---
name: adversarial-validation
description: Execute the Adversarial Validation policy from the root AGENTS.md — run the applicable reviewer roles with RustFS-specific attack probes. Use on every behavior-affecting code change, bug fix, design proposal, or agent-instruction change that alters execution before declaring it done.
---

# Adversarial Validation Playbooks

The policy — risk tiers, role list, protocol, exit criteria — lives in the
root `AGENTS.md` under "Adversarial Validation (Default On)". Read it first;
this skill does not restate it. This file adds the RustFS-specific probe
playbook for each role: concrete attacks, where they apply, and the real
shipped bug or rule that earns each probe its place.

## How to run a role

1. Pick the tier and the applicable roles per the root `AGENTS.md`.
2. Run each role as an independent pass over the final diff — a parallel
   reviewer agent where the tooling supports it, otherwise a fresh
   sequential pass that starts from the diff and the nearest scoped
   `AGENTS.md`, discarding the writing session's assumptions.
3. Within a role, execute the probes whose domain the diff touches, plus any
   attack the diff obviously invites that no probe lists — the playbook is a
   floor, not a ceiling.
4. Report findings (concrete failure scenario or named missing test, with
   file:line) or the role's null report: "attacked X, Y, Z — no break
   found". A bare pass is not a result.

## Role playbooks

### Correctness adversary

- For any change touching error aggregation or quorum decisions, build the exact disk-error slice at the quorum boundary: N disks where successes == quorum, then flip one success to an error (quorum-1) and separately inject None/nil placeholder entries into the slice. Trace whether reduce_errs (or the new equivalent) picks the placeholder as the dominant error or lets quorum-1 pass as success. Also check heal/write paths: does a per-target failure at quorum-1 return an explicit error, or silently degrade to success?
  - Where: crates/ecstore/src/disk/error_reduce.rs; crates/ecstore/src/set_disk/{core,ops}; crates/heal
  - Evidence: Commit 20d61c73b 'stop reduce_errs leaking nil placeholder as dominant error' (#4551) and 47c1e730c 'make erasure heal write quorum best-effort per target' (#4545); crates/ecstore/AGENTS.md: 'Do not weaken quorum checks... Prefer explicit failure over silent data corruption or implicit success.'
- For any change in EC read/reconstruct/streaming code, trace the mid-stream error path: the first K shards read fine, then a shard turns out bitrot-corrupt or inconsistent after N bytes have already been sent to the client. Verify the error propagates as a stream error (client sees failure), not a clean end-of-body — a silently truncated GET body is data corruption. Also re-check byte accounting: sum of per-part bytes vs object size, and partNumber-to-offset routing for the first and last part.
  - Where: crates/ecstore/src/set_disk/read.rs (reconstruct-read, inconsistent_source_indexes handling ~line 3827); codec streaming paths in crates/ecstore/src/erasure_coding
  - Evidence: Known live bug: EC reconstruct-read failing on inconsistent shards mid-stream silently truncates GET body → client 'unexpected EOF'; commit 15808254d 'correct codec-streaming byte accounting and partNumber routing' (#4535).
- For any listing/pagination change, construct the exact-boundary inputs: (a) exactly max_keys/max_uploads matching entries — assert the response contains max and is_truncated=false, then max+1 entries — assert exactly max returned with is_truncated=true and a correct continuation marker; (b) a delimiter listing where folding into CommonPrefixes re-fills a full page; (c) an object 'a' coexisting with prefix dir 'a/'. Off-by-one and dropped-truncation bugs live exactly at these boundaries.
  - Where: crates/ecstore listing/merge paths (metacache, list_objects, list_multipart_uploads); rustfs/src/storage
  - Evidence: Three recent real bugs: fefa70b31 'stop ListMultipartUploads from returning one upload past max-uploads' (#4447), d91f4d455 'report truncation when delimiter list re-folds a full page' (#4538), 7e1f7f242 'preserve CommonPrefixes when an object and same-named prefix dir coexist' (#4563).
- Run the diff's logic with a directory object key (trailing slash, e.g. 'pre/dir/') as input. Check which layer encodes/decodes __XLDIR__ — set_disk never sees the trailing slash, so any trailing-slash branch added below the store layer is dead code and a wrong-layer bug. For delete paths, check whether options force a nil versionId onto directory keys: the resulting miss surfaces as version-not-found, not object-not-found, so callers matching only ObjectNotFound leak ghost directory entries.
  - Where: crates/ecstore/src/store*.rs (store layer) vs crates/ecstore/src/set_disk/*; delete option construction (del_opts) and its callers
  - Evidence: trailing-slash branches must live at store layer; del_opts injects nil version for dir keys, PR#4220 ghost-directory cleanup never fired on the real path (rustfs#4307, backlog#798 still OPEN).
- Feed every new binary-UUID metadata read the three degenerate values: key absent, zero-length bytes, and 16 zero bytes (nil UUID). All three must mean 'no value' — any path that produces Uuid::nil() and then acts on it (e.g. sends it as a versionId) is a finding. For tier code specifically: with remote-tier version None or "", assert the tier GET/DELETE request carries no versionId parameter at all — sending versionId="" or nil yields NoSuchVersion against unversioned tier buckets.
  - Where: crates/filemeta/src/filemeta/version.rs; crates/ecstore/src/bucket/lifecycle/; crates/ecstore/src/services/tier/; any new consumer of crates/utils/src/http/metadata_compat.rs
  - Evidence: AGENTS.md Cross-Cutting Domain Invariants (defensive Uuid read pattern + unversioned-tier rule); docs/operations/tier-ilm-debugging.md ('nil transition_ver_id = corrupt legacy write-back, readers must filter'); commit 726f3dc18 'accept empty remote version_id in tier recovery paths' (#4552).
- For each match/if-let on an error or algorithm enum touched by the diff, enumerate what the wildcard/else arm swallows. Inject the variants the author didn't think of — DiskNotFound during listing, an unsupported checksum algorithm, an Err from a cleanup rename/delete — and trace whether they degrade into 'not found', a wrong-but-plausible value, or silent success. Any error path that converges with the success path without logging and propagating is a finding.
  - Where: crates/ecstore (listing, delete/rename cleanup); crates/checksums; error-mapping layers in rustfs/src/storage
  - Evidence: Three recent real bugs of this exact shape: e0619e355 'stop treating DiskNotFound as object not-found in listing' (#4536), afaf8c681 'reject md5 instead of silently returning crc32' (#4513), f7d2b2563 'propagate disk delete/rename failures instead of swallowing them' (#4546).
- For any change to version ordering, index lookup, or shard/part indexing: (a) call the accessor with index == len() and len()-1 — a get_idx-style bound must reject, not panic or wrap; (b) construct two versions with identical mod-times and check the sort tie-break is total and deterministic (equal keys must not compare as both before each other); (c) for EC shard math, compute shard size for object sizes 0, 1, blockSize-1, blockSize, blockSize+1 and cross-check total reconstructed length against the object size.
  - Where: crates/filemeta/src/filemeta/*.rs (version sort, get_idx); crates/ecstore/src/erasure_coding shard-size math
  - Evidence: Commit 8bfb00bc0 'guard get_idx bound and fix sorts_before tie-break' (#4509) — both bug classes shipped before; ecstore AGENTS.md high-risk designation for read/write/repair correctness.
- Exercise the zero/empty end of every new size or count parameter: zero-length object PUT then GET (body must be empty, not error), part count 0, empty Vec of disks/entries into aggregation functions, and env/config values of 0 (must clamp or reject, never divide-by-zero or 'scan nothing and report zero usage'). Anywhere the diff computes a ratio, capacity, or progress percentage, plug in 0 and the max value.
  - Where: crates/ecstore aggregation and scanner paths; crates/object-capacity; config/env parsing in touched crates
  - Evidence: Commits 787cc77a7 'clamp zero capacity env values to safe defaults' (#4559) and 32b1094ec 'resolve a symlinked scan root instead of silently counting zero' (#4564) — zero-as-silent-wrong-answer is a recurring repo bug class.
- For any diff touching multipart or object commit paths, order the operations on paper and attack the failure point between them: kill the process (or return Err) after the commit rename but before cleanup, and after cleanup but before commit. Verify the earlier-failure case leaves the object readable and the later-failure case leaves no half-visible object; part meta files must never be deleted before the commit is durable.
  - Where: crates/ecstore multipart commit/cleanup (set_disk/ops); rustfs/src/storage multipart handlers
  - Evidence: Commit c77c5f047 'defer multipart part.N.meta cleanup until after commit' (#4548) — cleanup-before-commit ordering already caused a real data-loss window; the #4221 durability work shows fsync/ordering bugs are endemic here.

