fix(zbin): a nullable field must not inflate an object's minBytes

[Zixer1]

Summary

zbin's object codec computes each object's minBytes (its minimum possible
encoded size) by summing every non-optional field's minBytes — but it does
not exclude nullable fields. A nullable field encodes to zero value
bytes when null (its presence/null state rides the object's header bitmap), so
counting its non-null minBytes over-states the object's true minimum size.

That over-statement is unsound for the array decoder's DoS guard. readCount
rejects a length prefix when n * minElemBytes > remaining. So a valid,
compact array whose elements are mostly null — each smaller than the inflated
minBytes — is falsely rejected at decode with:

ZbDecodeError: $…: N elements exceeds the remaining M byte(s)

decodeClientMessageUnvalidated then throws, and the game server treats it as a
malformed frame and kicks the sender (INVALID_MESSAGE).

Impact

Any large array of objects with nullable fields that are null in practice hits
this. It surfaced with the admin-bot live-stats snapshot: PlayerLiveStats
has three nullable fields (killedBy / deathPosition / team) that are all
null before combat. Once a lobby had enough players, the first post-spawn
snapshot exceeded the inflated n * minElemBytes bound, every reporting client
was kicked, and the lobby emptied ~30 s in. (Games without liveStatsEnabled
were unaffected — they send no such frame.)

Root cause

zbin/zb.ts, object codec minBytes:

for (const p of plans) {
  if (p.mode === "body" && !p.optionalLike && p.codec !== null) {
    minBytes += p.codec.minBytes;   // nullable fields counted at non-null size
  }
}

minBytes must be a true lower bound on the encoded size for readCount
(n * minElemBytes > remaining) to be sound. A nullable field's minimum is 0
value bytes, so it must be excluded — exactly like an optional field.

Fix

Exclude nullable fields (nullBit >= 0) from the minBytes sum. Zero-width
elements remain bounded by the per-message element budget (takeItems), which
the existing zero-width tests already cover, so the DoS guard is unweakened for
the cases it was designed for.

Test

Adds a regression test in tests/zbin/hardening.test.ts: a compact 44-element
array of objects whose nullable fields are all null. It throws
N elements exceeds the remaining M byte(s) before the fix and round-trips
after. Full zbin suite green (152 tests).

How it was found

Bisected a lobby-collapse regression to the binary-protocol rollout, isolated it
to liveStatsEnabled with a live A/B test, then reproduced the exact decode
failure offline by re-simulating a real recorded game and round-tripping the
actual per-tick live-stats snapshots through this codec.

[coderabbitai]

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📒 Files selected for processing (2)

• tests/zbin/hardening.test.ts
• zbin/zb.ts

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• zbin/zb.ts
• tests/zbin/hardening.test.ts

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---

Walkthrough

The change updates readCount to avoid false rejection of compact arrays when minBytes overstates the smallest encoding. Regression tests verify round-trips for nullable, optional, and float-plus-nullables array elements.

Changes

Compact array decoding

Layer / File(s)	Summary
Count guard and regression coverage zbin/zb.ts, tests/zbin/hardening.test.ts	readCount compares the claimed count with remaining input bytes. The minBytes documentation describes its nonzero indicator use. Regression tests cover three compact array element shapes.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 1adaf

This change corrects nullable-field size accounting and adds regression coverage for compact object arrays; no actionable merge-blocking risk remains beyond normal checks and review.

Poem

Compact arrays pass the byte check
Nullable fields encode as light
Counts match the bytes that remain
Three shapes round-trip again
The decoder stays correct

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise and directly related to the nullable-field and minBytes false-rejection issue. The final implementation broadens the readCount guard instead of changing minBytes, but the title st…
Description check	✅ Passed	The description is directly related to the changeset and clearly explains the failure, impact, root cause, and intended fix. Some implementation details are outdated because the final patch changes re…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Title check

Explanation

The title is concise and directly related to the nullable-field and minBytes false-rejection issue. The final implementation broadens the readCount guard instead of changing minBytes, but the title still describes the addressed problem.

Full details: Description check

Explanation

The description is directly related to the changeset and clearly explains the failure, impact, root cause, and intended fix. Some implementation details are outdated because the final patch changes readCount and reverts the nullable minBytes change, but the description remains on topic.

