Function local read-only vs. global read-only variable with the same name

Question

I get a suprising behaviour when I have a function local read-only variable and global read-only variable with the same name.

When read-only option is removed from global declaration. I.e.

declare -r var="main"

is changed to:

declare var="main"

I get the expected behaviour. I've been reading bash man page but I can't find an explanation to this behaviour. Could you please point me to the section(s) of the manual explaining the issue ?

I think this is a similar kind of issue than How does lexical scoping supported in different shell languages? but more specific.

Details:

$ cat readonly_variable.sh 
#!/bin/bash

# expected output:
#
# BASH_VERSION = 3.2.25(1)-release
# function
# main
#
# but instead getting:
#
# BASH_VERSION = 3.2.25(1)-release
# ./readonly_variable.sh: line 6: local: var: readonly variable
# main
# main
#
# when read-only option (-r) is removed from global declaration (*), the output
# is expected

set -o nounset

function func {
  local -r var="function"
  echo "$var"
}

declare -r var="main" # (*)

echo BASH_VERSION = $BASH_VERSION
echo $(func)
echo $var

exit 0

I'm stucked to this particular Bash version.

$ ./readonly_variable.sh
BASH_VERSION = 3.2.25(1)-release
./readonly_variable.sh: line 24: local: var: readonly variable
main
main
$

Answer 1

Actually, making local copies of readonly global variables is explicitely forbidden for security reasons, as documented in bash source code (in variables.c:make_local_variable):

The test against old_var's context level is to disallow local copies of readonly global variables (since "I" believe that this could be a security hole).

(where "I" is not me, I'm merely quoting)

/* Since this is called only from the local/declare/typeset code, we can
   call builtin_error here without worry (of course, it will also work
   for anything that sets this_command_name).  Variables with the `noassign'
   attribute may not be made local.  The test against old_var's context
   level is to disallow local copies of readonly global variables (since I
   believe that this could be a security hole).  Readonly copies of calling
   function local variables are OK. */
if (old_var && (noassign_p (old_var) ||
   (readonly_p (old_var) && old_var->context == 0)))
{
  if (readonly_p (old_var))
    sh_readonly (name);
  return ((SHELL_VAR *)NULL);
}