MobileGL/docs/Disaggregated/P6-CONTRACT-DRAFT.md

CONTRACT-P6 (DRAFT) — the backend runs in a second process

⏸ Blocked on P5f (2026-09-19): this draft assumes P6 is only a transport swap, which is not true today - 15 BARRIER_PULLED fields are still filled by the client into a shared gPipeInputs, and Magma has not moved at all. See P5F-WIRE-COMPLETENESS.md.

Draft, written before its own audit. Package a6 (P6-SPAWN-PLAN.md §5) is a read-only audit whose output is this file's input. Rows that depend on it are marked PENDING a6. c6 moves this file to MobileGL/MG_Remote/CONTRACT-P6.md, where it becomes normative.

Authority once landed: this file, beside CONTRACT-P5.md (table 0, byte carriers, R-1…R-17), CONTRACT-P5B.md (class-C slots), CONTRACT-P5C.md (rule E, SEG_EVENT, the guards) and CONTRACT-P5E.md (rule F, the barriered predicate, the wait rule). Disagreements: this file is newer and wins; §9 lists them. Base feat/disaggregated @ f23fbc1b; every file:line re-resolves at c6's head. Paths under MobileGL/ unless they start with docs/.

Changes go through the integrator, in c6's copy. Packages compile against these rows from day one.

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§0 Rule G

Rules A–F bind unchanged. P6 adds:

Rule G — a role may not name a process-local handle. Rule E forbade naming the other role's memory; rule G extends it to identity. A value whose meaning comes from the owning process's kernel or loader state — EGLDisplay, EGLSurface, EGLContext, ANativeWindow*, any void* window handle, an fd number, a pid, a mapped address — either crosses as a token defined below, or does not cross. Descriptors cross only by SCM_RIGHTS (Transport/FdPassing.h), where the kernel renumbers them.

Rule G's teeth are §8's arm-proof gate: a spawn lane that silently ran monolith satisfies every other gate here.

§1 Table 0 additions

row	encoding	invalid	consumer
DisplayToken / SurfaceToken / ContextToken	Uint64, minted by the client, dense from 1, never reused in a session. Server keeps token → native handle; it never sees the client's EGL values. protocol.fbs:186-196's display / surface are already ulong and are these.	0 on any op but ReleaseCurrent → Fatal{ProtocolCorruption}	the server's surface map
SurfaceOpKind additions	Append-only to protocol.fbs:166-175. Expected SetSwapInterval, ReleaseResources, SetWindowHandle — PENDING a6 (audit item 4).	unknown tag → Fatal{ProtocolCorruption}, never ignored	ServerMain's control pump
WindowKind::AndroidNativeWindow at the server	Refusal: Fatal{UnmigratedSurface, "AndroidNativeWindow@P12"}.	—	§4.3
FatalCode::ServerCrashed / DeviceLost	Already allocated (protocol.fbs:234-242); P6 is the first phase that can produce either.	—	§5
ABI fingerprint	Unchanged. CapsCodec.h:23-29: spawn is same-machine, same-binary, so the existing sizeof(DynamicBackendParameters) / sizeof(MGPCaps) / build-fingerprint handshake is inherited. Fixed-width rewrite stays P7's.	mismatch → Fatal{AbiMismatch}	SessionHandshakeTest.cpp:115

§2 The transport (so)

1. SocketTransport implements ITransport and adds nothing to it. The two doorbell accessors stay on the session (Transport/SessionRings.h:279-285, Server/ServerSession.cpp:670-690).
2. ReceiveFrame's BUFFER_TOO_SMALL keeps the message queued. ITransport.h records that the earlier branch dropped it, wedging the stream at the first oversized message.
3. SocketTransport must be green before a second process exists: one socketpair(), two threads, the whole InProcessTransportTest suite. Same discipline as Transport/SessionRings.h:22-28.
4. The server creates the four segments (SessionRings.h:120-130); the client adopts them by SCM_RIGHTS. ShmSegment::Adopt (Transport/ShmSegment.h:57, ShmSegmentPosix.cpp:120) and the aux socket (Transport/FdPassing.cpp:92) exist and are tested (MG_Test/Wire/FdPassingTest.cpp:114, :187). AttachInProcess (SessionRings.h:147-159) is replaced by an adopt on the received fds.
5. Doorbells are SocketDoorbell. Doorbell.h:385-420 chose a socket byte over a futex so peer death arrives as POLLIN|POLLHUP with recv()==0 rather than as a hang. One syscall per wake is the price; §8.8 measures it.
6. Windows: Adopt is POSIX-only. UnixSocket and NamedPipe stay named refusals (ConfigLoader.cpp:310-348). P6 lands POSIX only.

