Composite: don't undo writes made while a full sync is scanning

A write made during a full sync could be scanned in the replica but not in
the (read-ahead) source scan, so the diff removed it, and echo tracking then
consumed the server's echo. Keys written during a sync are now left out of
the diff. Deterministic regression test with a read-ahead source.

Also exclude -0 from the Extended JSON property test (JSON has no -0).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017Qxbkp4tA7G5SCWjzWuxmE

test/kv-storage/kv-storage-composite.test.ts

@@ -3,7 +3,7 @@ import { KVStorageComposite } from '../../src/kv-storage/kv-storage-composite.js
 import { KVStorageInMemory } from '../../src/kv-storage/kv-storage-in-memory.js'
 import type { BaseKVStorage } from '../../src/kv-storage/base-kv-storage.js'
 import { DocumentStorageKV } from '../../src/document-storage/document-storage-kv.js'
-import { collect } from '../helpers.js'
+import { collect, KVStorageCounting } from '../helpers.js'
 import { runKVStorageConformance } from './kv-storage.conformance.js'
 import { TestNetwork } from './sync-helpers.js'
 
@@ -141,4 +141,39 @@ describe('KVStorageComposite sync', () => {
     network.online = false
     expect((await collect(users.find({ age: { $gt: 35 } }))).map((r) => r.key)).toEqual(['2'])
   })
+
+  it('does not undo writes made while a full sync is scanning', async () => {
+    // A source whose scan reads ahead (like the HTTP stream) and pauses after its first entry.
+    let release!: () => void
+    let reached!: () => void
+    const gate = new Promise<void>((r) => (release = r))
+    const paused = new Promise<void>((r) => (reached = r))
+    const inner = new KVStorageInMemory()
+    await inner.writeMany([
+      ['a', 1],
+      ['c', 3],
+    ])
+    const source = new KVStorageCounting(inner)
+    const scan = source.scan.bind(source)
+    source.scan = async function* (options) {
+      let first = true
+      for (const entry of await collect(scan(options))) {
+        yield entry
+        if (first && options?.values) ((first = false), reached(), await gate)
+      }
+    }
+    const persistent = new KVStorageInMemory()
+    await persistent.writeMany([['a0', 'stale']]) // a replica copy from before, without sync metadata
+    const db = new KVStorageComposite({ source, replicas: [persistent] })
+    void db.start()
+    await paused // the diff has passed 'b' in the source but not in the replica
+    await db.writeMany([['b', 2]])
+    release()
+    await db.synced()
+    expect(await collect(db.scan({ values: true }))).toEqual([
+      ['a', 1],
+      ['b', 2],
+      ['c', 3],
+    ])
+  })
 })

test/utils/extended-json.test.ts

@@ -16,7 +16,7 @@ describe('ExtendedJson', () => {
         fc.bigInt(),
         fc.uint8Array(),
         fc.string(),
-        fc.double({ noNaN: true, noDefaultInfinity: true }),
+        fc.double({ noNaN: true, noDefaultInfinity: true }).map((x) => (x === 0 ? 0 : x)) /* JSON has no -0 */,
         fc.boolean(),
         fc.constant(null),
         fc.array(tie('value')),