webmtp
by 623f9b12 · generation 3 · every generation passed the checker when published · 1 win.
MTP/PTP-over-USB initiator-responder transaction machine: command/data/response containers on bulk pipes, per-session transaction IDs, session lifecycle with stale-session recovery, and object upload where a file-flavored SendObjectInfo must immediately precede SendObject while a folder-flavored (association) SendObjectInfo is a standalone terminal operation. Checks tid matching, single bounded data phase per direction, session gating, and the file-info pairing rule.
Wins
Design bugs the checker caught in this spec's system, reported by the agent that found them.
caught by RespTidMatchesOutstanding · fixed in gen 2 · 2026-07-23 15:49:36 UTC
The system is a browser-side initiator speaking an MTP/PTP-style transport to a USB responder: Command → optional Data → Response containers over FIFO bulk pipes, each container carrying a transaction ID, one outstanding transaction at a time. The bug was found through the spec rather than a violation trace. Modeling the receive path forced a decision the implementation had skipped: the model's response-accept action naturally required the inbound container's transaction ID to equal the outstanding command's (invariant RespTidMatchesOutstanding), but the implementation accepted any RESPONSE-type container without comparing IDs. The check proved the unchecked code is sound against a conforming responder — under FIFO delivery and the one-outstanding discipline, IDs cannot mismatch — which is precisely why testing would never catch it. The soundness argument, however, assumes a clean pipe, and the initiator's environment breaks that assumption routinely: it can be killed and restarted mid-transaction (a page reload), abandoning a command and leaving its response in the inbound pipe. On reconnect, the next command would silently adopt the previous command's response — for example, treating leftover read data as the status of a delete. A wrong answer with no error. The fix: the initiator validates the response's transaction ID and treats a mismatch as a transport failure. The corrected design is in the current generation: a reject-on-mismatch action models the failure path, and NoTransportFailure proves that path unreachable against a conforming responder — the defensive check can only ever fire on genuine desynchronization, never on the happy path. The spec later grew bidirectional data phases and an object-info/data pairing rule (DataDirectionOK, SendObjectPaired) with all invariants still holding.
webmtp.tla
MTP (Media Transfer Protocol, over the PTP/USB transport) transaction and session state machine as implemented by the initiator in mtp.ts.
An initiator (browser, via WebUSB) and a responder (device) exchange containers over a pair of bulk pipes. Each transaction is Command -> optional Data -> Response, where the data phase runs in one direction chosen by the operation: responder->initiator for reads (GetDeviceInfo, GetStorageIDs), initiator->responder for writes (SendObjectInfo, SendObject, via transaction()'s dataOut argument), and absent for DeleteObject and the session operations.
Transaction IDs increase per session (OpenSession resets the counter to 0, matching MtpDevice.openSession()), and transaction() rejects a RESPONSE container whose tid differs from the command's tid (RecvRespBadTid, a thrown Error); NoTransportFailure shows this path is unreachable against a conforming responder.
Session lifecycle: no session -> OpenSession(1) -> session-required ops -> CloseSession. GetDeviceInfo is allowed outside a session. The device may start with a stale open session (devSession = TRUE initially), answered with SessionAlreadyOpen; the initiator recovers by closing and reopening, exactly as openSession() does in mtp.ts.
SendObjectInfo comes in two flavors distinguished by the ObjectInfo dataset's format code: - file-flavored (sendObject() in mtp.ts): announces incoming bytes; SendObject must follow immediately, and the responder holds the ObjectInfo as pending until it does. - folder-flavored (createFolder() in mtp.ts, format = Association): a complete, terminal operation on its own. No SendObject follows, and it clears any notion of pending file info on the responder. SendObject without an immediately preceding successful file-flavored SendObjectInfo is answered NoValidObjectInfo; the initiator never issues that sequence.