§3 The process (sm)

1. The child must not be able to become a client. Both required: (a) MOBILEGL_TRANSPORT and every role-selecting MOBILEGL_IPC_* removed from the child's envp; (b) the child's config forced to Monolith regardless. Either alone fails as a fork bomb.
2. Image lookup: MOBILEGL_IPC_SERVER_PATH (already parsed, Config.h IpcTable::ServerPath), then dladdr to find libMobileGLServer.so beside the loaded library. An unresolvable path is a named refusal, never a monolith fallback — ConfigLoader.cpp:315-317 names that accident.
3. ServerMain. Today's MobileGLServer (CMakeLists.txt:923) is the P0 spike stub (tools/spikes/server_stub/main.cpp). PENDING a6: whether the server image can link without MG_Impl (MG_Backend/MGPipe/PipeInputs.h claims it). If not, this file says so and the debt is recorded against P13 rather than left as a comment.
4. Handshake: Hello → Welcome → segment descriptors → first CapsSnapshot (protocol.fbs:59-153). Bounded retry; an exhausted budget is a named refusal.
5. Child exits on EOF; parent reaps. §8.4 makes this mechanical.

§4 The control plane (cp)

1. Server/ServerLoop.cpp:802-1022's twelve Server* forwarders each post a function pointer plus a stack-local void* to a one-slot mailbox (ServerLoop.cpp:533-568, :645) — P5c audit row G4, recorded as P6's (CONTRACT-P5C.md:537). Neither has meaning across a process, so each becomes a SurfaceOp / SurfaceReply pair.
2. Reconciliation against the seven existing enumerators: ServerSwapEGLBuffers stays a record with its own present credit (P5e ruling, Client/BackendObject_Remote.cpp); ServerInitCapabilities is answered by CapsSnapshot; ServerInitWindowSurface is a client-side no-op. The remaining nine map onto the seven plus §1's additions. PENDING a6.
3. Real windows are refused, by name. WindowHandle::Handle (MG_Backend/BackendObject.h:561-566) is an ANativeWindow* on Android. P6 lands pbuffer / surfaceless / offscreen — what HeadlessGL, the split-equivalent lane and trace replay need, and what P6's exit gate measures. Real window arrival is P12.
4. ForgetCurrentTuple's N-3 rule (ServerLoop.cpp:318-330) is a backend property, not a transport one, and must hold identically on the frame path.

§5 Death, and what P5e changed

1. The latch is already designed and is implemented unchanged: docs/Disaggregated/ARCHITECTURE.md:504 — server dies → client reads EOF/EPIPE → device-lost latch (GL calls no-op, eglSwapBuffers returns EGL_FALSE + EGL_CONTEXT_LOST, glGetGraphicsResetStatus returns GL_UNKNOWN_CONTEXT_RESET); MOBILEGL_IPC_RESPAWN=1 restarts and re-pushes (default off). Client dies → server reads EOF and exits; MOBILEGL_IPC_IDLE_EXIT_S (default 30) is a last resort.
2. What P5e added, and no existing document covers. Under lockstep a dead apply thread took the process with it, so "the client kept drawing" was unreachable. Under run-ahead the client is several records ahead by construction. Therefore:
   • A published, never-applied record is discarded at the latch, not waited on.
   • Slow and dead are distinguished without a timeout threshold. A server one frame behind is P5e's intended steady state; a server that is gone is Fatal{ServerCrashed}. SocketDoorbell already latches m_dead on EOF (Doorbell.cpp's Drain) — take the fact from there.
   • A frozen peer produces no hangup. Android's cached-app freezer suspends the child without closing anything. P6 gives a named, bounded diagnostic; full handling is P12's.
3. This is P6's only new design; everything else is carriage. It shares a predicate with E1's outstanding re-specification (ID-122) — design the two together, and settle neither by choosing a threshold.

