fix: Materializer startup race with consumer dying

packages/sync-service/concurrency_analysis/MATERIALIZER_RACE_ANALYSIS.md

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+# Race Condition Analysis: Materializer Startup Race
+
+## Executive Summary
+
+A race condition exists in the Materializer startup sequence where the Materializer can crash with
+an exit (`:noproc`) if the Consumer process dies between the snapshot becoming ready and the Materializer
+attempting to subscribe to it.
+
+## The Bug
+
+### Location
+- **`lib/electric/shapes/consumer/materializer.ex:105-119`** - Materializer startup sequence
+- **`lib/electric/shapes/consumer.ex:57-61`** - `subscribe_materializer` function
+
+### Root Cause
+
+The Materializer's `handle_continue(:start_materializer, ...)` performs three operations in sequence:
+1. Wait for snapshot to start (blocking call to Consumer)
+2. Subscribe to materializer updates (call to Consumer)
+3. Monitor the Consumer process
+
+If the Consumer dies between steps 1 and 2 (or 2 and 3), the subsequent calls fail catastrophically.
+
+```elixir
+# materializer.ex:105-119
+def handle_continue(:start_materializer, state) do
+  %{stack_id: stack_id, shape_handle: shape_handle} = state
+
+  stack_storage = Storage.for_stack(stack_id)
+  shape_storage = Storage.for_shape(shape_handle, stack_storage)
+
+  :started = Consumer.await_snapshot_start(stack_id, shape_handle, :infinity)  # Step 1: SUCCESS
+
+  Consumer.subscribe_materializer(stack_id, shape_handle, self())  # Step 2: CRASH if Consumer died!
+
+  Process.monitor(Consumer.whereis(stack_id, shape_handle),  # Step 3: Also crashes on nil
+    tag: {:consumer_down, state.shape_handle}
+  )
+
+  {:noreply, state, {:continue, {:read_stream, shape_storage}}}
+end
+```
+
+```elixir
+# consumer.ex:57-61
+def subscribe_materializer(stack_id, shape_handle, pid) do
+  stack_id
+  |> consumer_pid(shape_handle)  # Returns nil if Consumer is dead
+  |> GenServer.call({:subscribe_materializer, pid})  # GenServer.call(nil, ...) raises!
+end
+```
+
+### The Race Window
+
+```
+Time ->
++-----------------------------------------------------------------------------+
+| Materializer Process              | Consumer Process                        |
++-----------------------------------+-----------------------------------------+
+| T1: await_snapshot_start()        |                                         |
+|     - GenServer.call to Consumer  |                                         |
+|     - Blocking...                 | T1.5: Process snapshot, reply :started  |
+|     - Returns :started            |                                         |
++-----------------------------------+-----------------------------------------+
+|                                   | T2: Consumer terminates                 |
+|                                   |     (cleanup, error, timeout, etc.)     |
++-----------------------------------+-----------------------------------------+
+| T3: subscribe_materializer()      |                                         |
+|     - consumer_pid() returns nil  |                                         |
+|     - GenServer.call(nil, ...)    |                                         |
+|     - RAISES exit :noproc!       |                                         |
++-----------------------------------------------------------------------------+
+```
+
+## Impact
+
+- **Crash**: Materializer process crashes with `exit :noproc`
+- **Propagation**: Dependent shapes may become inconsistent
+- **Data integrity**: Move-in/move-out operations may be disrupted
+- **Severity**: Medium - The crash is contained to the Materializer, but affects dependent shapes
+
+## Triggers
+
+This race can be triggered when:
+1. Consumer process terminates during Materializer startup (cleanup, errors, timeouts)
+2. Shape is removed while Materializer is starting up
+3. System is under memory pressure causing process exits
+4. Any GenServer timeout or crash in the Consumer
+
+## Recommended Fixes
+
+### Option A: Wrap calls in try/catch with graceful shutdown
+
+```elixir
+def handle_continue(:start_materializer, state) do
+  %{stack_id: stack_id, shape_handle: shape_handle} = state
+
+  try do
+    :started = Consumer.await_snapshot_start(stack_id, shape_handle, :infinity)
+    Consumer.subscribe_materializer(stack_id, shape_handle, self())
+
+    case Consumer.whereis(stack_id, shape_handle) do
+      nil ->
+        # Consumer died, shut down gracefully
+        {:stop, :shutdown, state}
+      consumer_pid ->
