Artifact 21ca364fb326dc7e17640f159248d4b11ddd5b61b9d62681f7dc51b68402979b:
Silent merge corruption is the worst failure a version-control system can have — it produces a tree that looks right — and merge3 had no fuzz target. This adds one, driving the merger with generated base/current/other triples and asserting the properties that must hold for any input.
Oracle 4 (no invented lines) needs a carve-out, because the merger is supposed to fabricate lines inside a conflict envelope. That carve-out is only sound if the recorded spans really are envelopes, so expectNoInventedLines validates the claim before subtracting anything: each span must be in-bounds, open at a line boundary with conflict_sentinel, close with conflict_terminator, start at or after the previous span's end, and carry no second terminator inside it.
The ordering and interior-terminator checks are load-bearing, not belt-and- braces. inAnySpan matches on overlap, so an over-long span exempts every output line up to its end. End-pinning alone admits a single span stretched from the first envelope's start to the last one's end — it opens and closes correctly and overlaps nothing — which would exempt the clean lines between two envelopes from the oracle entirely. closes_once catches that case on the first envelope's terminator. merge3 asserts ordering itself, but only under runtime_safety, which a ReleaseFast sweep — the build a fuzz run uses — erases; this loop holds in every optimize mode.
merge3.zig gains one export: conflict_terminator, the closing counterpart of the existing conflict_sentinel, with a comptime tripwire pinning its shape. Additive only, no behavior change — it lets the oracle pin both envelope ends against the module that produces them instead of re-typing the literal, so the two cannot drift apart.
Battery green in the worktree: zig fmt --check ., zig build, zig build test, zig build e2e, zig build dst (6 seeds x 1000 steps). The new target runs clean at seed 1 over 2000 rounds; no bug was found, so no regression seed is pinned. by dmestas on 2026-07-19 01:54:32.
C Add\smerge3_properties:\salgebraic-property\sfuzzing\sover\sthe\sthree-way\smerge\n\nSilent\smerge\scorruption\sis\sthe\sworst\sfailure\sa\sversion-control\ssystem\scan\shave\n—\sit\sproduces\sa\stree\sthat\slooks\sright\s—\sand\smerge3\shad\sno\sfuzz\starget.\sThis\nadds\sone,\sdriving\sthe\smerger\swith\sgenerated\sbase/current/other\striples\sand\nasserting\sthe\sproperties\sthat\smust\shold\sfor\sany\sinput.\n\nOracle\s4\s(no\sinvented\slines)\sneeds\sa\scarve-out,\sbecause\sthe\smerger\sis\ssupposed\nto\sfabricate\slines\sinside\sa\sconflict\senvelope.\sThat\scarve-out\sis\sonly\ssound\sif\nthe\srecorded\sspans\sreally\sare\senvelopes,\sso\sexpectNoInventedLines\svalidates\sthe\nclaim\sbefore\ssubtracting\sanything:\seach\sspan\smust\sbe\sin-bounds,\sopen\sat\sa\sline\nboundary\swith\sconflict_sentinel,\sclose\swith\sconflict_terminator,\sstart\sat\sor\nafter\sthe\sprevious\sspan's\send,\sand\scarry\sno\ssecond\sterminator\sinside\sit.\n\nThe\sordering\sand\sinterior-terminator\schecks\sare\sload-bearing,\snot\sbelt-and-\nbraces.\sinAnySpan\smatches\son\soverlap,\sso\san\sover-long\sspan\sexempts\severy\soutput\nline\sup\sto\sits\send.\sEnd-pinning\salone\sadmits\sa\ssingle\sspan\sstretched\sfrom\sthe\nfirst\senvelope's\sstart\sto\sthe\slast\sone's\send\s—\sit\sopens\sand\scloses\scorrectly\sand\noverlaps\snothing\s—\swhich\swould\sexempt\sthe\sclean\slines\sbetween\stwo\senvelopes\sfrom\nthe\soracle\sentirely.\scloses_once\scatches\sthat\scase\son\sthe\sfirst\senvelope's\nterminator.\smerge3\sasserts\sordering\sitself,\sbut\sonly\sunder\sruntime_safety,\swhich\na\sReleaseFast\ssweep\s—\sthe\sbuild\sa\sfuzz\srun\suses\s—\serases;\sthis\sloop\sholds\sin\nevery\soptimize\smode.\n\nmerge3.zig\sgains\sone\sexport:\sconflict_terminator,\sthe\sclosing\scounterpart\sof\sthe\nexisting\sconflict_sentinel,\swith\sa\scomptime\stripwire\spinning\sits\sshape.\sAdditive\nonly,\sno\sbehavior\schange\s—\sit\slets\sthe\soracle\spin\sboth\senvelope\sends\sagainst\sthe\nmodule\sthat\sproduces\sthem\sinstead\sof\sre-typing\sthe\sliteral,\sso\sthe\stwo\scannot\ndrift\sapart.\n\nBattery\sgreen\sin\sthe\sworktree:\szig\sfmt\s--check\s.,\szig\sbuild,\szig\sbuild\stest,\nzig\sbuild\se2e,\szig\sbuild\sdst\s(6\sseeds\sx\s1000\ssteps).\sThe\snew\starget\sruns\sclean\nat\sseed\s1\sover\s2000\srounds;\sno\sbug\swas\sfound,\sso\sno\sregression\sseed\sis\spinned. 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