§6 Process-lifetime statics (st)

g_syncedRenderStateParameters (MG_Backend/DirectGLES/DirectGLES.cpp:4586) and ScopedDefaultUnpackState::s_synced are process-level statics with per-context semantics, recorded as P6's by CONTRACT-P5C.md:538-539. In a server process reused across client contexts they hand the next context the previous one's synced state.

Rule: any static whose meaning is per-context or per-session is keyed by the context generation and reset at applier_reset. PENDING a6 for the complete list (audit item 2); the two above are the known members. This class is structurally invisible to inproc.

§7 Unchanged by P6

Rings, codec, record layout, applier, both verb sinks, both backends, the barriered predicate and the wait rule (CONTRACT-P5E.md §2). P6's lane runs the same reduced path as integration-split and must produce the same name set and the same numbers, not more (§8.2). The 511 UnmigratedVerb class-C entries are not P6's.

§8 The gate

Beyond the five-part gate (docs/Disaggregated/ARCHITECTURE.md §13.2):

1. G1 — pull build 0/0/0/0, .text unchanged.
2. G2/G14 — integration-spawn's name set identical to integration-split's.
3. integration-spawn green at the same count as integration-split (today 179/179).
4. Process tree, mechanical — child count before and after every entry: exactly one while running, zero after. HeadlessGL's fork pre-check leaves no orphan.
5. Arm proof (ID-124) — every spawn entry records the child pid and transport=spawn in its own private log. A spawn lane that fell back to monolith passes items 1-4.
6. Red-once per package (R-16), one named pair each.
7. Device — Redmi 2f7cbe2e, reboot-clean, paired A/B in one thermal window; OpenRA on Adreno 830 at split SSIM ≥ 0.99.
8. Performance recorded, not gated, but one number is mandatory: the socket doorbell's cost against inproc's condvar. P5d took the client thread 15.0 → 9.2 ms/frame and part of that was the doorbell path. P6's only expected regression.

Negative controls, each run red once:

#	knob	what it must falsify
S1	MOBILEGL_IPC_SERVER_PATH=/nonexistent	named refusal, not a silent monolith fallback (§3.2)
S2	kill -9 the server mid-frame	§5.1's latch; the client never hangs and never keeps submitting
S3	child envp not scrubbed	§3.1's second safety catches it, by name
S4	st's generation reset disabled	a second context inherits the first's synced state (§6)
S5	WindowKind::AndroidNativeWindow reaches the server	named Fatal, not a black screen (§4.3)

S2's shape is settled before it is written. E1 demanded a Fatal that occurs zero times in the entire run (ID-122); S2 must falsify "the client can tell slow from dead".

§9 Amendments to the earlier contracts

1. CONTRACT-P5C.md:537-539 — the EGL forwarders' control plane and the s_synced / g_syncedRenderStateParameters resets are discharged here, §4 and §6.
2. CONTRACT-P5C.md:56 says P6's multi-context shape reads MGPApplierReset::ContextSerial for real. Declined: P6 stays single-context, multi-context moves to P12 where the Service lifecycle makes it concrete, and the assertion stands unchanged.
3. Transport/ReplySlot.h marks the 2 MiB reply cap and chunked readback "P6 debt". Declined: they are P9's. Taking them here would make "a transport swap and nothing else" false a second time.
4. CapsCodec.h:23-29 — inherited unchanged; recorded in §1 so it is not re-litigated.

§10 Not P6

Real window arrival and the android:process=":mgl" Service (P12); multi-context (P12, §9.2); chunked readback and the reply-slot pool (P9, §9.3); DynamicBackendParameters fixed-width rewrite (P7); MOBILEGL_IPC_POLL_ESCALATE (P10); object-class BARRIER-PULLED rows and the frontend-keyed twin registry (P3b/P4b, P7); Windows pipe: / unix: (§2.6); the 511 class-C entries (§7).

E1's re-specification (ID-122) is not discharged by P6 and does not become P6's because P6 starts. It appears here only because §5.3 argues it shares a predicate.