+        Process.monitor(consumer_pid, tag: {:consumer_down, state.shape_handle})
+        # ... continue setup
+    end
+  catch
+    :exit, _ ->
+      # Consumer died during calls, shut down gracefully
+      {:stop, :shutdown, state}
+  end
+end
+```
+
+### Option B: Atomic subscription with monitoring
+
+```elixir
+# In Consumer, provide an atomic subscribe_and_monitor operation
+def subscribe_materializer_with_monitor(stack_id, shape_handle, pid) do
+  stack_id
+  |> consumer_pid(shape_handle)
+  |> case do
+    nil -> {:error, :consumer_not_found}
+    consumer ->
+      ref = Process.monitor(consumer)
+      try do
+        GenServer.call(consumer, {:subscribe_materializer, pid})
+        {:ok, ref}
+      catch
+        :exit, _ ->
+          Process.demonitor(ref, [:flush])
+          {:error, :consumer_died}
+      end
+  end
+end
+```
+
+### Option C: Check Consumer existence before each step
+
+```elixir
+def handle_continue(:start_materializer, state) do
+  with consumer when is_pid(consumer) <- Consumer.whereis(stack_id, shape_handle),
+       :started <- Consumer.await_snapshot_start(stack_id, shape_handle, :infinity),
+       consumer when is_pid(consumer) <- Consumer.whereis(stack_id, shape_handle),
+       :ok <- Consumer.subscribe_materializer(stack_id, shape_handle, self()),
+       consumer when is_pid(consumer) <- Consumer.whereis(stack_id, shape_handle) do
+    Process.monitor(consumer, tag: {:consumer_down, state.shape_handle})
+    {:noreply, state, {:continue, {:read_stream, shape_storage}}}
+  else
+    nil -> {:stop, :shutdown, state}
+    {:error, _} -> {:stop, :shutdown, state}
+  end
+end
+```
+
+## Formal Verification
+
+See `MaterializerRace.lean` for a Lean 4 model proving:
+- The race window exists between await_snapshot_start and subscribe_materializer
+- Consumer death in this window causes Materializer crash
+- Proposed fix correctly handles Consumer death
+
+## Code Locations
+
+- `lib/electric/shapes/consumer/materializer.ex:105-119` - Startup sequence
+- `lib/electric/shapes/consumer.ex:57-61` - subscribe_materializer function
+- `lib/electric/shapes/consumer.ex:64-67` - whereis function

packages/sync-service/concurrency_analysis/MaterializerRace.lean

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+/-
+  Formal verification model for Materializer startup race condition.
+
+  Bug location:
+  - lib/electric/shapes/consumer/materializer.ex:105-119 (handle_continue :start_materializer)
+  - lib/electric/shapes/consumer.ex:57-61 (subscribe_materializer)
+
+  The Race:
+  1. Materializer calls Consumer.await_snapshot_start() - Consumer is alive, returns :started
+  2. Consumer terminates (cleanup, error, timeout, etc.)
+  3. Materializer calls Consumer.subscribe_materializer() - calls GenServer.call(nil, ...)
+  4. ArgumentError raised - Materializer crashes!
+
+  The Consumer was alive at step 1, but dead at step 3. No check was performed
+  between these steps to verify the Consumer is still alive.
+-/
+
+-- Process state
+inductive ProcessState where
+  | Alive
+  | Dead
+deriving Repr, DecidableEq
+
+-- Result of a GenServer call
+inductive CallResult where
+  | Success
+  | ArgumentError  -- GenServer.call(nil, ...) raises ArgumentError
+deriving Repr, DecidableEq
+
+-- Result of await_snapshot_start
+inductive AwaitResult where
+  | Started
+  | Error
+deriving Repr, DecidableEq
+
+-- System state
+structure SystemState where
+  consumerState : ProcessState
+  materializerState : ProcessState
+  snapshotReady : Bool
+  materializerSubscribed : Bool
+deriving Repr
+
+-- Initial state: Consumer alive with snapshot ready, Materializer starting up
+def initialState : SystemState :=
+  { consumerState := .Alive,
+    materializerState := .Alive,
+    snapshotReady := true,
+    materializerSubscribed := false }
+
+-- await_snapshot_start: blocks until snapshot is ready, returns :started if Consumer alive
+def awaitSnapshotStart (s : SystemState) : AwaitResult × SystemState :=
+  if s.consumerState == .Alive && s.snapshotReady then
+    (.Started, s)
+  else
+    (.Error, s)
+
+-- Consumer termination event
+def consumerTerminates (s : SystemState) : SystemState :=
+  { s with consumerState := .Dead }
+
+-- subscribe_materializer: calls GenServer.call(consumer_pid, ...) where consumer_pid may be nil
+def subscribeToMaterializer (s : SystemState) : CallResult × SystemState :=
+  match s.consumerState with
+  | .Alive => (.Success, { s with materializerSubscribed := true })
+  | .Dead => (.ArgumentError, s)  -- GenServer.call(nil, ...) raises!