Null report example: "Attacked quorum-1 error reduction, exact max-keys listing boundary, trailing-slash dir keys, nil-UUID tier versionId, and mid-stream reconstruct error propagation — no break found."

### Simplicity adversary

- Smaller-diff attack: inspect production growth separately from tests, fixtures, generated code, and documentation; test additions have no growth budget. Rewrite the production diff mentally (or in scratch) as the minimal equivalent edit. Report a finding only with a concrete smaller design that preserves correctness, compatibility, readability, and real boundaries; fewer lines alone are not evidence.
  - Where: Any diff; extra scrutiny for crates/ecstore, crates/lock, rustfs/src/storage where 'preserve the existing control-flow shape' is an explicit rule
  - Evidence: AGENTS.md 'Change Style for Existing Logic' (conditional extraction rule, preserve sensitive control flow, canonical modules) and 'Reuse Before You Write'; the Adversarial Validation roles list charters this attack.
- Reuse-and-necessity attack: for each new helper, search `crates/utils`, `crates/common`, the touched crate, the likely domain owner, and relevant direct dependencies. A reimplementation is a finding, but forced reuse with mismatched normalization, error, backoff, or durability semantics is also a finding. Demand a nameable trigger for new defensive branches. Tests remain subject to validity and near-duplicate coverage review, never a size limit.
  - Where: Any diff adding helpers, branches on decoded/peer data, or tests
  - Evidence: AGENTS.md 'Reuse Before You Write' and 'Necessary Code Only'; GHSA-f4vq-9ffr-m8m3 (normalization-asymmetry traversal — why forced reuse of normalizing helpers on raw keys is itself an attack); docs/operations/tier-ilm-debugging.md nil-versionId incident (why boundary re-checks are load-bearing).
- Replacement-and-comment attack: when the diff introduces a replacement path or representation, trace all callers and flag a superseded in-scope path left behind without a compatibility requirement. Keep one canonical core behind compatibility adapters. Comments must state non-obvious invariants completely without narration or change history. Never demand unrelated deletion or trade away correctness, compatibility, or readability to reduce the diff.

Null report example: "Separated production growth from tests/docs, tested a smaller equivalent, checked helper reuse and superseded paths, and found no break."