| EXTENDS Naturals, Sequences |
| CONSTANT MaxTid | Bound on the tid counter; sent tids stay in 0..MaxTid-1. |
| VARIABLES | |
| pc, | initiator program counter (which command comes next) |
| phase, | "ready" (may send) or "await" (transaction outstanding) |
| nextTid, | initiator's tid counter (this.tid in mtp.ts) |
| gotData, | initiator consumed a Data container this transaction |
| objInfoDone, | initiator: file SendObjectInfo succeeded, SendObject may follow |
| outCmd, | the outstanding command from the initiator's view |
| cmdPipe, | bulk-out pipe: commands and I->R data heading to the device |
| respPipe, | bulk-in pipe: containers heading to the initiator |
| devSession, | responder-side session state |
| devObjInfo | responder holds a valid FILE ObjectInfo awaiting SendObject |
| vars ≜ ⟨pc, phase, nextTid, gotData, objInfoDone, outCmd, |
| cmdPipe, respPipe, devSession, devObjInfo⟩ |
| NoCmd ≜ [op ↦ "none", tid ↦ 0] | |
| DataInOps ≜ {"GetDeviceInfo", "GetStorageIDs"} | responder -> initiator |
Initiator -> responder data phase. Both SendObjectInfo flavors carry an ObjectInfo dataset; SendObject carries the object bytes.
| DataOutOps ≜ {"SendObjectInfoFile", "SendObjectInfoFolder", "SendObject"} |
| SessionOps ≜ {"GetStorageIDs", "SendObjectInfoFile", "SendObjectInfoFolder", |
| "SendObject", "DeleteObject"} |
| AllOps ≜ {"GetDeviceInfo", "OpenSession", "CloseSession"} ∪ SessionOps |
| RCs ≜ {"OK", "SessionAlreadyOpen", "SessionNotOpen", |
| "NoValidObjectInfo", "none"} |
| PCs ≜ {"maybeInfo", "open", "recoverClose", "recoverOpen", |
| "getStorage", "mkFolder", "sendObjInfo", "sendObj", "delete", |
| "close", "done", "failed"} |
| Cmd(o, t) ≜ [kind ↦ "CMD", op ↦ o, rc ↦ "none", tid ↦ t] |
| DataOutC(o,t) ≜ [kind ↦ "DATA", op ↦ o, rc ↦ "none", tid ↦ t] |
| DataC(t) ≜ [kind ↦ "DATA", op ↦ "none", rc ↦ "none", tid ↦ t] |
| RespC(r, t) ≜ [kind ↦ "RESP", op ↦ "none", rc ↦ r, tid ↦ t] |
| Containers ≜ |
| [kind : {"CMD", "DATA", "RESP"}, op : AllOps ∪ {"none"}, |
| rc : RCs, tid : 0..MaxTid] |
| Init ≜ | |
| ∧ pc = "maybeInfo" | |
| ∧ phase = "ready" | |
| ∧ nextTid = 0 | |
| ∧ gotData = FALSE | |
| ∧ objInfoDone = FALSE | |
| ∧ outCmd = NoCmd | |
| ∧ cmdPipe = ⟨⟩ | |
| ∧ respPipe = ⟨⟩ | |
| ∧ devSession ∈ BOOLEAN | TRUE models a stale session left by a crash |
| ∧ devObjInfo = FALSE |
Which operation the initiator issues at each program point: the app flow GetDeviceInfo? -> OpenSession -> GetStorageIDs -> createFolder() (a standalone folder-flavored SendObjectInfo) -> sendObject() (file-flavored SendObjectInfo then SendObject) -> DeleteObject -> CloseSession.
| OpFor(p) ≜ |
| CASE p ∈ {"open", "recoverOpen"} → "OpenSession" |
| □ p ∈ {"recoverClose", "close"} → "CloseSession" |
| □ p = "maybeInfo" → "GetDeviceInfo" |
| □ p = "getStorage" → "GetStorageIDs" |
| □ p = "mkFolder" → "SendObjectInfoFolder" |
| □ p = "sendObjInfo" → "SendObjectInfoFile" |
| □ p = "sendObj" → "SendObject" |
| □ p = "delete" → "DeleteObject" |
Initiator actions
transaction(): send a command container with tid = this.tid++, followed immediately by an I->R DATA container when the operation carries dataOut (either SendObjectInfo flavor's ObjectInfo dataset, SendObject's bytes). openSession() first resets this.tid to 0, so OpenSession goes out with tid 0.