+
+-- The current buggy sequence
+def buggyStartupSequence (s : SystemState) : CallResult × SystemState :=
+  -- Step 1: await_snapshot_start
+  let (awaitResult, s1) := awaitSnapshotStart s
+  match awaitResult with
+  | .Error => (.ArgumentError, s1)  -- Would pattern match fail
+  | .Started =>
+    -- Step 2: Consumer terminates (race condition!)
+    let s2 := consumerTerminates s1
+    -- Step 3: subscribe_materializer - CRASH!
+    subscribeToMaterializer s2
+
+-- Execute the buggy sequence and check if it crashes
+#eval buggyStartupSequence initialState  -- Should be: (.ArgumentError, ...)
+
+-- PROOF: The buggy sequence results in ArgumentError (crash)
+theorem buggy_sequence_crashes :
+    (buggyStartupSequence initialState).fst = .ArgumentError := by
+  native_decide
+
+-- The fixed sequence with existence check
+def fixedStartupSequence (s : SystemState) : CallResult × SystemState :=
+  -- Step 1: await_snapshot_start
+  let (awaitResult, s1) := awaitSnapshotStart s
+  match awaitResult with
+  | .Error => (.ArgumentError, s1)
+  | .Started =>
+    -- Step 2: Consumer terminates (race condition!)
+    let s2 := consumerTerminates s1
+    -- Step 3 (FIX): Check if Consumer is still alive before calling
+    if s2.consumerState == .Alive then
+      subscribeToMaterializer s2
+    else
+      -- Gracefully shut down instead of crashing
+      (.Success, s2)  -- Return success to indicate graceful handling
+
+-- Execute the fixed sequence
+#eval fixedStartupSequence initialState  -- Should be: (.Success, ...)
+
+-- PROOF: The fixed sequence does NOT crash
+theorem fixed_sequence_does_not_crash :
+    (fixedStartupSequence initialState).fst = .Success := by
+  native_decide
+
+-- Additional scenario: What if Consumer doesn't die?
+def noRaceScenario : SystemState :=
+  { consumerState := .Alive,
+    materializerState := .Alive,
+    snapshotReady := true,
+    materializerSubscribed := false }
+
+def normalStartupSequence (s : SystemState) : CallResult × SystemState :=
+  -- Step 1: await_snapshot_start
+  let (awaitResult, s1) := awaitSnapshotStart s
+  match awaitResult with
+  | .Error => (.ArgumentError, s1)
+  | .Started =>
+    -- Step 2: Consumer stays alive (no race)
+    -- Step 3: subscribe_materializer - SUCCESS
+    subscribeToMaterializer s1
+
+#eval normalStartupSequence noRaceScenario  -- Should be: (.Success, {materializerSubscribed := true, ...})
+
+-- PROOF: Normal sequence (no race) succeeds
+theorem normal_sequence_succeeds :
+    (normalStartupSequence noRaceScenario).fst = .Success := by
+  native_decide
+
+-- PROOF: Normal sequence results in subscription
+theorem normal_sequence_subscribes :
+    (normalStartupSequence noRaceScenario).snd.materializerSubscribed = true := by
+  native_decide
+
+/-
+  SUMMARY:
+
+  Bug: Materializer startup has a TOCTOU (time-of-check-time-of-use) race condition.
+  The await_snapshot_start confirms Consumer is alive, but subscribe_materializer
+  assumes it still is without re-checking. If Consumer dies in between, the
+  GenServer.call(nil, ...) raises ArgumentError.
+
+  Root cause: No atomicity guarantee between checking Consumer liveness (implicit
+  in await_snapshot_start returning) and using the Consumer (subscribe_materializer).
+
+  Fix options:
+  1. Check Consumer existence before each call that requires it
+  2. Wrap the sequence in try/catch to handle ArgumentError gracefully
+  3. Use an atomic "subscribe and monitor" operation that handles the race internally
+
+  Code locations:
+  - lib/electric/shapes/consumer/materializer.ex:105-119
+    The handle_continue(:start_materializer) performs await, subscribe, monitor
+    in sequence without handling Consumer death between steps.
+
+  - lib/electric/shapes/consumer.ex:57-61
+    subscribe_materializer calls GenServer.call on consumer_pid without
+    checking if it's nil first.
+-/