Reap orphaned rewrite-rpc node servers to bound the live count

[timtebeek]

Problem

OpenRewrite runs Python/JavaScript/TypeScript (and Go/C#) parsing and printing through out-of-process Node rewrite-rpc servers. RewriteRpcProcessManager held one server per thread in a ThreadLocal, started lazily by getOrStart() and only ever torn down by shutdown()/shutdownCurrent() — which is also thread-local and can only reach the calling thread's server.

A node server is a separate OS process with no finalizer: letting its owning RewriteRpc become garbage does not stop the process. So any server whose owning thread terminates before calling shutdown() is orphaned and survives until the JVM-exit shutdown hook fires. Threads that come and go during a run do exactly this:

• ForkJoinPool ManagedBlocker compensation threads (the hazard already called out in RewriteRpc.send()),
• cached / elastic pool threads that retire on their idle timeout.

Each spawns its own server via getOrStart(); none of them is the orchestrator thread that calls shutdownCurrent(). The orphaned processes accumulate without bound across a long-lived host JVM. Worse, each orphaned RewriteRpcProcess keeps its entire RewriteRpc object graph (object/ref caches, prepared recipes) heap-reachable via the shutdown-hook list. On a large run (a DevCenter recipe over ~4200 repos, the tail of which were Python) this produced many live node servers at once and contributed to an OOM.

The create↔dispose accounting was asymmetric: servers are created on any thread, but disposed from one thread.

Fix

RewriteRpcProcessManager now tracks every started server in a process-wide, thread-keyed registry (replacing the bare ThreadLocal) so the full set of live servers can be enumerated:

• Per-thread reuse is unchanged — a thread still gets and reuses exactly one server.
• getOrStart() and shutdown() reap servers whose owning thread is no longer alive. Reaping a dead thread's server is always safe (a terminated thread cannot be mid-RPC), so it is sound even while other threads are doing concurrent RPC work. Reaping is activity-driven: it runs whenever a new RPC thread appears and whenever the existing per-run shutdownCurrent() is called — no call-site changes required. So a dead thread's server lingers only until the next such event (rather than until JVM exit), and the residual live-server count is bounded by the number of RPC threads that have retired since the last reap, not by the total number of threads ever created.
• New shutdownAll() (exposed as PythonRewriteRpc.shutdownAll(), JavaScriptRewriteRpc.shutdownAll(), etc.) tears down every live server, driving the count to zero. It stops servers on currently-live threads too, so it is documented for use only when no RPC work is in flight (JVM/service shutdown). It is not required for correctness on JVM exit — each subprocess already registers its own JVM-exit shutdown hook — it only bounds the count earlier within a long-running JVM.
• liveCount() exposes the live-server count for diagnostics and tests.

Tests

RewriteRpcProcessManagerTest uses a counting fake server (no real subprocess) to assert:

• one server per thread, reused;
• across 200 sequential runs the live count never exceeds one and ends at zero;
• 50 transient threads that die without cleanup are all reaped on the next getOrStart(), leaving exactly one live server;
• shutdown() on the orchestrator thread reaps a dead-thread orphan as well as its own server;
• shutdownAll() drives the count to zero even with servers held open on live threads;
• resilience: when one server's teardown throws, the remaining servers are still torn down — covered for both shutdownAll() and shutdown()'s reap path (via a ThrowingRpc stub).

The fake server chains super.shutdown() so each stub's backing JsonRpc ForkJoinPool is torn down rather than leaking worker threads across the suite, and the live-threads test uses a guarded wait + bounded join so a future regression fails fast instead of hanging.

Out of scope

• The Moderne worker's RECIPE_EXECUTOR is an unbounded newCachedThreadPool(); this PR bounds orphaned (dead-thread) servers but not concurrently-live ones, so bounding the executor's parallelism (so the host spawns "no more servers than needed") is a separate worker-side change.
• DevCenterStarter memory use on Python repos, observed in the same run, is not addressed here.

[timtebeek]

@kmccarp as you've explored the rpc servers running in longer running processes, would you be comfortable taking a first pass through this if it aligns with your findings and thoughts?

[timtebeek]

Unassigning myself as this likely needs more scrutiny than I can fit in for now; I've logged an internal issue to follow up.

[timtebeek]

Closing the PR; we'll track internally and have someone else pick it up fresh.