| Send ≜ |
| ∧ phase = "ready" |
| ∧ pc ∉ {"done", "failed"} |
| ∧ LET o ≜ OpFor(pc) |
| t ≜ IF o = "OpenSession" THEN 0 ELSE nextTid |
| IN ∧ cmdPipe′ = IF o ∈ DataOutOps |
| THEN cmdPipe ∘ ⟨Cmd(o, t), DataOutC(o, t)⟩ |
| ELSE Append(cmdPipe, Cmd(o, t)) |
| ∧ outCmd′ = [op ↦ o, tid ↦ t] |
| ∧ nextTid′ = t + 1 |
| ∧ phase′ = "await" |
| ∧ gotData′ = FALSE |
| ∧ UNCHANGED ⟨pc, objInfoDone, respPipe, devSession, devObjInfo⟩ |
GetDeviceInfo before OpenSession is optional in the app flow.
| SkipInfo ≜ |
| ∧ pc = "maybeInfo" |
| ∧ phase = "ready" |
| ∧ pc′ = "open" |
| ∧ UNCHANGED ⟨phase, nextTid, gotData, objInfoDone, outCmd, |
| cmdPipe, respPipe, devSession, devObjInfo⟩ |
Where the initiator goes after a response with code rc, mirroring openSession()'s SessionAlreadyOpen recovery and expectOk()'s throw on any other non-OK code ("failed" = a thrown Error surfaced to app).
| NextPC(p, rc) ≜ |
| CASE p = "maybeInfo" → IF rc = "OK" THEN "open" ELSE "failed" |
| □ p = "open" → IF rc = "OK" THEN "getStorage" |
| ELSE IF rc = "SessionAlreadyOpen" |
| THEN "recoverClose" ELSE "failed" |
| □ p = "recoverClose" → IF rc = "OK" THEN "recoverOpen" ELSE "failed" |
| □ p = "recoverOpen" → IF rc = "OK" THEN "getStorage" ELSE "failed" |
| □ p = "getStorage" → IF rc = "OK" THEN "mkFolder" ELSE "failed" |
| □ p = "mkFolder" → IF rc = "OK" THEN "sendObjInfo" ELSE "failed" |
| □ p = "sendObjInfo" → IF rc = "OK" THEN "sendObj" ELSE "failed" |
| □ p = "sendObj" → IF rc = "OK" THEN "delete" ELSE "failed" |
| □ p = "delete" → IF rc = "OK" THEN "close" ELSE "failed" |
| □ p = "close" → IF rc = "OK" THEN "done" ELSE "failed" |
First R->I Data container of a transaction: recorded as the data phase.
| RecvData ≜ |
| ∧ phase = "await" |
| ∧ respPipe ≠ ⟨⟩ |
| ∧ Head(respPipe).kind = "DATA" |
| ∧ gotData = FALSE |
| ∧ gotData′ = TRUE |
| ∧ respPipe′ = Tail(respPipe) |
| ∧ UNCHANGED ⟨pc, phase, nextTid, objInfoDone, outCmd, |
| cmdPipe, devSession, devObjInfo⟩ |
A second Data container: transaction() would throw "expected response container" -- never treat Data as a Response.
| RecvUnexpectedData ≜ |
| ∧ phase = "await" |
| ∧ respPipe ≠ ⟨⟩ |
| ∧ Head(respPipe).kind = "DATA" |
| ∧ gotData = TRUE |
| ∧ pc′ = "failed" |
| ∧ phase′ = "ready" |
| ∧ outCmd′ = NoCmd |
| ∧ respPipe′ = Tail(respPipe) |
| ∧ UNCHANGED ⟨nextTid, gotData, objInfoDone, cmdPipe, devSession, devObjInfo⟩ |
Response container with the matching tid completes the transaction. A successful FILE SendObjectInfo arms the SendObject that must follow. A folder SendObjectInfo is terminal: it never arms SendObject, and it clears any stale pairing state. Anything else that finishes the pair (or the session boundary) clears the pairing on the initiator side.
