fix(sync-service): cold nested subquery shape times out subscribing to a dependency materializer (#4715)

[robacourt]

Summary

A cold nested subquery shape could return an initial HTTP 500 while its dependency snapshots were still in progress. A retry succeeded once the dependency snapshots finished.

Fixes #4715

Problem

When an outer shape's consumer initializes, Consumer.all_materializers_alive?/1 subscribes to each dependency materializer via Materializer.subscribe/1. That was the only materializer call using the default 5s GenServer.call timeout — wait_until_ready/1 and new_changes/3 both use :infinity.

A materializer can't answer any call until handle_continue(:start_materializer) returns, and that blocks on Consumer.await_snapshot_start(..., :infinity) until its own snapshot starts. For a cold nested topology, the intermediate materializer itself waits on a deeper dependency, so its snapshot can start later than 5s after the outer consumer subscribes. The subscribe then times out, Electric removes the outer shape, and the client's first request 500s:

Removing shape "..." due to abnormal shutdown: {:timeout, {GenServer, :call, [#PID, :subscribe, 5000]}}
  ... consumer.ex: Electric.Shapes.Consumer.all_materializers_alive?/1
  ... consumer.ex: Electric.Shapes.Consumer.finish_initialization/3

This matches the logs reported in #4715.

Solution

Materializer.subscribe/* now waits with :infinity, consistent with the other materializer calls. This matches the intended semantics from the issue — "a single cold nested shape waits for dependency materializers and completes without an externally visible 500."

Why this is safe:

• Liveness is already handled by the caller: all_materializers_alive?/1 does Process.monitor(pid, ...) before subscribing, so a dead materializer surfaces as a call exit rather than being masked by a short timeout.
• No deadlock: the shape-dependency graph is a DAG (outer → inner), so no dependency can transitively wait on the outer consumer.
• Replication is not stalled by the longer block: the outer shape is registered with the ShapeLogCollector's EventRouter (via SetupEffects → ShapeLogCollector.add_shape) only after all_materializers_alive?/1 returns, inside initialize_event_handler. While the consumer is blocked in the subscribe it is not yet a routing target, so the collector never broadcasts a transaction to it. The overall wait is bounded by the request-level await_snapshot_start(outer, 45000) budget.

Reproduction / Test Plan

Added a :slow router test (test/electric/plug/router_test.exs) that deterministically reproduces the bug by stalling only the dependency shape's snapshot past the 5s subscribe window (via an injected :create_snapshot_fn), then asserting the cold nested shape still serves 200.

• Test fails on main with the {:timeout, {GenServer, :call, [_, :subscribe, 5000]}} signature
• Test passes with the fix
• materializer_test.exs, consumer_test.exs, and the /v1/shapes - subqueries router tests pass

Run the reproduction:

mix test test/electric/plug/router_test.exs --include slow --only line:3505

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Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.01%. Comparing base (4051e0f) to head (b2a0d7a).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4729      +/-   ##
==========================================
- Coverage   60.04%   60.01%   -0.03%     
==========================================
  Files         397      397              
  Lines       43766    43766              
  Branches    12586    12588       +2     
==========================================
- Hits        26281    26268      -13     
- Misses      17403    17416      +13     
  Partials       82       82              

Flag	Coverage Δ
packages/agents	72.64% <ø> (ø)
packages/agents-mcp	77.70% <ø> (ø)
packages/agents-mobile	80.67% <ø> (ø)
packages/agents-runtime	83.73% <ø> (+0.01%)	⬆️
packages/agents-server	75.47% <ø> (-0.21%)	⬇️
packages/agents-server-ui	8.32% <ø> (ø)
packages/electric-ax	51.06% <ø> (ø)
packages/experimental	87.73% <ø> (ø)
packages/react-hooks	86.48% <ø> (ø)
packages/start	82.83% <ø> (ø)
packages/typescript-client	91.78% <ø> (ø)
packages/y-electric	56.05% <ø> (ø)
typescript	60.01% <ø> (-0.03%)	⬇️
unit-tests	60.01% <ø> (-0.03%)	⬇️

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