[PATCH v1 0/5] fib6: fix tbl8 reservation drift

[Maxime Leroy]

This v1 supersedes the earlier RFC. The RFC dropped rsvd_tbl8s and
used tbl8_pool_pos in the pre-check, which loses the worst-case
envelope: a compressed /48 under a /28 allocates zero tbl8s but must
reserve the boundaries the /48 would need if the /28 is later
removed (DEL forces mid-flight decompression in modify_dp() with no
rollback).

This v1 keeps rsvd_tbl8s and computes it the way dir24_8 already
does for IPv4. dir24_8 counts /24 supernets that contain at least
one /25..32 prefix: that count is invariant under unrelated RIB
changes, so the counter cannot drift. trie6 has the same need at
13 levels instead of 1 (byte boundaries 24, 32, ..., 120), so v1
counts, for each L in that set, the /L supernets containing at
least one prefix with depth > L. ADD/DEL pairs are symmetric by
construction.

Patch 1 is the minimal self-contained fix (Fixes: + Cc: stable).
Patches 2-3 add the reproducer and extended regression tests.
Patches 4-5 are an optimization (not for stable): valid_descendants
in rte_rib6 + single-descent helper, so trie_modify() walks once
instead of up to 13 times per ADD/DEL.

Validated on a live BGP router (grout + FRR, 127 IPv6 prefixes):
RSVD_TBL8 returned to its pre-cycle value after a zebra-kill /
reconverge cycle.

Maxime Leroy (5):
  fib6: fix tbl8 reservation drift in trie
  test/fib6: add reproducer for tbl8 reservation drift
  test/fib6: extended drift test cases
  rib: track valid descendant count per node
  fib6: speed up tbl8 reservation accounting

 app/test/test_fib6.c    | 335 ++++++++++++++++++++++++++++++++++++++++
 app/test/test_rib6.c    |  92 +++++++++++
 lib/fib/trie.c          |  47 +-----
 lib/rib/rib6_internal.h |  37 +++++
 lib/rib/rte_rib6.c      |  80 ++++++++++
 5 files changed, 552 insertions(+), 39 deletions(-)
 create mode 100644 lib/rib/rib6_internal.h

---
v1:
* Keep rsvd_tbl8s; recompute it via topology-stable empty-supernet
  count (dir24_8 pattern at 13 levels) instead of RIB-derived
  depth_diff.
* Drop RFC patch 3/3 (no longer needed).
* Add extended regression tests.
* Add patches 4-5: RIB valid_descendants + single-descent helper
  (optional perf optimization; not for stable).
* Production-validated on a live BGP router.

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2.43.0