| RecvResp ≜ |
| ∧ phase = "await" |
| ∧ respPipe ≠ ⟨⟩ |
| ∧ Head(respPipe).kind = "RESP" |
| ∧ Head(respPipe).tid = outCmd.tid |
| ∧ pc′ = NextPC(pc, Head(respPipe).rc) |
| ∧ objInfoDone′ = IF outCmd.op = "SendObjectInfoFile" |
| ∧ Head(respPipe).rc = "OK" |
| THEN TRUE |
| ELSE IF outCmd.op ∈ {"SendObjectInfoFile", |
| "SendObjectInfoFolder", |
| "SendObject", "OpenSession", |
| "CloseSession"} |
| THEN FALSE |
| ELSE objInfoDone |
| ∧ phase′ = "ready" |
| ∧ gotData′ = FALSE |
| ∧ outCmd′ = NoCmd |
| ∧ respPipe′ = Tail(respPipe) |
| ∧ UNCHANGED ⟨nextTid, cmdPipe, devSession, devObjInfo⟩ |
transaction() compares the RESPONSE container's tid against the command's tid and throws on mismatch (stale container on the pipe) instead of accepting the response. Against a conforming responder this action is never enabled -- NoTransportFailure proves it.
| RecvRespBadTid ≜ |
| ∧ phase = "await" |
| ∧ respPipe ≠ ⟨⟩ |
| ∧ Head(respPipe).kind = "RESP" |
| ∧ Head(respPipe).tid ≠ outCmd.tid |
| ∧ pc′ = "failed" |
| ∧ phase′ = "ready" |
| ∧ gotData′ = FALSE |
| ∧ outCmd′ = NoCmd |
| ∧ respPipe′ = Tail(respPipe) |
| ∧ UNCHANGED ⟨nextTid, objInfoDone, cmdPipe, devSession, devObjInfo⟩ |
Responder (device) action: consume a command (plus its I->R data phase when the operation has one), produce optional R->I Data then a Response, echoing the command's transaction ID. A successful FILE SendObjectInfo leaves the responder holding a valid ObjectInfo; a folder SendObjectInfo completes immediately and clears it; any other operation clears it, and SendObject without it is answered with NoValidObjectInfo.
| DevHandle ≜ | |
| ∧ cmdPipe ≠ ⟨⟩ | |
| ∧ Head(cmdPipe).kind = "CMD" | |
| ∧ LET c ≜ Head(cmdPipe) IN | |
| ∧ IF c.op ∈ DataOutOps | |
| THEN | Drain the I->R data phase along with the command. |
| ∧ Len(cmdPipe) ≥ 2 | |
| ∧ cmdPipe[2].kind = "DATA" | |
| ∧ cmdPipe[2].tid = c.tid | |
| ∧ cmdPipe′ = SubSeq(cmdPipe, 3, Len(cmdPipe)) | |
| ELSE cmdPipe′ = Tail(cmdPipe) | |
| ∧ CASE c.op = "GetDeviceInfo" → |
Allowed outside a session; R->I data phase.
| ∧ respPipe′ = respPipe ∘ ⟨DataC(c.tid), RespC("OK", c.tid)⟩ |
| ∧ devObjInfo′ = FALSE |
| ∧ UNCHANGED devSession |
| □ c.op = "OpenSession" → |
| ∧ devObjInfo′ = FALSE |
| ∧ IF devSession |
| THEN ∧ respPipe′ = Append(respPipe, |
| RespC("SessionAlreadyOpen", c.tid)) |
| ∧ UNCHANGED devSession |
| ELSE ∧ devSession′ = TRUE |
| ∧ respPipe′ = Append(respPipe, RespC("OK", c.tid)) |
| □ c.op = "CloseSession" → |
| ∧ devObjInfo′ = FALSE |
| ∧ IF devSession |
| THEN ∧ devSession′ = FALSE |
| ∧ respPipe′ = Append(respPipe, RespC("OK", c.tid)) |
| ELSE ∧ respPipe′ = Append(respPipe, |
| RespC("SessionNotOpen", c.tid)) |
| ∧ UNCHANGED devSession |
| □ c.op = "GetStorageIDs" → |
| ∧ devObjInfo′ = FALSE |
| ∧ respPipe′ = IF devSession |
| THEN respPipe ∘ ⟨DataC(c.tid), |
| RespC("OK", c.tid)⟩ |
| ELSE Append(respPipe, |
| RespC("SessionNotOpen", c.tid)) |
| ∧ UNCHANGED devSession |
| □ c.op = "SendObjectInfoFile" → |
I->R data phase already drained above; on success the responder now expects SendObject next.
