Fix IPv6 symmetric difference and add IPv6 multi-input tests

[ktsaou]

Summary

Fixes a correctness bug in IPv6 --diff (symmetric difference) and adds tests covering the IPv6 multi-input set operations, which were largely or entirely unexercised.

The bug

--diff is documented as symmetric difference — IPs in either set but not both. In IPv6 mode it wrongly included addresses common to both sets:

A = 2001:db8::1, 2001:db8::2, 2001:db8::5
B = 2001:db8::1, 2001:db8::3
iprange -6 A --diff B
  before:  2001:db8::1  2001:db8::2  2001:db8::3  2001:db8::5   # ::1 is common — wrong
  after:                2001:db8::2  2001:db8::3  2001:db8::5   # correct

The IPv4 path (ipset_diff) handles the same case correctly.

Root cause

In ipset6_diff(), when one range extended past the other, the code emitted the partial-overlap prefix, set the surviving range's lo to hi_other + 1, advanced the consumed index — and then unconditionally reloaded lo/hi for the range that did not advance, clobbering the adjusted lo. The common head of that range was then re-emitted. Both the hi1>hi2 and hi2>hi1 branches had the mirror defect.

The fix mirrors the IPv4 algorithm: keep the survivor's adjusted lo, advance/reload only the consumed range. (hi1>hi2 implies hi2<IPV6_ADDR_MAX, so the increment can't overflow and the previous max-guard is unnecessary.)

Verification

Beyond the new fixed cases, I ran differential/property fuzzing on the IPv6 engine (~25k checks, all passing after the fix):

• algebraic set identities on random native IPv6 sets — e.g. A△B = (A\\B)∪(B\\A), A∪B = (A∩B)∪(A△B), (A\\B)∩B = ∅;
• an independent oracle comparing the IPv6 engine against the well-tested IPv4 engine on embedded inputs;
• binary round-trip and count-unique invariants.

No other correctness issues were found.

Tests added (tests.d/95–100)

Multi-input IPv6 coverage for merge, common, exclude, symmetric difference, count-unique (merged) and compare-first. 98-ipv6-diff specifically guards against the regression (the common address must not appear in the result).

Coverage impact (line coverage)

file	before	after
ipset6_merge.c	0%	67%
ipset6_diff.c	34%	56%
ipset6_exclude.c	49%	64%
ipset6_common.c	56%	66%
iprange6_main.c	47%	53%

All 100 CLI tests pass.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 0 duplication

Metric	Results
Duplication	0

View in Codacy

🟢 Coverage 50.00% diff coverage · +2.21% coverage variation

Metric	Results
Coverage variation	✅ +2.21% coverage variation (-1.00%)
Diff coverage	✅ 50.00% diff coverage

View coverage diff in Codacy

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (5ed9649)	3310	2235	67.52%
Head commit (6b32487)	3304 (-6)	2304 (+69)	69.73% (+2.21%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#55)	4	2	50.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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