### Security reviewer

- For every admin handler in the diff, grep the exact AdminAction constant it passes to validate_admin_request and confirm it names the operation the handler actually performs. Construct the escalation: a low-privileged user whose policy grants the wrong-but-adjacent action (e.g. Export while the handler Imports, or Update while it Lists) — if the mismatched constant lets them through, that is the bug. Also confirm read-only endpoints (metrics, list, server-info, diagnostics) still call an operation-specific admin authz path and not a mere 'credentials exist' check.
  - Where: rustfs/src/admin/handlers/**, rustfs/src/admin/router registration; check_permissions / validate_admin_request / AdminAction::* call sites
  - Evidence: GHSA-vcwh-pff9-64cc (ImportIam checked ExportIAMAction), GHSA-mm2q-qcmx-gw4w (ListServiceAccount used UpdateServiceAccountAdminAction), GHSA-f5cv-v44x-2xgf (/admin/v3/metrics accepted any authenticated IAM user). advisory-patterns.md 'Admin authorization and route exposure'; rustfs/src/admin/AGENTS.md 'route registration, whitelist, handler authz must agree'.
- If the diff touches service-account or IAM import/update, treat parent, claims, accessKey, secretKey, status, policy names, and groups as attacker-controlled. Construct an ImportIam/create payload where parent points at root (or another user) and prove the code writes credentials without proving caller ownership or root authority. Separately, set deny_only=true (or 'no explicit deny') on a restricted account and check it does not skip the required allow check, letting it mint an unrestricted child.
  - Where: crates/iam/, rustfs/src/admin/handlers (service account / import IAM), rustfs/src/auth.rs
  - Evidence: GHSA-566f-q62r-wcr8 (attacker-controlled parent/claims/accessKey/secretKey → persistent backdoor under root), GHSA-xgr5-qc6w-vcg9 (deny_only=true skipped allow checks → privilege creation). crates/iam/AGENTS.md security boundaries; advisory-patterns.md 'IAM import, service accounts'.
- For a changed protocol-frontend handler (FTP/FTPS/SFTP/WebDAV/gateway), enumerate ALL sibling command handlers in the same driver — not just the changed one. For each, confirm it calls the per-operation IAM authorize hook mapped to the correct S3 action (RETR→GetObject, SIZE/MDTM→HeadObject, MKD→CreateBucket, bucket probe→ListBucket/HeadBucket) BEFORE touching storage. Construct a denied-authz case and prove the storage backend is never reached.
  - Where: crates/protocols/ (FtpsDriver, SftpDriver, WebDAV), authorize_operation call sites
  - Evidence: GHSA-3g29-xff2-92vp (FTP RETR/SIZE/MDTM authenticated but skipped IAM), GHSA-g3vq-vv42-f647 (FTPS MKD called create_bucket without s3:CreateBucket). advisory-patterns.md: 'RustFS advisories show mixed guarded and unguarded siblings in the same driver.'
- For any secret/token/signature/password comparison in the diff, check it uses a constant-time compare (e.g. subtle/constant_time_eq), not == or early-return byte loops. Then check the failure-response paths: construct an invalid-user request and an invalid-secret request and confirm they are indistinguishable (same error, no early length short-circuit) so an attacker cannot enumerate valid users or time-side-channel the secret.
  - Where: crates/protocols/ (FTPS/WebDAV/FormPost auth), crates/credentials/, rustfs/src/auth.rs, RPC signature verification
  - Evidence: GHSA-3p3x-734c-h5vx (FTPS/WebDAV early-return string equality + distinguishable invalid-user vs invalid-password). Fix commits 3c3113619 (constant-time FTPS/WebDAV) and c41062f27 (constant-time FormPost signature). 3p3x was fixed by PR #4403.
- If the diff parses or transports secret-bearing config (env vars, key files, connection strings), grep every error-construction and format site on that value's path (`format!` feeding `Error::other`/`configuration_error`/`panic!`/`expect`) for interpolation of the raw value or of variables named like secret material. Construct the likeliest misconfiguration: the operator supplies the bare secret without the expected `<name>:` prefix (or with a stray newline) — if the parse-failure hint echoes the input, the secret lands in startup logs. Error strings are log content; the hint may name the env var and expected format, never the value. If the diff re-implements an existing parse helper, diff the two error paths — the duplicate is where the leak hides.
  - Where: rustfs/src/init.rs (env plumbing), crates/kms/src/config.rs, crates/credentials/, any from_env/parse on secret values; mechanical backstop in scripts/check_logging_guardrails.sh (secret-interpolation check)
  - Evidence: PR #5222 introduced `got: {secret_str}` in build_static_kms_config's format-hint error — a bare base64 key (the secret itself) would have been echoed into startup logs; fixed by PR #5243. The parallel parse in KmsConfig::from_env already omitted the value: the leak lived only in the duplicated copy (AGENTS.md 'Reuse Before You Write').
- If the diff touches internode/RPC auth secret handling, trace whether the RPC HMAC secret can fall back to a public default (e.g. 'rustfsadmin', 'rustfs rpc') or be derived deterministically from the S3 root credentials. Construct the case where RUSTFS_RPC_SECRET is unset and confirm the code fails closed rather than silently using a default or a root-derived key. Verify RPC signing keys are independent random secrets, not reused across S3-root/RPC-HMAC/STS-JWT roles.
  - Where: crates/credentials/, crates/ecstore/src/rpc/, internode auth setup
  - Evidence: GHSA-r5qv-rc46-hv8q (fell back to 'rustfsadmin'), GHSA-75fx/68cw (RPC secret derivable from root creds → forgeable signatures), GHSA-h956 (hard-coded 'rustfs rpc'), GHSA-m77q (STS JWT reused root secret). Fix commit 7b2055405 (fail closed when deriving RPC secret from default credentials, PR#4402).
- If the diff touches RPC/NodeService authentication, verify the HMAC payload binds the EXACT concrete gRPC method path (not a service prefix), the HTTP method surrogate, and a fresh timestamp. Construct captured valid metadata for method A and replay it to method B within the timestamp window — if it authorizes, the signature is under-bound. Also test stale timestamp, wrong path, wrong method, wrong secret.
  - Where: crates/ecstore/src/rpc/, verify_rpc_signature / NodeServiceServer, x-rustfs-signature handling
  - Evidence: GHSA-c667-rgrv-99vj (signed service prefix instead of concrete method path → cross-method replay in timestamp window). advisory-patterns.md 'RPC input validation and panic safety'; Minimum Regression Test Expectations lists replay across two methods.