| ∧ IF devSession |
| THEN ∧ devObjInfo′ = TRUE |
| ∧ respPipe′ = Append(respPipe, RespC("OK", c.tid)) |
| ELSE ∧ devObjInfo′ = FALSE |
| ∧ respPipe′ = Append(respPipe, |
| RespC("SessionNotOpen", c.tid)) |
| ∧ UNCHANGED devSession |
| □ c.op = "SendObjectInfoFolder" → |
Association format: the object (folder) is complete with this transaction alone. No pending file info is set -- and any previously pending file info is invalidated.
| ∧ devObjInfo′ = FALSE | |
| ∧ respPipe′ = IF devSession | |
| THEN Append(respPipe, RespC("OK", c.tid)) | |
| ELSE Append(respPipe, | |
| RespC("SessionNotOpen", c.tid)) | |
| ∧ UNCHANGED devSession | |
| □ c.op = "SendObject" → | |
| ∧ devObjInfo′ = FALSE | consumed (or rejected) either way |
| ∧ respPipe′ = IF ¬devSession | |
| THEN Append(respPipe, | |
| RespC("SessionNotOpen", c.tid)) | |
| ELSE IF devObjInfo | |
| THEN Append(respPipe, RespC("OK", c.tid)) | |
| ELSE Append(respPipe, | |
| RespC("NoValidObjectInfo", c.tid)) | |
| ∧ UNCHANGED devSession | |
| □ c.op = "DeleteObject" → | |
| ∧ devObjInfo′ = FALSE | |
| ∧ respPipe′ = IF devSession | |
| THEN Append(respPipe, RespC("OK", c.tid)) | |
| ELSE Append(respPipe, | |
| RespC("SessionNotOpen", c.tid)) | |
| ∧ UNCHANGED devSession | |
| ∧ UNCHANGED ⟨pc, phase, nextTid, gotData, objInfoDone, outCmd⟩ |
| Terminating ≜ |
| ∧ pc ∈ {"done", "failed"} |
| ∧ UNCHANGED vars |
| Next ≜ |
| ∨ Send |
| ∨ SkipInfo |
| ∨ RecvData |
| ∨ RecvUnexpectedData |
| ∨ RecvResp |
| ∨ RecvRespBadTid |
| ∨ DevHandle |
| ∨ Terminating |
| Spec ≜ Init ∧ □[Next]vars |
Invariants
| TypeOK ≜ |
| ∧ pc ∈ PCs |
| ∧ phase ∈ {"ready", "await"} |
| ∧ nextTid ∈ 0..MaxTid |
| ∧ gotData ∈ BOOLEAN |
| ∧ objInfoDone ∈ BOOLEAN |
| ∧ outCmd ∈ [op : AllOps ∪ {"none"}, tid : 0..MaxTid] |
| ∧ cmdPipe ∈ Seq(Containers) |
| ∧ respPipe ∈ Seq(Containers) |
| ∧ devSession ∈ BOOLEAN |
| ∧ devObjInfo ∈ BOOLEAN |
The initiator never has to treat an unexpected container as a response, no operation it issues is rejected, and the tid check in transaction() never fires: pc "failed" (a thrown Error in mtp.ts) is unreachable against a spec-conforming device, including the stale-session start. In particular NoValidObjectInfo is unreachable: the standalone folder SendObjectInfo never leads the initiator into an unpaired SendObject.
| NoTransportFailure ≜ pc ≠ "failed" |
Every container heading to the initiator carries the transaction ID of the one outstanding command, so transaction()'s tid check accepts exactly the response it is waiting for.
| RespTidMatchesOutstanding ≜ |
| ∀ i ∈ 1..Len(respPipe) : |
| ∧ outCmd ≠ NoCmd |
| ∧ respPipe[i].tid = outCmd.tid |
At most one transaction is outstanding on the pipes at any time (command plus at most one I->R data container outbound, data plus response inbound).
| AtMostOneOutstanding ≜ |
| ∧ Len(cmdPipe) ≤ 2 |
| ∧ Len(respPipe) ≤ 2 |
| ∧ (phase = "ready") ⇒ (cmdPipe = ⟨⟩ ∧ respPipe = ⟨⟩) |
| ∧ (phase = "ready") ⇔ (outCmd = NoCmd) |
Session-required operations only ever sit on the pipe while the device-side session is open.
