feat(avm)!: Context id constraining in the tx trace

Tracks context ids properly in the tx trace, and adds context id emission to tx_context events in simulation

Author: sirasistant

barretenberg/cpp/pil/vm2/tx.pil

@@ -201,8 +201,16 @@ namespace tx;
     pol commit should_process_call_request;
     should_process_call_request = is_public_call_request * (1 - is_padded);
 
-    pol commit context_id; // TODO: Constrain
-    pol commit next_context_id; // TODO: Constrain
+    // When sending a call to execution, next context id is incremented by one, since we use next_context_id for the call.
+    pol commit next_context_id_sent_to_enqueued_call;
+    // When coming back from a call, the next context id may have incremented an amount we can't predict from the tx trace.
+    pol commit next_context_id_from_enqueued_call;
+
+    // When issuing a call, context id is used for the call, making the next context id it plus one.
+    should_process_call_request * ((next_context_id + 1) - next_context_id_sent_to_enqueued_call) = 0;
+    // When comming back from the call, the newly available context id is the next_context_id that comes from the end of the call.
+    should_process_call_request * (next_context_id_from_enqueued_call - next_context_id') = 0;
+
     pol commit is_teardown_phase; // TODO: Constrain
     is_teardown_phase * (1 - is_teardown_phase) = 0;
 
@@ -224,8 +232,8 @@ namespace tx;
     // Commented out for now since we dont have enqueued call flags in execution set
     // #[DISPATCH_EXEC_START]
     // should_process_call_request {
-    //     context_id,
     //     next_context_id,
+    //     next_context_id_sent_to_enqueued_call,
     //     discard,
     //     msg_sender,
     //     contract_addr,
@@ -284,8 +292,8 @@ namespace tx;
     // Commented out for now since we dont have enqueued call flags in execution set
     //#[DISPATCH_EXEC_GET_REVERT]
     //should_process_call_request {
-    //    context_id,
     //    next_context_id,
+    //    next_context_id_from_enqueued_call,
     //    reverted,
     //    discard,
     //    // Tree State

barretenberg/cpp/pil/vm2/tx_context.pil

@@ -396,3 +396,16 @@ namespace tx;
         precomputed.clk,
         public_inputs.cols[0]
     };
+
+    // ===== EXECUTION CONTEXT ID =====
+
+    // The next available context id. When peforming an enqueued call, this is the context id used by it.
+    pol commit next_context_id;
+    // Initial next context id is 1
+    #[NEXT_CONTEXT_ID_INITIAL_VALUE]
+    start_tx * (1 - next_context_id) = 0;
+
+    // We normally propagate the next context id, unless a call is issued, where the execution trace
+    // controls the next row's next context id.
+    #[NEXT_CONTEXT_ID_CONTINUITY]
+    NOT_LAST * (1 - should_process_call_request) * (next_context_id' - next_context_id) = 0;

barretenberg/cpp/src/barretenberg/vm2/constraining/relations/tx.test.cpp

@@ -359,6 +359,7 @@ TEST(TxContextConstrainingTest, InitialStateChecks)
             { C::tx_next_l2_gas_used, start_gas_used.l2Gas },
             { C::tx_next_da_gas_used, start_gas_used.daGas },
             { C::tx_setup_phase_value, static_cast<uint8_t>(TransactionPhase::SETUP) },
+            { C::tx_next_context_id, 1 },
         },
     });
 
@@ -480,5 +481,44 @@ TEST(TxContextConstrainingTest, EndStateChecks)
                       lookup_tx_context_restore_state_on_revert_settings>(trace);
 }
 
+TEST(TxContextConstrainingTest, NegativeContextIdChecks)
+{
+    TestTraceContainer trace({
+        {
+            // Row 0
+            { C::precomputed_first_row, 1 },
+        },
+        {
+            // Row 1
+            { C::tx_sel, 1 },
+            { C::tx_start_tx, 1 },
+            { C::tx_next_context_id, 1 },
+        },
+        {
+            // Row 2
+            { C::tx_sel, 1 },
+            { C::tx_next_context_id, 1 },
+            { C::tx_should_process_call_request, 1 },
+        },
+        {
+            // Row 3
+            { C::tx_sel, 1 },
+            { C::tx_next_context_id, 5 },
+        },
+    });
+    check_relation<tx_context>(
+        trace, tx_context::SR_NEXT_CONTEXT_ID_CONTINUITY, tx_context::SR_NEXT_CONTEXT_ID_INITIAL_VALUE);
+
+    // Negative test: initial context id should be 1
+    trace.set(C::tx_next_context_id, 1, 2);
+    EXPECT_THROW_WITH_MESSAGE(check_relation<tx_context>(trace, tx_context::SR_NEXT_CONTEXT_ID_INITIAL_VALUE),
+                              "NEXT_CONTEXT_ID_INITIAL_VALUE");
+
+    // Negative test: continuity check
+    trace.set(C::tx_next_context_id, 2, 42);
+    EXPECT_THROW_WITH_MESSAGE(check_relation<tx_context>(trace, tx_context::SR_NEXT_CONTEXT_ID_CONTINUITY),
+                              "NEXT_CONTEXT_ID_CONTINUITY");
+}
+
 } // namespace
 } // namespace bb::avm2::constraining

barretenberg/cpp/src/barretenberg/vm2/constraining/relations/tx_context.test.cpp

@@ -129,10 +129,10 @@ namespace bb::avm2 {
 
 struct AvmFlavorVariables {
     static constexpr size_t NUM_PRECOMPUTED_ENTITIES = 133;
-    static constexpr size_t NUM_WITNESS_ENTITIES = 2921;
-    static constexpr size_t NUM_SHIFTED_ENTITIES = 314;
+    static constexpr size_t NUM_WITNESS_ENTITIES = 2922;
+    static constexpr size_t NUM_SHIFTED_ENTITIES = 315;
     static constexpr size_t NUM_WIRES = NUM_WITNESS_ENTITIES + NUM_PRECOMPUTED_ENTITIES;
-    static constexpr size_t NUM_ALL_ENTITIES = 3368;
+    static constexpr size_t NUM_ALL_ENTITIES = 3370;
 
     // Need to be templated for recursive verifier
     template <typename FF_>