• Fix all pre-merge checks with AI

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

---

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[evanpelle]

[
{
"file": "tests/zbin/fuzz.test.ts",
"line": 155,
"summary": "No test pins the soundness invariant serialize(v).length >= codec.minBytes; the fuzz suite masked this exact bug and the PR adds only a single-shape regression, so the next over-stated minBytes (any codec) will ship undetected.",
"failure_scenario": "Verified against pre-fix zb.ts: a bare zb.object({ xs: Mix.array() }) with 7 all-null 'mix' items throws '$.xs: 7 elements exceeds the remaining 21 byte(s)', yet the identical items inside FuzzSchema.items pass 2000 fuzz iterations because taggedUnionCodec's minBytes: 1 + minOf(codecs) (zb.ts:780) picks the smaller 'txt' variant and hides mix's inflated 4-vs-3. grep minBytes tests/ returns nothing. Also, the existing wire.test.ts live_stats fixture (2 players, 30-char gold) does NOT reproduce the incident even pre-fix; the real PlayerLiveStatsSchema only trips with a mapped ctx and a short gold (e.g. gold:'0' → '$#live_stats.stats.players: 1 elements exceeds the remaining 13 byte(s)'), so there is no wire-level regression for the actual failure either. A generic 'minimal witness' property (all-null/all-absent/empty values per builder, length >= minBytes) needs minBytes reachable from tests — it is module-private today (zbin/index.ts exports only the Codec type and zb)."
},
{
"file": "zbin/zb.ts",
"line": 627,
"summary": "readCount's precise per-element minBytes is a hand-maintained lower bound across ~9 codecs that only buys an earlier failure over the trivially-sound n > r.remaining; the PR fixes one arithmetic slip but leaves the mechanism that turns any future over-statement into a client kick.",
"failure_scenario": "Every codec with minBytes >= 1 reads >= 1 byte per element and ByteReader.need throws 'unexpected end of input' on underflow (bytes.ts:218-221); arrays are deliberately not preallocated (zb.ts:650-653) and takeItems charges MAX_DECODE_ITEMS before any element decodes (bytes.ts:230-236), so decode work is O(remaining) with or without the exact sum. The next layout change that lets a field write zero body bytes (e.g. small ints packed into the header, a sparse-object mode) re-inflates some object's minBytes and valid frames are again rejected as INVALID_MESSAGE (src/server/SocketIngress.ts:109). Clamping in readCount (Math.min(minElemBytes, 1)) or reducing minBytes to a 0/1 'consumes a byte' flag removes the class of bug; flagged as a scope-expanding design concern, not a defect in this PR."
},
{
"file": "zbin/zb.ts",
"line": 503,
"summary": "The new condition mixes schema-flag and header-bit vocabularies (!p.optionalLike && p.nullBit < 0) although FieldPlan already carries nullable: boolean, and keeps a p.mode === \"body\" test that p.codec !== null already implies.",
"failure_scenario": "FieldPlan declares nullable: boolean (zb.ts:434), enc already branches on it (if (!p.nullable) at :529), and nullBit: nullable ? bits++ : -1 (:485) makes p.nullBit < 0 exactly !p.nullable. codec is assigned only at :464/:474, both leaving mode 'body', so p.mode === \"body\" is redundant. The strictly equivalent one-liner if (p.codec !== null && !p.optionalLike && !p.nullable) avoids the prettier-forced five-line wrap and lets a reader see the two real conditions without cross-checking plan construction; minBytes is identical for every plan so no test changes."
},
{
"file": "zbin/zb.ts",
"line": 1048,
"summary": "zb.custom accepts a caller-supplied minBytes but neither its doc comment nor the README states the lower-bound contract or the consequence of over-stating it, so the same false-reject can be reintroduced from outside the library.",
"failure_scenario": "custom()'s comment (zb.ts:1044-1047) says only 'Register a hand-written codec ... returns a clone'; README mentions zb.custom only at lines 71/88/98 and grep minBytes zbin/README.md is empty; the Codec interface comment (:50-54) describes the role but not that over-stating rejects valid input and kicks the client. The repo's one custom codec (src/core/StatsSchemas.ts:111, minBytes: 1 over w.bigint) is sound, but an author copying that pattern for a codec that can emit zero bytes for some value would false-reject arrays of it with nothing in tests or docs flagging it. One sentence at the exported builder ('must not exceed the smallest encoding; 0 is always safe') closes this."
},
{
"file": "tests/zbin/hardening.test.ts",
"line": 32,
"summary": "The regression test is dressed as an incident replica (44 elements, five PlayerLiveStats-named fields, 8-line comment narrating the admin-bot kick) when the property is per-element and a 2-field, 1-element array discriminates identically.",
"failure_scenario": "Pre-fix rejection is n-independent: element = 10 bytes (1 header + 1 varint id + 8 f64) vs inflated min 13, so n=1 already throws '$.xs: 1 elements exceeds the remaining 10 byte(s)'; nothing in the test says 44 is incidental, so a reader will hunt for a threshold. zb.object({ a: zb.uint(), b: zb.uint().nullable() }) with [{ a: 1, b: null }] throws '1 elements exceeds the remaining 2 byte(s)' pre-fix and round-trips post-fix. The look-alike already diverges from the real schema (no clientID/gold/isAlive, uint vs MappedID) and the narrative (INVALID_MESSAGE, first post-spawn tick) will go stale unnoticed; sibling tests in the describe block use 1-3 line property comments and the file imports only ../../zbin, so a minimal synthetic shape is the file's convention."
},
{
"file": "zbin/zb.ts",
"line": 500,
"summary": "Lines 4-6 of the new minBytes comment restate readCount's guard expression and the false-reject narrative that the test comment also carries, so the same explanation now lives in three places.",
"failure_scenario": "The first three lines are the unique WHY (a null writes zero body bytes; not stated anywhere else near minBytes) and should stay. The last three ('makes the array readCount guard (n * minElemBytes > remaining) FALSE-REJECT a valid compact array...') literally copy the expression from readCount:629 and the sentence from hardening.test.ts:33-36; changing the guard's form later stales two comments. Multi-line rationale is house style in this file, so this is about duplication, not length."
}
]