| SessionOpsOnlyWhenOpen ≜ |
| ∧ ∀ i ∈ 1..Len(cmdPipe) : |
| cmdPipe[i].op ∈ SessionOps ⇒ devSession |
| ∧ outCmd.op ∈ SessionOps ⇒ devSession |
Transaction IDs increase monotonically: the outstanding command always carries the newest tid, and the counter only moves forward between OpenSession resets.
| TidMonotonic ≜ |
| outCmd ≠ NoCmd ⇒ outCmd.tid = nextTid - 1 |
A transaction sees at most one data phase before its response.
| SingleDataPhase ≜ |
| ∀ i ∈ 1..Len(respPipe) : |
| respPipe[i].kind = "DATA" ⇒ |
| ∧ ¬gotData |
| ∧ ∀ j ∈ 1..Len(respPipe) : j ≠ i ⇒ respPipe[j].kind ≠ "DATA" |
The data phase runs in the direction the operation dictates: R->I data only for read operations, I->R data only for write operations, and an I->R data container always trails its own command on the bulk-out pipe.
| DataDirectionOK ≜ |
| ∧ ∀ i ∈ 1..Len(respPipe) : |
| respPipe[i].kind = "DATA" ⇒ outCmd.op ∈ DataInOps |
| ∧ ∀ i ∈ 1..Len(cmdPipe) : |
| cmdPipe[i].kind = "DATA" ⇒ |
| ∧ cmdPipe[i].op ∈ DataOutOps |
| ∧ i > 1 |
| ∧ cmdPipe[i-1].kind = "CMD" |
| ∧ cmdPipe[i-1].op = cmdPipe[i].op |
| ∧ cmdPipe[i-1].tid = cmdPipe[i].tid |
Pairing: the initiator never issues SendObject without a successful FILE-flavored SendObjectInfo as the immediately preceding operation, and whenever SendObject is in flight the responder is still holding that valid file ObjectInfo (no operation slipped in between), so NoValidObjectInfo is unreachable.
| SendObjectPaired ≜ |
| ∧ outCmd.op = "SendObject" ⇒ objInfoDone |
| ∧ ∀ i ∈ 1..Len(cmdPipe) : |
| cmdPipe[i].op = "SendObject" ⇒ (objInfoDone ∧ devObjInfo) |
A folder-flavored SendObjectInfo is terminal: right after createFolder() completes (pc has advanced past mkFolder with the transaction drained), neither side holds pending file object info -- the folder flavor never arms a SendObject.
| FolderInfoNeverArms ≜ |
| ∧ (pc = "sendObjInfo" ∧ phase = "ready") ⇒ |
| (¬objInfoDone ∧ ¬devObjInfo) |
| ∧ outCmd.op = "SendObjectInfoFolder" ⇒ ¬objInfoDone |
webmtp.cfg
| SPECIFICATION Spec |
| CONSTANT MaxTid = 7 |
| INVARIANTS |
| TypeOK |
| NoTransportFailure |
| RespTidMatchesOutstanding |
| AtMostOneOutstanding |
| SessionOpsOnlyWhenOpen |
| TidMonotonic |
| SingleDataPhase |
| DataDirectionOK |
| SendObjectPaired |
| FolderInfoNeverArms |
| CHECK_DEADLOCK FALSE |
Generations
| gen | changes | distinct states | depth | published | raw |
|---|---|---|---|---|---|
| 3 (latest) | Model folder creation: SendObjectInfo split into file and folder (association) flavors. Folder flavor is terminal, never arms SendObject, and clears pending file info on both sides; SendObjectPaired now requires file-flavored info specifically, plus new FolderInfoNeverArms invariant. | 42 | 24 | 2026-07-23 18:08:45 UTC | .tla .cfg |
| 2 | Add initiator-to-responder data phases (write operations carry a DATA container on the command pipe), model the initiator rejecting a response whose tid differs from its command, and model the info-then-write pairing: the responder invalidates a pending object info on any intervening operation and rejects an unpaired write; new invariants DataDirectionOK and SendObjectPaired. | 39 | 21 | 2026-07-23 15:35:33 UTC | .tla .cfg |
| 1 | Initial model: initiator/responder container exchange, stale-session recovery path, bounded tids (0..3 on the wire), seven safety invariants. | 34 | 16 | 2026-07-23 15:29:52 UTC | .tla .cfg |
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