Fix sync client dropping lines (#960)

Author: simolus3

.changeset/short-gifts-repeat.md

@@ -0,0 +1,5 @@
+---
+'@powersync/common': patch
+---
+
+Fix internal client skipping sync lines, causing checksum mismatch errors.

packages/common/src/utils/stream_transform.ts

@@ -50,6 +50,7 @@ export function injectable<T>(source: SimpleAsyncIterator<T>): InjectableIterato
   let waiter: Waiter | undefined = undefined; // An active, waiting next() call.
   // A pending upstream event that couldn't be dispatched because inject() has been called before it was resolved.
   let pendingSourceEvent: ((w: Waiter) => void) | null = null;
+  let sourceFetchInFlight = false;
 
   let pendingInjectedEvents: T[] = [];
 
@@ -61,6 +62,8 @@ export function injectable<T>(source: SimpleAsyncIterator<T>): InjectableIterato
 
   const fetchFromSource = () => {
     const resolveWaiter = (propagate: (w: Waiter) => void) => {
+      sourceFetchInFlight = false;
+
       const active = consumeWaiter();
       if (active) {
         propagate(active);
@@ -69,6 +72,7 @@ export function injectable<T>(source: SimpleAsyncIterator<T>): InjectableIterato
       }
     };
 
+    sourceFetchInFlight = true;
     const nextFromSource = source.next();
     nextFromSource.then(
       (value) => {
@@ -101,7 +105,9 @@ export function injectable<T>(source: SimpleAsyncIterator<T>): InjectableIterato
 
         // Nothing pending? Fetch from source
         waiter = { resolve, reject };
-        return fetchFromSource();
+        if (!sourceFetchInFlight) {
+          fetchFromSource();
+        }
       });
     },
     inject: (event) => {

packages/common/tests/utils/stream_transform.test.ts

@@ -92,6 +92,32 @@ describe('injectable', () => {
       expect(await next).toStrictEqual(doneResult);
     }
   });
+
+  test('does not start a second source fetch while one is already in flight', async () => {
+    const outstandingEvents: Array<(value: IteratorResult<number>) => void> = [];
+
+    const stream = injectable<number>({
+      next() {
+        expect(outstandingEvents).toHaveLength(0);
+        return new Promise((r) => outstandingEvents.push(r));
+      }
+    });
+
+    // First downstream call starts an event
+    const p1 = stream.next();
+    expect(outstandingEvents).toHaveLength(1);
+
+    // inject() steals the waiter; event remains in flight
+    stream.inject(-1);
+    expect(await p1).toStrictEqual(valueResult(-1));
+
+    // Second downstream call while event is still pending. Should wait for it instead of starting another one.
+    const p2 = stream.next();
+
+    // Clean up
+    outstandingEvents[0](valueResult(10));
+    expect(await p2).toStrictEqual(valueResult(10));
+  });
 });
 
 describe('bson objects', () => {