- For any RPC/gRPC handler or deserialization touched, feed empty bytes, truncated MessagePack/protobuf, invalid enum discriminants, and stale timestamps. Grep the deserialization path for unwrap()/expect()/panic-prone decode and prove malformed attacker payloads return a typed error, not a panic (remote DoS). Weak internode auth makes reachability worse, so combine with the RPC-secret probe.
  - Where: crates/ecstore/src/rpc/, any #[derive(Deserialize)] decoded from wire bytes, RPC handler bodies
  - Evidence: GHSA-gw2x-q739-qhcr (malformed GetMetrics reached unwrap() → remote DoS). advisory-patterns.md 'Treat all RPC payload bytes as attacker-controlled.' rust-code-quality skill: unwrap abuse.
- Take any object key, RPC disk path, or archive/tar/zip entry name introduced or handled in the diff and construct traversal payloads: '../', URL-encoded '%2e%2e%2f', absolute paths, platform separators, empty components. Trace the value through parse → authz check → final storage path and prove (a) authz and storage normalize the SAME way, and (b) the canonicalized path cannot escape the bucket/prefix root. Attack the case where authz sees the raw attacker bucket but storage cleaning crosses into a victim bucket.
  - Where: crates/ecstore (path join/canonicalize), rustfs/src/storage/, Snowball auto-extract / normalize_extract_entry_key, rpc read_file_stream
  - Evidence: GHSA-pq29-69jg-9mxc (read_file_stream joined untrusted paths, no canonical boundary check), GHSA-8r6f-hmq2-28rg (traversal object keys bypassed authz), GHSA-f4vq-9ffr-m8m3 (Snowball '../victim-bucket/object' authorized raw path then storage crossed boundary). Note __XLDIR__ trailing-slash encoding is store-layer only.
- If the diff touches multipart/copy or presigned POST, verify UploadPartCopy enforces source GetObject AND destination PutObject semantics equivalent to CopyObject, including copy-source policy conditions (not just independent source-read + dest-write). Construct a cross-bucket UploadPartCopy from a bucket the caller cannot read. For presigned POST, submit an upload that violates content-length-range, key prefix, or exact content-type and prove the server rejects it.
  - Where: rustfs/src/storage / S3 API handlers: upload_part_copy, CompleteMultipartUpload, PostObject policy enforcement
  - Evidence: GHSA-mx42-j6wv-px98 (UploadPartCopy missed source authz → cross-bucket exfil), GHSA-wfxj-ph3v-7mjf (missed destination copy-source policy constraint), GHSA-w5fh-f8xh-5x3p (presigned POST didn't enforce signed policy conditions). advisory-patterns.md 'S3 copy, multipart, and upload policy validation'.
- Grep the diff for debug!/trace!/info!/error! and any ?value / {:?} on structs or response bodies that can carry secret_key, session_token, JWT claims, HMAC secrets, expected signatures, access keys beyond safe identifiers, or raw credential-bearing responses. Check custom Debug impls and merged-config dumps too. Construct the error path (invalid signature, failed auth) and confirm it does not log the secret or the derived authenticator. Verify audit/notify entries redact credential request headers.
  - Where: rustfs/src/**, crates/iam/, crates/audit/, crates/notify/, crates/targets/, RPC signature error paths
  - Evidence: GHSA-r54g (STS creds logged at info), GHSA-8cm2 (debug logs leaked tokens/secrets/JWT claims/raw STS bodies), GHSA-333v (invalid RPC signature log included HMAC secret + expected signature). Fix commit ee6f79110 (redact credential request headers from audit/notify, backlog#963). rustfs-logging-governance skill.
- For any struct in the diff deserialized from untrusted input (S3 XML/JSON, lifecycle rules, bucket policy, replication config, RPC payload), check for #[serde(deny_unknown_fields)]. Construct a payload with a typo'd field (e.g. 'NoncurentDays') or an extra field and prove it is rejected, not silently ignored. Flag #[serde(default)] on security-critical fields (retention days, limits, permissions) lacking explicit post-deserialize validation, and any user-controlled integer cast with `as` (i32 as u32) — feed a negative value and check it doesn't wrap to a huge positive.
  - Where: crates/policy/ (bucket policy), crates/ecstore lifecycle/ILM config, replication config structs, crates/protocols XML parsing
  - Evidence: AGENTS.md 'Serde Safety'; advisory-patterns.md 'Serde deserialization' (no deny_unknown_fields found repo-wide; 'NoncurentDays' typo silently accepted; i32 as u32 wrap). Fix commit 1acd47f15 (SSE crash-loop DoS + credential reserved-char bypass, backlog#806).
- If the diff touches SSE / encryption reader-writer composition, do not trust API metadata claiming encryption. Trace the reader wrapper order (HashReader / EncryptReader / compression / warp) and confirm EncryptReader is actually in the chain that writes to disk — construct the case where a helper unwraps a nested reader and bypasses encryption, storing plaintext. Require a regression test that inspects the ACTUAL stored bytes on disk, not just read-back. Also check encrypted-object checksums are not exposed.
  - Where: rustfs/src/storage/ecfs.rs, crates/rio/ (reader wrappers), crates/kms/, SSE-C replication
  - Evidence: GHSA-xrrf-67jm-3c2r (SSE metadata reported encryption while composition bypassed EncryptReader, stored plaintext). Fix commits a7b9659e7 (hide encrypted object checksums, #4529), 80cc3b1fc (preserve SSE-C checksum state, #4410). advisory-patterns.md 'SSE and on-disk storage invariants'.
- If the diff touches CORS or the console/browser/object-preview surface: confirm default CORS does not reflect an arbitrary Origin while also sending Access-Control-Allow-Credentials: true — construct a request with a spoofed Origin and check the response. For preview, confirm attacker-controlled object content is origin-isolated (not rendered in a same-origin iframe with console creds), served with nosniff/CSP, and that preview trust derives from validated content-type + sandboxing, NOT from object name/extension (.pdf, .html). Separately, if aws:SourceIp is evaluated, spoof X-Forwarded-For / X-Real-IP as a direct (non-trusted-proxy) client and confirm the socket peer IP is used instead.
  - Where: rustfs/src/server/layer.rs (CORS), console preview/auth code, aws:SourceIp / policy condition evaluation, X-Forwarded-For handling
  - Evidence: GHSA-x5xv-223c-8vm7 (default CORS reflected arbitrary origins with credentials), GHSA-v9fg-3cr2-277j (preview rendered attacker HTML same-origin, exposed localStorage creds), GHSA-7gcx-wg4x-q9x6 (extension-based PDF detection bypassed sandbox), GHSA-fc6g-2gcp-2qrq (aws:SourceIp trusted client XFF). advisory-patterns.md 'Browser, CORS' + 'Trusted proxy'.

