Dictionary opts, Error *err)
   // Parse command line
   exarg_T ea;
   CmdParseInfo cmdinfo;
-  char_u *cmdline = vim_strsave((char_u *)str.data);
+  char_u *cmdline = (char_u *)string_to_cstr(str);
 
   if (!parse_cmdline(cmdline, &ea, &cmdinfo)) {
     api_set_error(err, kErrorTypeException, "Error while parsing command line");

src/nvim/lua/executor.c

@@ -1647,9 +1647,7 @@ int nlua_expand_pat(expand_T *xp, char_u *pat, int *num_results, char_u ***resul
       goto cleanup_array;
     }
 
-    GA_APPEND(char_u *,
+    GA_APPEND(char_u *, &result_array, (char_u *)string_to_cstr(v.data.string));
-              &result_array,
-              vim_strsave((char_u *)v.data.string.data));
   }
 
   xp->xp_pattern += prefix_len;