[donbowman]

Summary

Parallel test workers (for example pytest-xdist with -n 8) each install the PGlite npm packages into their own work directory today, so a test run does one npm install per worker. This adds a node_modules_dir config option that installs them once, guarded by a cross-process file lock, and links the result into each work directory:

config = PGliteConfig(
    work_dir=Path(f"/tmp/pglite-worker-{worker_id}"),
    node_modules_dir=Path("/tmp/py-pglite-npm"),
)

• The first worker installs; the others wait on the lock and reuse it.
• The lock (flock / msvcrt.locking) is released by the OS if the installer dies, so a crashed process cannot leave a stale lock.
• Changing the dependency set (enabling an extension, upgrading py-pglite) invalidates and reinstalls the shared copy automatically.
• Behaviour is unchanged when node_modules_dir is not set (install into the work directory).

Package alignment

The branch moves the npm pins to the latest compatible releases:

• @electric-sql/pglite ^0.5.8
• @electric-sql/pglite-socket ^0.2.11
• @electric-sql/pglite-pgvector ^0.0.9

Since pglite 0.5.x moved vector out of the main bundle, SUPPORTED_EXTENSIONS["pgvector"] now points at @electric-sql/pglite-pgvector, matching #52's approach.

Readiness fix

With the newer socket server, the existing readiness probe (bare connect + immediate close) causes the next connection to be dropped. Two changes make startup deterministic:

• the generated script awaits db.waitReady before server.start(), so the socket only accepts once PGlite can serve queries;
• the Python readiness check performs a real PostgreSQL startup handshake (StartupMessage, AuthenticationOk / cleartext password, ReadyForQuery, graceful Terminate) instead of a bare socket probe.

Without this, tests/test_extensions.py::test_pgvector_extension fails with server closed the connection unexpectedly on the new packages.

Overlap

• fix: update pglite 0.5.0, allow multiple concurrent connections #52 (update-pglite) already brings the 0.5.0 migration and pgvector package; this branch carries the newer 0.5.8 pin set plus the readiness changes. Happy to rebase once fix: update pglite 0.5.0, allow multiple concurrent connections #52 lands.
• make package.json content a variable #42 changes package.json content to be configurable; _package_json_content() here is the single place that content is generated, which composes cleanly with that.

Testing

• tests/test_shared_node_modules.py: shared install reuse, stale-install invalidation, linking over an existing directory, lock timeout, package pins, startup probe, generated-script ordering.
• tests/test_extensions.py::test_pgvector_extension passes with the new packages.
• Core manager / advanced / connection-management slices: 89 passed locally.