Null report example: "Attacked admin action-constant matching in the two changed handlers (both call validate_admin_request with the exact AdminAction), the FTPS RETR/SIZE authz parity, and the new lifecycle struct's serde surface (has deny_unknown_fields) — no break found."

### Concurrency/durability reviewer

- For every new or moved lock acquisition, enumerate all other code paths that take any overlapping subset of those locks and construct the concrete ABBA interleaving (thread 1 holds A wants B, thread 2 holds B wants A). If the diff acquires 2+ locks without a comment documenting acquisition order, that alone is a finding.
  - Where: crates/ecstore/** (namespace locks, set_disk, disk registry), crates/audit/**, crates/lock/**, any Mutex/RwLock pair in a diff
  - Evidence: crates/ecstore/AGENTS.md 'Lock Ordering' (document order; same set in different orders = deadlock); real ABBA deadlock fixed in c0d5f938f (#4421, audit registry vs stream_cancellers)
- If the diff touches the object write/commit path or a lock guard's lifetime, construct the timeline where the distributed lock is lost (heartbeat refresh fails / expiry) after shard writes but before the xl.meta rename commit — verify the commit is fenced on guard.is_lock_lost() (set_disk/ops/object.rs:874-880) and the diff does not move the commit outside the fenced region or drop the guard early.
  - Where: crates/ecstore/src/set_disk/ops/object.rs, ops/multipart.rs, crates/lock/**
  - Evidence: 1e6207c08 (#4406) fence write commit on lock loss; ddf197ba5 (#4388) heartbeat lock refresh; backlog#899 fencing comment in object.rs
- For any change to file creation or write-then-rename: write out the exact syscall order (write tmp -> fdatasync tmp -> rename -> fsync parent dir -> fsync ancestor dirs on first object under a prefix) and simulate a power cut after each step. Flag any dropped/reordered sync, and check the change honors the durability gate (RUSTFS_DRIVE_SYNC_ENABLE, strict/relaxed/none modes, per-bucket overrides) instead of hardcoding one mode. The 'skip tmp parent fsync' optimization is only sound when the file is renamed out of tmp — verify that precondition still holds.
  - Where: crates/ecstore/src/disk/local.rs, disk/os.rs, disk/fs.rs, crates/ecstore/src/bucket/durability.rs, set_disk/core/io_primitives.rs
  - Evidence: PR #4221 (the repo previously had no fsync anywhere); 2df315baf/c081586e7 (#4493) fsync ancestor dirs; 062a68d15 (#4387) tmp-parent-fsync skip is rename-conditional; eaff17cad (#4397) durability modes; 54872d52d (#4478) rename_data crash harness exists — extend it for the diff
- If the diff touches quorum counting or per-disk error aggregation, construct adversarial error vectors for reduce_errs: nil/placeholder entries, DiskNotFound mixed with FileNotFound, exactly quorum-1 agreeing errors — and show which dominant error wins. Specifically attack the case where offline-disk errors get counted as 'object does not exist', flipping a read/heal decision into data loss. Also check quorum monotonicity: a retry or heal pass must never conclude with a LOWER quorum than the original write.
  - Where: crates/ecstore/src/disk/error_reduce.rs, crates/ecstore/src/api/mod.rs, set_disk read/heal paths
  - Evidence: 20d61c73b (#4551) reduce_errs leaked nil placeholder as dominant error; e0619e355 (#4536) DiskNotFound treated as object-not-found in listing; quorum monotonicity flagged as open follow-up to #4221 (#4221 follow-up list); crates/ecstore/AGENTS.md 'Do not weaken quorum checks'
- For any multi-disk fan-out (delete, rename, heal write, cleanup), trace each per-disk Result: find any `let _ =`, `.ok()`, or best-effort collapse that keeps a failed disk out of the quorum math. Construct the run where exactly write_quorum-1 disks succeed and prove the op still returns success — that is the bug. Conversely, for heal writes, check one bad target cannot fail the whole heal (best-effort per target).
  - Where: crates/ecstore/src/set_disk/ops/*.rs, disk/disk_store.rs, crates/heal/**
  - Evidence: f7d2b2563 (#4546) disk delete/rename failures were swallowed; 47c1e730c (#4545) heal write quorum made best-effort per target; 2b063b0c4 (#4400) disk-replacement heal missed versions
- For every new .await placed between a state mutation and its cleanup/commit (or inside select!/timeout/spawned task that can be aborted), construct the cancellation point: client disconnects and the future is dropped exactly there. Enumerate what is left behind — tmp files, incremented counters never decremented, half-written xl.meta, a held permit/waiter — and verify cleanup runs in Drop or the state is re-entrant. Background loops the diff adds must have a hard outer timeout so a wedged awaitee cannot pin them forever.
  - Where: crates/ecstore/** write paths, crates/object-capacity/** scanners, crates/audit/**, anything using tokio::select! or spawn+abort
  - Evidence: d608e320f io_uring cancel-safety spike (cancellation known-hard here); 5a372557e (#4533) wedged scans needed hard outer timeout; 7b87d4d13 (#4520) waiter-count leak; e44bece00 (#4497) audit start race + paused drops
- If the diff touches metacache/list producers or cursor resume, construct the interleaving where the producer task completes (or errors) while a reader with a saved cursor comes back for the next page — verify a completed producer is tolerated (no error, no hang) and that a re-folded/full page reports truncation instead of silently ending the listing early. Also feed a corrupt/oversized length prefix into any metacache decode the diff touches.
  - Where: crates/ecstore/src/cache_value/metacache_set.rs, crates/ecstore/src/store/list_objects.rs, crates/filemeta/**
  - Evidence: 91a23361e (#4531) tolerate completed metacache producers; d91f4d455 (#4538) delimiter re-fold dropped truncation flag; d2c100fd3 (#4226) corrupt length-prefix guard
- For multipart changes, construct concurrent operations on the SAME uploadId: put_object_part racing put_object_part (same part number), abort racing complete between the parts listing and the commit rename, and list-parts racing cleanup. Verify every metadata read/list/abort holds the per-uploadId lock, and that part.N.meta cleanup is deferred until AFTER the commit rename — cleanup before commit loses parts on a crash between the two.
  - Where: crates/ecstore/src/set_disk/ops/multipart.rs, rustfs/src/storage/
  - Evidence: 3bc8d79fe (#4329) serialize put_object_part per uploadId; 7fb95d4fc (#4428) unlocked upload metadata reads/aborts; 93ffbdb9b (#4437) unlocked part listings; c77c5f047 (#4548) part.N.meta cleanup moved after commit
- Any cleanup/rollback logic added near a commit: verify it runs strictly AFTER the commit is durable, is best-effort (its failure must not fail an already-committed write), and is safe under retry — i.e., re-running it after a partial first attempt must never delete the newly-committed data dir or the last surviving copy.
  - Where: crates/ecstore/src/set_disk/ops/object.rs (rename_data tail), ops/multipart.rs, disk cleanup helpers
  - Evidence: afc7f1d6f/d908243e6 (#4386, backlog#898) post-commit old-data-dir cleanup had to be made best-effort; e7cc719c1 (#4389) speculative tmp cleanup moved off hot path
- If the diff does read-modify-write on any persisted shared state (bucket metadata, notify/target config, queue store), construct two concurrent writers: show whether the second write silently discards the first (lost update) — RMW must be serialized or CAS-guarded. For persisted queues/replay, construct crash-mid-replay and prove entries are neither lost nor delivered twice without an idempotency key.
  - Where: crates/notify/**, crates/targets/** (queue store, SQL backends), crates/ecstore/src/bucket/metadata_sys.rs
  - Evidence: 2490d4ee2 (#4425) persisted config RMW lost updates; 08e44b95f (#4505) queue store crash-safety + replay lifecycle; e008cc5da (#4500) SQL backend idempotency
- If the diff touches erasure decode/reconstruct or streaming GET, construct the failure mid-stream: shards become inconsistent (or a disk read fails) after N bytes of the body have already been sent — verify the stream surfaces an error to the client instead of ending cleanly at a truncated length. Silent truncation on a 200 response is the known failure mode.
  - Where: crates/ecstore/src/set_disk/read.rs (reconstruct-read validation, historically ~line 3117), crates/ecstore/src/erasure/coding/decode.rs
  - Evidence: Known open bug: EC reconstruct-read 'inconsistent shards' mid-GET silently truncates body -> client unexpected EOF; crates/ecstore/AGENTS.md 'explicit failure over silent corruption'

