[Feat] (P13 W4): MG_Config::DataArmIsRecord and the W4-W6 MOBILEGL_PIPE_DATA_ARM knob

MobileGL/Config.h

@@ -735,10 +735,33 @@ namespace MobileGL::MG_Config {
     inline Bool ServerOwnedWindowSurfaces() {
         return Transport == TransportMode::Spawn && Ipc.Surface == IpcSurface::Server;
     }
+
+    // P13 W4: THE DATA-ARM QUESTION - do the backends read RECORDS (the applier's twins, the staged
+    // stores, the verb's own handles) rather than frontend objects? Under a wire the answer has
+    // always been yes; P13 makes it yes for monolith too ("monolith is the push variant"), one
+    // record family at a time. Each family's arm sites ask THIS rather than `Transport`, which
+    // keeps answering the questions that really are about the transport (lifecycle, roles, whose
+    // caps, whether a store copies).
+    //
+    // MOBILEGL_PIPE_DATA_ARM=record (default) | frontend picks monolith's arm for the families
+    // already switched. It exists for W4-W6 only - bisecting a monolith red to the switch, and
+    // the per-thread CPU record (ID-P13-2) - and goes in W6 with the frontend arms. Latched once
+    // by MG_ConfigLoader::Init(): the two arms keep their twins in different places.
+    extern Bool MonolithTakesRecordArm;
+    inline Bool DataArmIsRecord() { return Transport != TransportMode::Monolith || MonolithTakesRecordArm; }
+    // ...and the record arm runs in this process over stores that ALIAS the frontend's shadows
+    // (monolith) instead of copying them (every wire).
+    inline Bool RecordArmAliasesFrontend() {
+        return Transport == TransportMode::Monolith && MonolithTakesRecordArm;
+    }
 #else
     // The whole point: in a build without MG_Remote this folds at compile time, so
     // `if (MG_Config::Transport != MG_Config::TransportMode::Monolith)` in Init.cpp is a
     // discarded statement and the pull build gains no symbol, no branch and no byte.
     inline constexpr TransportMode Transport = TransportMode::Monolith;
+    // P13 W4: a build without MG_Remote stays on the frontend arm until W5 moves the record arm
+    // out of the disaggregated gating (ID-P13-1 / A2).
+    inline constexpr Bool DataArmIsRecord() { return false; }
+    inline constexpr Bool RecordArmAliasesFrontend() { return false; }
 #endif
 } // namespace MobileGL::MG_Config

MobileGL/ConfigLoader.cpp

@@ -38,6 +38,7 @@ namespace MobileGL::MG_Config {
     String TransportEndpoint;
     Bool SplitTransportRequestedByConfig = false;
     IpcTable Ipc;
+    Bool MonolithTakesRecordArm = true;
 #endif
 } // namespace MobileGL::MG_Config
 
@@ -455,6 +456,33 @@ namespace MobileGL::MG_ConfigLoader {
     // rather than silent, because the failure this avoids is a P6 lane that set
     // MOBILEGL_TRANSPORT=spawn, fell back to monolith, and went green on the wrong arm.
     // The mode is left at Monolith so nothing half-initializes.
+    // P13 W4 (Config.h DataArmIsRecord). Read after InitTransport: `frontend` names monolith's
+    // arm only - a wire has no frontend objects on its server half to read, so there it is
+    // ignored by name rather than half-honoured.
+    inline void InitDataArm() {
+        String value;
+        QueryEnvVariable("MOBILEGL_PIPE_DATA_ARM", value, "record");
+        String lowered = value;
+        std::transform(lowered.begin(), lowered.end(), lowered.begin(),
+                       [](unsigned char c) { return static_cast<char>(std::tolower(c)); });
+        MG_Config::MonolithTakesRecordArm = true;
+        if (lowered.empty() || lowered == "record") return;
+        if (lowered == "frontend") {
+            if (MG_Config::Transport != MG_Config::TransportMode::Monolith) {
+                MGLOG_W("Config: MOBILEGL_PIPE_DATA_ARM=frontend is ignored under a transport - the "
+                        "record arm is the only arm a wire has");
+                return;
+            }
+            MG_Config::MonolithTakesRecordArm = false;
+            MGLOG_I("Config: MOBILEGL_PIPE_DATA_ARM=frontend - monolith reads frontend objects in "
+                    "the record families switched so far");
+            return;
+        }
+        MGLOG_W("Config: Ignoring invalid env variable MOBILEGL_PIPE_DATA_ARM='%s'; expected "
+                "record|frontend, using record",
+                value.c_str());
+    }
+
     inline void InitTransport() {
         String value;
         QueryEnvVariable("MOBILEGL_TRANSPORT", value, "monolith");
@@ -702,6 +730,7 @@ namespace MobileGL::MG_ConfigLoader {
         // After InitFeatures, so the one line InitIpc logs is the last word on this run's
         // configuration, and before the accepted-env map is destroyed just below.
         InitTransport();
+        InitDataArm();
         InitIpc();
 #endif
 

MobileGL/MG_IntegrationTest/Harness/SplitRuntimePeek.cpp

@@ -128,6 +128,7 @@ namespace MGITest {
             default: state.transportName = "non-monolith"; break;
         }
         state.transportResolved = MobileGL::MG_Config::Transport != TransportMode::Monolith;
+        state.dataArmIsRecord = MobileGL::MG_Config::DataArmIsRecord();
 
         // Active() is c0's one deliberately non-aborting accessor: "does a session exist" has a
         // legitimate no. Everything below it is only reached through a live session, so nothing

MobileGL/MG_IntegrationTest/Harness/SplitRuntimePeek.h

@@ -63,6 +63,9 @@ namespace MGITest {
         bool transportResolved = false;
         // The resolved transport, for a message: "monolith", "inproc", "spawn", "unix", "pipe".
         std::string transportName = "monolith";
+        // P13 W4: MG_Config::DataArmIsRecord() - false in a build without the peek (and without
+        // MG_Remote), which stays on the frontend arm until W5.
+        bool dataArmIsRecord = false;
         // ClientSession::Active() != nullptr.
         bool sessionActive = false;
         // ImplementedVerbCount(), out of kRemoteEmitSlotCount (71).