[Zixer1]

Sounds good.
@evanpelle

pivoted to #2: readCount now bounds by n > remaining (any element that costs ≥1 byte), killing the whole over-stated-minBytes class rather than the one live_stats slip; reverted the per-field minBytes change.
That moots #1/#3/#6, resolves #4 (over-stating is now harmless — noted on Codec.minBytes), and #5 is minimized to a parametrized shape test. Amended in 3183b8b, zbin suite green (154).

[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@zbin/zb.ts`:
- Around line 55-57: Update the `custom()`/`readCount()` contract documentation
in `zb.ts` to state that any positive `minBytes` requires each encoded element
to consume at least one byte, since `readCount()` uses it to bound collection
counts against remaining input. Make the wording explicit near the `minBytes`
comment so custom codecs with zero-byte encodings are not treated as valid when
`minBytes > 0`, and add an empty-encoding test covering the `custom()` path to
verify such a codec is rejected or handled consistently.

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---

ℹ️ Review info

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Review profile: CHILL

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Run ID: 0543acc4-6d0a-456f-b576-5ad9e3464a25

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Reviewing files that changed from the base of the PR and between 011c29c and 1adafbe.

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• tests/zbin/hardening.test.ts
• zbin/zb.ts

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[github-actions]

🤖 Claude Code Review

Verdict: No issues found — the fix is sound. 0 blocking, 0 major, 0 minor findings.

Review notes

The shipped diff takes a different (and more robust) approach than the one described in the PR's Summary/Root cause/Fix sections: rather than excluding nullable fields from the object codec's minBytes sum, it changes readCount in zbin/zb.ts to only check whether minElemBytes is nonzero (n > r.remaining) instead of multiplying by the possibly-inflated exact value (n * minElemBytes > r.remaining). The PR description appears to predate this pivot (also noted by CodeRabbit's pre-merge check).

I independently traced every codec constructor in zbin/zb.ts (objectCodec, arrayCodec, recordCodec, taggedUnionCodec, untaggedUnionCodec, tupleCodec, presenceCodec, lazyCodec, leaf codecs) to verify the invariant the new guard relies on: minBytes > 0 must imply a real value of that codec always encodes to ≥ 1 byte. This holds throughout — in particular, objectCodec always allocates its header byte(s) whenever any field is optional/nullable/boolean (i.e., whenever the header contributes to minBytes), so a positive minBytes can never correspond to a true zero-byte encoding. This confirms:

• The fix eliminates the false-rejection bug (compact arrays of objects with nullable fields now round-trip).
• The weakened guard does not reintroduce the DoS risk it exists for: arrays are never preallocated from n, decode work stays bounded by actual remaining bytes, and the separate whole-message MAX_DECODE_ITEMS budget (bytes.ts) still caps total allocations regardless of this per-array check.

Four independent review passes (2x CLAUDE.md compliance, 2x bug-hunting, one with full file context) all reached the same conclusion: no CLAUDE.md violations, no significant bugs. The only sub-finding — that the new "optional field" test case in tests/zbin/hardening.test.ts happens to pass both before and after the fix (i.e., doesn't itself demonstrate the regression, unlike the nullable and live-stats-shape cases which do) — is a test-coverage nuance, not a functional defect, so it isn't listed as a finding here.