Null report example: "Attacked lock ordering on the new disk-registry mutex pair, lock-loss fencing across the moved commit, power-cut points around the added rename, and cancellation at the two new awaits — no break found; quorum math and metacache paths untouched by this diff."

### Compatibility reviewer

- Grep the diff for raw 'x-rustfs-internal-' or 'x-minio-internal-' string literals used with map.insert/remove/get instead of the metadata_compat helpers. If found, construct the MinIO-written object case: a metadata map containing ONLY 'X-Minio-Internal-<suffix>' (mixed case, no RustFS key) and trace the diff's read path — does it miss the value? Then construct the removal case: does remove leave the twin key behind so a stale MinIO-key value resurrects on next read? Also check the value-type trap: get_bytes has NO case-insensitive fallback (unlike get_str), so a diff that moves a suffix from FileInfo.metadata (String) to meta_sys (Vec<u8>) silently loses mixed-case MinIO keys.
  - Where: Any code touching FileInfo.metadata / user_defined / meta_sys: crates/ecstore/, crates/filemeta/, rustfs/src/storage/, crates/utils/src/http/metadata_compat.rs
  - Evidence: Repo-wide invariant in AGENTS.md 'Cross-Cutting Domain Invariants' and CLAUDE.md; helpers and the asymmetry are pinned by tests test_str_lookup_accepts_minio_metadata_case and test_get_bytes_no_case_insensitive_fallback in crates/utils/src/http/metadata_compat.rs
- For any diff reading a binary UUID from internal metadata (transitioned-versionID, tier-free-versionID, data_dir), trace the three degenerate inputs — key absent, value empty (b""), value nil UUID — through to the outgoing tier/S3 request. The bug shape to hunt: unwrap_or_default() or Uuid::from_slice(..).unwrap_or(Uuid::nil()) turning 'no value' into Uuid::nil(), which then gets serialized as ?versionId=00000000-... and the remote tier returns NoSuchVersion. The required pattern is .and_then(|v| Uuid::from_slice(&v).ok()).filter(|u| !u.is_nil()).
  - Where: crates/ecstore/src/bucket/lifecycle/ (bucket_lifecycle_ops.rs, tier_sweeper.rs), crates/ecstore/src/services/tier/warm_backend_*.rs, crates/filemeta/src/filemeta/version.rs
  - Evidence: Historical production bug documented in docs/operations/tier-ilm-debugging.md ('Nil-UUID versionId sent to tier'); regression tests live in crates/filemeta/src/filemeta/version.rs; follow-up fix 726f3dc18 'accept empty remote version_id in tier recovery paths' (#4552)
- For any diff touching tier or replication GET/DELETE against a remote S3 target, enumerate BOTH directions of the versionId contract and trace each: (a) remote version None/"" means the tier bucket is unversioned — the request must carry NO versionId parameter at all (not an empty one, not nil); (b) remote version Some(v) on a versioned target — the versionId MUST be sent, especially on version-purge deletes, or the delete lands on the wrong version / creates a delete marker instead of purging. Check whether the diff collapses these cases through a single Option/String conversion that loses the distinction.
  - Where: crates/ecstore/src/services/tier/warm_backend*.rs, crates/ecstore/src/bucket/lifecycle/tier_sweeper.rs, replication code under crates/ecstore/src/bucket/
  - Evidence: Invariant in AGENTS.md and docs/operations/tier-ilm-debugging.md; real bug fixed by 0fad35645 'send versionId on version-purge deletes to generic S3 targets' (#4401) — the versioned direction, and #4552 — the unversioned direction
- If the diff changes xl.meta encoding (adds/reorders msgpack header fields, touches FileMeta::marshal_msg or codec.rs encode paths), attack downgrade and cross-vendor parse: encode an object with the new code and decode it with (a) the meta_ver<=3 read path and (b) the real-MinIO fixture tests from #4377. Then check the header signature: is it recomputed over the new bytes, or copied/hardcoded? MinIO validates it; a stale or zero signature makes MinIO reject the file. Finally verify XL_META_VERSION was not silently bumped — old RustFS/MinIO nodes reject meta_ver > 3 during a rolling upgrade.
  - Where: crates/filemeta/src/filemeta.rs (XL_HEADER_VERSION/XL_META_VERSION, lines ~46-54), crates/filemeta/src/filemeta/codec.rs (check_xl2_v1, decode_xl_headers)
  - Evidence: Real bug 073bc9675 'compute header signature instead of hardcoding zero' (#4343); format contract pinned in docs/architecture/minio-file-format-compat.md (write meta_ver 3, read <=3, XL2 magic); parity fixtures from a91d9cefc (#4377)
- If the diff touches xl.meta / FileInfo decode (into_fileinfo, version parsing, part arrays), construct hostile foreign input: a MinIO- or corruption-shaped msgpack with a parts-count that disagrees with the etags/sizes array lengths, missing optional fields, and a meta_ver 2 object with legacy checksum. Trace whether the new code indexes past an array, panics, or fabricates default values instead of returning a decode error. Run the pinned legacy fixtures (test_issue_2265_legacy_meta_v2_object_compatibility, test_issue_2288) plus the #4377 real-MinIO xl.meta parse tests against the diff.
  - Where: crates/filemeta/src/ (fileinfo.rs, filemeta.rs, filemeta/codec.rs, filemeta/version.rs)
  - Evidence: Real bug 7efacbdf9 'validate part array lengths in into_fileinfo' (#4382); legacy meta_ver 2 regression fixtures at crates/filemeta/src/filemeta.rs (~:1130-:1174) cited by docs/architecture/minio-file-format-compat.md
- If the diff 'corrects' a formula, constant, or layout that is a byte-for-byte MinIO port (shard-size math, bitrot hash interleaving, erasure distribution, inline-data prefix), treat the correction itself as the bug: verify against legacy on-disk data before accepting. Concretely: run crates/ecstore/tests/legacy_bitrot_read_test.rs and the ECA-18 pinning tests; check whether existing objects written by old RustFS or MinIO still verify byte-for-byte. Known trap examples: bitrot_shard_file_size's bare return for non-streaming algorithms is CORRECT MinIO whole-file behavior, and the 32-byte prefix on inline data is the HighwayHash256 bitrot hash, not corruption.
  - Where: crates/ecstore/src/erasure/coding/bitrot.rs, crates/ecstore/src/io_support/bitrot.rs, crates/filemeta/src/fileinfo.rs
  - Evidence: f96314a1d 'pin streaming-only bitrot layout invariant (ECA-18)' (#4553) — audit explicitly decided NOT to change the formula because it breaks legacy interop; #4377 proved the inline-data prefix is the bitrot hash
- For any diff that copies object metadata into an S3-client-visible surface (GET/HEAD response headers, notification event userMetadata, ListObjects/replication payloads, copy-object metadata directives), construct an object carrying internal keys under BOTH prefixes and in non-canonical casing ('X-Minio-Internal-Compression') and confirm every one is stripped via is_internal_key (which is case-insensitive) — not by an exact-match filter on one prefix. Leaked internal keys are an API-semantics break and an information leak.
  - Where: rustfs/src/storage/, crates/notify/, replication and copy_object paths in crates/ecstore/
  - Evidence: Real bug cf8929189 'strip rustfs/minio internal metadata from event userMetadata' (#4419); is_internal_key contract in crates/utils/src/http/metadata_compat.rs
- If the diff touches crates/protos (node.proto, models.fbs) or internode RPC request/response structs, attack the rolling-upgrade interleaving: an old node sends a message without the new field to a new node, and a new node sends the extended message to an old node. Verify proto field numbers are only appended (never reused/renumbered), FlatBuffers tables are only extended at the end, and that an absent new field decodes to a safe default on the receiving side — 'safe' meaning it must not be interpreted as success/authorization (RPC errors fail closed) and must not flip a quorum decision.
  - Where: crates/protos/ (node.proto, models.fbs, generated/), gRPC transport and dispatch in the internode layer
  - Evidence: 6f613317f 'optimize gRPC transport' (#4337) shows the wire layer churns; security advisory 68cw fixed by PR#4402 established RPC fail-closed as a repo rule (see .agents/skills/security-advisory-lessons)
- For S3 handler diffs, replay the request shapes real clients actually send, not just the canonical one: mc and aws-sdk differ on path normalization (root '//' ListBuckets), virtual-host vs path style, and header casing. Then attack every pagination boundary the diff touches: request exactly max-keys/max-uploads/max-parts items and verify the response returns exactly N (not N+1), sets IsTruncated correctly, and yields a NextMarker/KeyMarker that resumes without skipping or duplicating — construct the N+1st-item case explicitly.
  - Where: rustfs/src/storage/ S3 handlers, listing paths in crates/ecstore/src/store/ and set_disk/
  - Evidence: Real bugs 511ad31ba 'normalize root double-slash ListBuckets requests' (#4336) and fefa70b31 'stop ListMultipartUploads from returning one upload past max-uploads' (#4447); docs/architecture/s3-compatibility-matrix.md is the compat source of truth
- If the diff changes bucket-metadata (.metadata.bin) or IAM/config parsing structs, run it against the real MinIO RELEASE.2025-07-23 fixtures: the msgpack blob uses PascalCase field names, so any serde rename, field-type change, or derive tweak silently drops MinIO-written fields instead of erroring. Verify parse_all_configs still loads all ten config types from the fixture without loss, and that drop-in migration still decrypts MinIO-encrypted IAM/server config rather than treating ciphertext as corrupt.
  - Where: crates/ecstore/src/bucket/metadata*, crates/ecstore/src/bucket/migration.rs, IAM/config load paths in crates/iam/ and crates/config/
  - Evidence: Fixture parity test parses_real_minio_bucket_metadata_blob_without_loss from a91d9cefc (#4377); real bug 717cdd2ab 'decrypt MinIO IAM & server config on drop-in migration' (#4358); format matrix in docs/architecture/minio-file-format-compat.md
- If the diff adds a compatibility shim, legacy fallback, wrapper, or old-endpoint alias (grep the diff for 'legacy', 'fallback', 'compat', 'deprecated'), verify two things: (1) it carries a RUSTFS_COMPAT_TODO(<task-id>) marker with an exact removal condition and a matching entry in the register — an unmarked shim becomes permanent dead weight; (2) the fallback's default direction is safe for old data: e.g. a new decode path must fall back to the legacy decode for old objects by default, not gate legacy reads behind an opt-in flag that makes existing data unreadable after upgrade.
  - Where: Anywhere in the diff; register at docs/architecture/compat-cleanup-register.md; recent example: allow_inplace_legacy_fallback flag in the ecstore erasure codec streaming path
  - Evidence: docs/architecture/compat-cleanup-register.md review checklist; d232a46b4 wired legacy decode prefetch behind a default-OFF gate while keeping legacy reads working (#4542), with arity fallout fixed in 05890d6e2 (#4573)

