fix: Ensure MessageContext and its children are actually cleared between queries

packages/pglite/.changeset/README.md

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+# Changesets
+
+Hello and welcome! This folder has been automatically generated by `@changesets/cli`, a build tool that works
+with multi-package repos, or single-package repos to help you version and publish your code. You can
+find the full documentation for it [in our repository](https://github.com/changesets/changesets)
+
+We have a quick list of common questions to get you started engaging with this project in
+[our documentation](https://github.com/changesets/changesets/blob/main/docs/common-questions.md)

packages/pglite/.changeset/big-lobsters-bathe.md

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+---
+'@electric-sql/pglite': minor
+---
+
+Ensure MessageContext and its children are actually cleared between queries

packages/pglite/.changeset/config.json

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+{
+  "$schema": "https://unpkg.com/@changesets/config@3.0.3/schema.json",
+  "changelog": "@changesets/cli/changelog",
+  "commit": false,
+  "fixed": [],
+  "linked": [],
+  "access": "restricted",
+  "baseBranch": "main",
+  "updateInternalDependencies": "patch",
+  "ignore": []
+}

packages/pglite/tests/message-context-leak.test.ts

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+import { describe, it, expect, beforeEach } from 'vitest'
+import { testDTC } from './test-utils.js'
+import { PGlite } from '../dist/index.js'
+
+// This test isolates the MessageContext leak reported in
+// https://github.com/electric-sql/pglite/issues/779
+// It inserts many rows with large JSON literals and then inspects
+// pg_backend_memory_contexts to ensure MessageContext has been reset
+// between queries and does not accumulate unbounded allocations.
+
+const KB = 1024
+
+function makeJsonBlob(size: number): string {
+  // Keep the SQL literal simple (mostly "x" payload) to simulate
+  // large messages while avoiding excessive parsing overhead.
+  return JSON.stringify({ padding: 'x'.repeat(size) })
+}
+
+testDTC(async (defaultDataTransferContainer) => {
+  describe('MessageContext reset between queries', () => {
+    let db: PGlite
+
+    beforeEach(async () => {
+      db = new PGlite({ defaultDataTransferContainer })
+      await db.exec(`
+        CREATE TABLE IF NOT EXISTS leak_test (
+          id SERIAL PRIMARY KEY,
+          blob jsonb NOT NULL
+        );
+      `)
+    })
+
+    it('does not accumulate allocations in MessageContext', async () => {
+      // Choose sizes to expose the leak without taking too long.
+      const blobSize = 100 * KB // ~100KB per row
+      const rows = 300 // ~30MB of total SQL literal payload
+
+      const blob = makeJsonBlob(blobSize)
+
+      for (let i = 0; i < rows; i++) {
+        await db.exec(`INSERT INTO leak_test (blob) VALUES ('${blob}')`)
+      }
+
+      // After the loop, the next query should see a freshly reset
+      // MessageContext (reset happens at the start of command read),
+      // so used_bytes should remain small (well below the total data inserted).
+      const mem = await db.query<{ used_bytes: number }>(`
+        SELECT used_bytes
+        FROM pg_backend_memory_contexts
+        WHERE name = 'MessageContext'
+        ORDER BY level
+        LIMIT 1
+      `)
+
+      expect(mem.rows).toHaveLength(1)
+      const used = Number(mem.rows[0].used_bytes)
+
+      // On a correctly resetting build, MessageContext should typically be in the
+      // low kilobytes to a few megabytes range. Set an upper bound that will fail
+      // if allocations accumulated across the INSERTs (~30MB of literal payload).
+      // Using 5MB as a generous ceiling for transient allocations of this SELECT.
+      expect(used).toBeLessThan(5 * 1024 * 1024)
+    })
+  })
+})
+