Refactor non-compliant/overlapping YANG definition for BGP MED

[earies]

• (M) release/models/bgp/openconfig-bgp-policy.yang
  • Remove string type from bgp-set-med-type union
  • Add new bgp-set-med-action typedef to separate out action on MED
    values
  • Add new set-med-action leaf to be optionally utilized with set-med

Change Scope

YANG 1.0 (RFC 6020) as well as YANG 1.1 (RFC 7950) specify that the legal
lexical representation for built-in integer types can be represented with an
optional sign.

https://www.rfc-editor.org/rfc/rfc6020.html#section-9.2.1

   An integer value is lexically represented as an optional sign ("+" or
   "-"), followed by a sequence of decimal digits.  If no sign is
   specified, "+" is assumed.

Example:

$ cat main.yang
module main {
  yang-version "1";
  namespace urn:m;
  prefix m;

  leaf value {
    type uint64;
  }
}

$ cat main.xml
<value xmlns="urn:m">+100</value>

$ yanglint -f xml main.xml main.yang
<value xmlns="urn:m">100</value>

The current union as specified interleaves a built-in integer type with a
string pattern regex that just so happens to overlap with allowable characters
for the uint32 type.

  type union {
    type uint32;
    type string {
      pattern '[+-][0-9]+';
      oc-ext:posix-pattern '^[+-][0-9]+$';
    }
    type enumeration {
      enum IGP {
        description "set the MED value to the IGP cost toward the
        next hop for the route";
      }
    }
  }

Therefore a "+100" would be matched by the uint32 and the "+" character
dropped. Since the integer is unsigned in this case, a "-100" would
fallthrough to the 2nd match.

Rather than encode the "action" and value into a string, this proposal suggests
separating out the action to a distinct optional node. It appears that this is
the only such usage where there is a string with a pattern statement that
overlaps that of another permissible type encoding when also used in a union
with that type.

Platform Implementations

This is a NBC however necessary to fix to remove ambiguity and improper handling

[dplore]

/gcbrun

[OpenConfigBot]

Major YANG version changes in commit 333b8ad:
openconfig-bgp-policy.yang: 7.1.0 -> 8.0.0

[dplore]

/gcbrun

[dplore]

This will be reviewed at the Aug 20, 2024 OC operators meeting

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[dplore]

I am extending last call to Oct 10th 2024 so this can be brought up at the Oct 3rd community meeting.

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