Null report example: "Attacked dual-key metadata writes/removals against MinIO-only-key objects, nil/empty transitioned-versionID paths to the tier, xl.meta encode against meta_ver<=3 decoders and the #4377 real-MinIO fixtures, and proto field-number evolution for old-node/new-node RPC — no compatibility break found."

### Performance reviewer

- For each `.clone()` or allocation added to a per-request/per-object path, identify the copied data and execution frequency. Report a finding only for a concrete repeated cost or benchmark regression. Recommend borrowing, moving, `Bytes`/`Arc`, `Cow`, or capacity reservation only when it reduces that cost without obscuring ownership or APIs.
  - Where: crates/ecstore/src/set_disk/**, crates/ecstore/src/store*.rs, rustfs/src/storage/, crates/filemeta/, request handlers in rustfs/src/
  - Evidence: crates/ecstore/AGENTS.md 'Allocation Discipline in Hot Paths'; .agents/skills/rust-code-quality/SKILL.md requires a concrete hot-path cost rather than a proxy metric
- For every new sync_all/sync_data/fdatasync/flush/File::sync call in the diff, trace the call chain to DurabilityMode / RUSTFS_DRIVE_SYNC_ENABLE resolution (crates/ecstore/src/disk/local.rs:291 DurabilityMode, :347 resolve_durability_mode) and to per-bucket durability overrides. Construct the run where the operator sets mode=none (or legacy RUSTFS_DRIVE_SYNC_ENABLE=false) and the new fsync still fires — that is an ungated durability cost and a regression on 4KiB writes.
  - Where: crates/ecstore/src/disk/local.rs, crates/ecstore/src/bucket/durability.rs, crates/ecstore/src/set_disk/** (rename_data/commit paths), any crate doing tokio::fs or std::fs writes
  - Evidence: #4221 fsync work caused a measured -10% 4KiB write regression (#814 investigation), later gated; durability modes added in eaff17cad (#4397), per-bucket tier overrides in 13e48d93a (#4407); 2df315baf (#4493) shows even ancestor-dir fsyncs are routed through the gate
- Attack blocking-work placement from both directions: (a) find new synchronous fs calls, hashing, or EC encode/decode executed directly on an async runtime thread without spawn_blocking/block_in_place — construct the stall (a slow disk blocks a worker thread and every task queued on it); (b) find new code that splits one logical disk operation into multiple spawn_blocking hops per object — each hop is a threadpool round-trip, so K hops x N objects multiplies latency. Demand the author justify the placement with the size of the work, not habit.
  - Where: crates/ecstore/src/disk/local.rs, crates/ecstore/src/erasure_coding/, crates/ecstore/src/bitrot/, crates/rio/
  - Evidence: 608ab14d7 (#4554, HP-12) folded metadata open+fstat+read into a single spawn_blocking because per-op hops were measurably slow; 8fc637fb1 (#4484) moved the short EC encode inline because block_in_place cost exceeded the work — direction depends on measured work size
- For each lock acquisition the diff adds or relocates, mark the guard's live range and list every .await and disk/RPC call inside it. Construct the contention interleaving: N concurrent requests serialize on the guard while the holder waits on IO; for namespace/multipart commit locks, compute worst-case hold time (fsync + rename per disk) against the lock's timeout. Also diff the acquisition order against other paths taking the same locks (ABBA).
  - Where: crates/ecstore/src/set_disk/** (commit/rename paths), crates/lock/, crates/audit/ registry, any RwLock/Mutex in per-request paths
  - Evidence: crates/ecstore/AGENTS.md 'Lock Ordering'; c0d5f938f (#4421) fixed a real ABBA deadlock between registry and stream_cancellers; #4370 history: fsync-heavy serial cross-disk commits held a lock long enough to blow test timeouts (#4370)
- Trace exactly what executes inside the PUT commit critical section (under the object write lock, between tmp write and rename_data completion) before vs after the diff. Any newly added work there — cleanup, extra stat, additional rename, O_DIRECT write, logging — is an attack target: construct the per-PUT latency delta and demand it be moved off the critical section or parallelized across disks.
  - Where: crates/ecstore/src/set_disk/ops/*, crates/ecstore/src/disk/local.rs rename_data path
  - Evidence: Three real optimizations removed exactly this class of regression: e7cc719c1 (#4389) moved speculative PUT-tail tmp cleanup off the hot path, 92c8c6db7 (#4411) moved O_DIRECT shard-writes off the commit critical section, 651ccac13 (#4487) parallelized tmp xl.meta write and shard fdatasync on commit
- Find any new loop in a batch API that performs a per-item stat/read/RPC sequentially. Construct the concrete blowup: a 1000-key DeleteObjects or a full listing page -> 1000 serial round-trips added by the diff. Demand either a gate (skip when not needed) or bounded parallelism; for startup/load paths, check for accidental O(n^2) (re-scanning the full list per item).
  - Where: crates/ecstore/src/store_delete*.rs / batch object APIs, listing/metacache paths, crates/iam/ store loading, crates/heal/
  - Evidence: a413729b1 (#4398) had to gate and parallelize the DeleteObjects per-object stat fanout after it shipped serial; 16a91c35e (#4537) fixed O(n^2) IAM startup load by chunking — both were diff-introduced fanouts of this exact shape
- For every buffer the diff allocates on the encode/decode/shard path, check: is it sized with with_capacity to the EC-expanded block (not the logical size, not default-grown)? Does it copy into a fresh Vec where Bytes::slice/clone (refcount) or the io-core buffer pool would avoid the copy? Does the diff read hash and data in separate passes where one pass suffices? Construct the per-block byte-copy count before vs after. If the diff touches the io-core pool, verify gauge accounting still balances.
  - Where: crates/ecstore/src/erasure_coding/, crates/ecstore/src/bitrot/, crates/io-core/src/pool.rs, crates/rio/
  - Evidence: 92bf55ce6 (#4396) fixed a real regression by right-sizing BytesMut encode ingest capacity to the EC-expanded block; 47bee8b31 (#4475) merged bitrot hash+data into one read pass; 7fa3d0d4b (#4534) shows pool gauge accounting is easy to drift when touching buffer reuse
- For every logging or instrumentation statement the diff adds, classify the call site frequency: per-request, per-object, per-shard, or per-block. Anything info!/warn!/error! at per-object frequency or higher is a finding — construct the flood (one listing under client cancellation, one 10k-object heal) and count emitted lines. New metrics/timers on the data path must be feature-gated, not always-on. Run scripts/check_logging_guardrails.sh on the diff.
  - Where: any per-request/per-object code, especially crates/ecstore listing and heal loops, rustfs/src/storage/ handlers; scripts/check_logging_guardrails.sh
  - Evidence: .agents/skills/rustfs-logging-governance/SKILL.md (trace level for hot-path/repetitive success events); d25ddb0e1 (#4372) fixed real listing-cancellation error-log noise; hotpath instrumentation is deliberately feature-gated (3f13d098b #4394, f262fcfce #4541 HP-14)
- Count how many times the diff's request path parses or fetches the same metadata: xl.meta/FileMeta decoded more than once per object, bucket metadata (metadata_sys) re-fetched inside a per-object loop, or the dual x-rustfs-internal/x-minio-internal key lookup re-run repeatedly on the same map. Construct the per-request parse count before vs after; a second full FileMeta decode per GET is a finding.
  - Where: crates/filemeta/, crates/ecstore/src/set_disk/** read paths, crates/ecstore/src/bucket/metadata_sys.rs, crates/utils/src/http/metadata_compat.rs
  - Evidence: 608ab14d7 (#4554) exists because redundant metadata-read syscall sequences per object were measurable; CLAUDE.md dual-key metadata convention makes repeated get_bytes lookups an easy hidden double-parse
- If the diff touches PUT/GET/commit/erasure paths and claims 'no perf impact', demand numbers, not assertion: run the criterion benches (cargo bench -p ecstore — comparison_benchmark, erasure_benchmark, rename_data_meta_benchmark, single_block_non_inline_benchmark per crates/ecstore/benches/) against origin/main, and for end-to-end paths the warp A/B relative-budget gate (scripts/run_hotpath_warp_ab.sh --baseline-ref origin/main, as .github/workflows/performance-ab.yml runs it). Probe specifically at 4KiB object size — that is where the last real regression hid.
  - Where: crates/ecstore/benches/, .github/workflows/performance-ab.yml, scripts/run_hotpath_warp_ab.sh
  - Evidence: crates/ecstore/AGENTS.md: 'Benchmark-sensitive changes should include measurable rationale'; performance-ab.yml (215747022 #4480) is the repo's own relative-budget gate; the #4221 regression was only visible at 4KiB writes (#814 bisect)

Null report example: "Attacked the new rename_data commit-section work, durability-gate routing of the added fdatasync, guard live-range across the shard-write awaits, and per-object clone count in the PUT path; ran comparison_benchmark + rename_data_meta_benchmark vs origin/main (4KiB delta within noise) — no break found."

### Test-coverage skeptic

- For every testable behavior claim in the PR description, revert that hunk and name the focused test or executable check that detects the revert. If no reasonable check exists, require the reason and residual risk from the validation floor. Especially verify the check exercises the real production path, not a lookalike helper.
  - Where: All crates; highest value in crates/ecstore, rustfs/src/storage, crates/heal
  - Evidence: AGENTS.md testable-behavior exit criterion. Real bug: PR #4220 (ghost-directory cleanup) merged with green tests but its fix never executed on the real delete path — required follow-up rustfs#4307, backlog#798 stayed OPEN. The tests exercised a path the production flow never took.
- Read each added/modified test and confirm it asserts the real outcome (returned value, stored bytes, error variant), not merely 'call succeeded' or 'no panic'. Flag any test whose only observable is that the function returned, and any `assert!(result.is_err())` that never checks WHICH error. Then check: does the test prove the exploit/failure form is denied, or only that the intended form still works?
  - Where: crates/e2e_test (security_boundary_test.rs pattern), and every #[cfg(test)] module in the diff
  - Evidence: Commit dee8e4e63 (#4466) had to rewrite 277 lines of crates/e2e_test/src/security_boundary_test.rs because 'security boundary tests' passed without asserting real outcomes. .agents/skills/rust-code-quality/SKILL.md requires an observable failure criterion; .agents/skills/security-advisory-lessons/SKILL.md asks whether the exploit form is denied.
- When the diff adds a boolean/mode parameter or config flag, find the test that fails if the flag's effect is INVERTED inside the changed function. Tests that were mechanically updated to pass `false`/default at every call site assert nothing about the new behavior. Execute the check: flip the flag's branch in the source and confirm at least one test goes red for each branch.
  - Where: crates/ecstore/src/set_disk/ (e.g. build_codec_streaming_part_reader), any function gaining a parameter
  - Evidence: Commit 05890d6e2 (#4573): PR #4560 added a 15th param allow_inplace_legacy_fallback; the arity tests were fixed by passing `false` everywhere — they assert Err outcomes independent of the flag, so the fallback behavior itself has no revert-detecting test at those sites.
- Mutation spot-check on error propagation: for each newly added `?`, `return Err`, or error-mapping line, mentally replace it with `Ok(default)`/ignore and ask which test fails. The swallowed-error bug class recurs in this repo and always ships with green tests — a fix that propagates errors needs a test that injects the failure (faulty disk, failed rename, dispatch error) and asserts the caller sees Err.
  - Where: crates/ecstore (disk delete/rename, reduce_errs), crates/audit, crates/notify, crates/targets
  - Evidence: Three recent fixes for the same class: f7d2b2563 (#4546, disk delete/rename failures swallowed), dbc628f16 (#4424, audit dispatch failures swallowed), 20d61c73b (#4551, reduce_errs leaking nil placeholder as dominant error). All existed while tests were green.
- Any test touching GET/read/reconstruct/stream paths must assert the FULL body content and exact length against a known value, not status-ok or first-bytes. Construct the degraded-read case (missing/inconsistent shards forcing EC reconstruction) and assert byte-for-byte equality; a mid-stream failure that truncates the body passes every test that only checks headers or the first chunk.
  - Where: crates/ecstore/src/set_disk/read.rs and ops/, crates/rio, crates/e2e_test GET scenarios
  - Evidence: Known live bug: EC reconstruct-read verification failure mid-GET at set_disk read path silently truncates the body → client 'unexpected EOF'; version-independent, undetected by existing suites because none assert full-body integrity under shard inconsistency.
- For on-disk / on-wire format changes (xl.meta, .metadata.bin, bitrot framing), reject round-trip-only tests: a struct serialized and deserialized by the same code under test cannot catch format drift. Demand the test parse a REAL captured fixture from crates/filemeta/tests/fixtures, crates/ecstore/tests/fixtures, or crates/rio-v2/tests/minio_fixture_lab — or capture a new one from a single-disk MinIO instance (RELEASE.2025-07-23 procedure from #4377).
  - Where: crates/filemeta, crates/ecstore (headers, msgpack bucket metadata), crates/rio-v2, migration code
  - Evidence: Commit 073bc9675 (#4343): filemeta header signature was hardcoded to zero — round-trip tests passed for months. Commit a91d9cefc (#4377) established the real-MinIO fixture convention (inline/versioned/multipart xl.meta, HighwayHash256-prefixed inline bodies) precisely because synthetic fixtures proved nothing about interop.
- Attack new concurrency tests for flakiness-by-construction: grep the added tests for `sleep(`, fixed timeouts under ~30s on lock acquisition, and use of shared global state (disk registry, lock client, GLOBAL_*). Serialized cross-disk commits exceed small lock timeouts under full-suite CI disk load. If the test shares global state or saturates IO, it must join the `ecstore-serial-flaky` nextest test-group in .config/nextest.toml (note: serial_test's #[serial] does NOT work — nextest runs each test in its own process). Require readiness polling, never fixed sleeps.
  - Where: crates/ecstore tests, crates/e2e_test, .config/nextest.toml
  - Evidence: Commit 2dfa3d3c3 (#4370): concurrent_resend test flaked with Lock(Timeout 5s) on CI — six legitimate serialized cross-disk commits under IO pressure needed 30s. Commit 7c701d9f2 (#4558) created the nextest test-group after bucket_delete_* raced make_bucket into InsufficientWriteQuorum. 65849740f (#4213) deflaked global-state contamination. crates/e2e_test/AGENTS.md: 'readiness checks and explicit polling over fixed sleep-based timing'.
- If the diff writes internal object metadata, run the dual-key mutation: delete the `x-minio-internal-<suffix>` write (keeping only `x-rustfs-internal-`) and check whether any test fails. Because `get_bytes` prefers the RustFS key, every read-back test stays green while MinIO interop is silently broken — coverage must include an assertion that BOTH keys are present in the stored metadata map.
  - Where: crates/utils/src/http/metadata_compat.rs and all its callers in crates/ecstore and rustfs/src/storage
  - Evidence: CLAUDE.md domain convention: metadata must be written under both x-rustfs-internal- and x-minio-internal- keys for MinIO interop; get_bytes prefers the RustFS key, making the MinIO-key half of the invariant invisible to read-back tests.
- For changed quorum/version/UUID logic, name the tests covering the specific poison values: quorum−1 disks, nil UUID, absent vs empty vs nil-serialized UUID bytes, remote-tier version_id of None/"" (unversioned tier bucket → no versionId sent), and the same metadata read on both MetaObject and MetaDeleteMarker version types. Mutation check: remove a `.filter(|u| !u.is_nil())` guard from the diff and confirm a test fails; if none does, the nil-UUID class is uncovered.
  - Where: crates/ecstore (tier recovery, heal, quorum paths), crates/filemeta, code reading UUIDs from xl.meta metadata
  - Evidence: Commit 726f3dc18 (#4552) fixed rejection of empty remote version_id in tier recovery. CLAUDE.md invariant: absent/empty/nil UUID all mean 'no value', not Uuid::nil(). docs/operations/tier-ilm-debugging.md: None/"" tier version means unversioned bucket. df9cbc4ed (#4427): unvalidated distribution values caused shuffle index panic — edge values reached production untested.
- For any pagination/limit/truncation change, construct the exact-boundary test: result count == max (page exactly full), max+1, and a delimiter re-fold that lands precisely on the page boundary — assert both the item count AND the is_truncated/continuation marker. Off-by-one at the page boundary is a recurring shipped bug here.
  - Where: crates/ecstore listing paths (list_objects, ListMultipartUploads, metacache), S3 handlers in rustfs/src/storage
  - Evidence: Two shipped boundary bugs: fefa70b31 (#4447) ListMultipartUploads returned one upload past max-uploads; d91f4d455 (#4538) delimiter re-fold of a full page lost the truncation flag. Both survived existing tests because no test pinned n == max exactly.
- A green focused test is evidence only for the targets it builds. Follow the `AGENTS.md` validation tier: add package-scoped Clippy or broader test-target compilation only when changed targets, features, or dependents remain uncovered; do not require a workspace-wide build by default.
  - Where: All crates; especially concurrent-branch merges into crates/ecstore
  - Evidence: #4322 broke main because only cargo test ran (field_reassign_with_default is clippy-only). b06f3df6b (#4441) and 05890d6e2 (#4573): test code broke the workspace test build (E0061) on main after textually-clean merges, failing CI for every open PR.

Null report example: "Attacked revert-detection for all 3 claimed behaviors (each has a named test that fails on revert), flag-inversion on the new fallback parameter (both branches covered in codec_streaming tests), full-body assertions on the changed GET path, and n==max pagination boundary — no coverage gap found."

## Sources and maintenance

Probes are distilled from shipped bugs in git history (commit/PR references
above), GitHub security advisories (see the security-advisory-lessons
skill), scoped `AGENTS.md` rules, and invariants under `docs/architecture/`
and `docs/operations/`. Line numbers drift; re-locate the invariant. Merge
new incidents into an existing probe when they share a failure class; add a
new probe only for a distinct attack, rather than growing the root policy.
