Merge pull request #558 from powersync-ja/fix-download-race-condition

simolus3 · web-flow · commit ec5137e9c09d · 2025-04-03T17:00:19.000+02:00
Fix checkpoints during uploads not being applied
diff --git a/.changeset/violet-falcons-build.md b/.changeset/violet-falcons-build.md
@@ -0,0 +1,5 @@
+---
+'@powersync/common': patch
+---
+
+Fix a race condition causing sync changes during uploads not to be applied.
diff --git a/packages/common/src/client/AbstractPowerSyncDatabase.ts b/packages/common/src/client/AbstractPowerSyncDatabase.ts
@@ -15,7 +15,7 @@ import { Schema } from '../db/schema/Schema.js';
 import { BaseObserver } from '../utils/BaseObserver.js';
 import { ControlledExecutor } from '../utils/ControlledExecutor.js';
 import { mutexRunExclusive } from '../utils/mutex.js';
-import { throttleTrailing } from '../utils/throttle.js';
+import { throttleTrailing } from '../utils/async.js';
 import { SQLOpenFactory, SQLOpenOptions, isDBAdapter, isSQLOpenFactory, isSQLOpenOptions } from './SQLOpenFactory.js';
 import { PowerSyncBackendConnector } from './connection/PowerSyncBackendConnector.js';
 import { runOnSchemaChange } from './runOnSchemaChange.js';
diff --git a/packages/common/src/client/sync/stream/AbstractStreamingSyncImplementation.ts b/packages/common/src/client/sync/stream/AbstractStreamingSyncImplementation.ts
@@ -1,9 +1,9 @@
 import Logger, { ILogger } from 'js-logger';
 
-import { SyncPriorityStatus, SyncStatus, SyncStatusOptions } from '../../../db/crud/SyncStatus.js';
+import { SyncStatus, SyncStatusOptions } from '../../../db/crud/SyncStatus.js';
 import { AbortOperation } from '../../../utils/AbortOperation.js';
 import { BaseListener, BaseObserver, Disposable } from '../../../utils/BaseObserver.js';
-import { throttleLeadingTrailing } from '../../../utils/throttle.js';
+import { onAbortPromise, throttleLeadingTrailing } from '../../../utils/async.js';
 import { BucketChecksum, BucketDescription, BucketStorageAdapter, Checkpoint } from '../bucket/BucketStorageAdapter.js';
 import { CrudEntry } from '../bucket/CrudEntry.js';
 import { SyncDataBucket } from '../bucket/SyncDataBucket.js';
@@ -161,6 +161,7 @@ export abstract class AbstractStreamingSyncImplementation
   protected abortController: AbortController | null;
   protected crudUpdateListener?: () => void;
   protected streamingSyncPromise?: Promise<void>;
+  private pendingCrudUpload?: Promise<void>;
 
   syncStatus: SyncStatus;
   triggerCrudUpload: () => void;
@@ -181,10 +182,16 @@ export abstract class AbstractStreamingSyncImplementation
     this.abortController = null;
 
     this.triggerCrudUpload = throttleLeadingTrailing(() => {
-      if (!this.syncStatus.connected || this.syncStatus.dataFlowStatus.uploading) {
+      if (!this.syncStatus.connected || this.pendingCrudUpload != null) {
         return;
       }
-      this._uploadAllCrud();
+
+      this.pendingCrudUpload = new Promise((resolve) => {
+        this._uploadAllCrud().finally(() => {
+          this.pendingCrudUpload = undefined;
+          resolve();
+        });
+      });
     }, this.options.crudUploadThrottleMs!);
   }
 
@@ -434,16 +441,8 @@ The next upload iteration will be delayed.`);
         if (signal?.aborted) {
           break;
         }
-        const { retry } = await this.streamingSyncIteration(nestedAbortController.signal, options);
-        if (!retry) {
-          /**
-           * A sync error ocurred that we cannot recover from here.
-           * This loop must terminate.
-           * The nestedAbortController will close any open network requests and streams below.
-           */
-          break;
-        }
-        // Continue immediately
+        await this.streamingSyncIteration(nestedAbortController.signal, options);
+        // Continue immediately, streamingSyncIteration will wait before completing if necessary.
       } catch (ex) {
         /**
          * Either:
@@ -501,8 +500,8 @@ The next upload iteration will be delayed.`);
   protected async streamingSyncIteration(
     signal: AbortSignal,
     options?: PowerSyncConnectionOptions
-  ): Promise<{ retry?: boolean }> {
-    return await this.obtainLock({
+  ): Promise<void> {
+    await this.obtainLock({
       type: LockType.SYNC,
       signal,
       callback: async () => {
@@ -552,7 +551,7 @@ The next upload iteration will be delayed.`);
           const line = await stream.read();
           if (!line) {
             // The stream has closed while waiting
-            return { retry: true };
+            return;
           }
 
           // A connection is active and messages are being received
@@ -582,30 +581,12 @@ The next upload iteration will be delayed.`);
             await this.options.adapter.removeBuckets([...bucketsToDelete]);
             await this.options.adapter.setTargetCheckpoint(targetCheckpoint);
           } else if (isStreamingSyncCheckpointComplete(line)) {
-            this.logger.debug('Checkpoint complete', targetCheckpoint);
-            const result = await this.options.adapter.syncLocalDatabase(targetCheckpoint!);
-            if (!result.checkpointValid) {
-              // This means checksums failed. Start again with a new checkpoint.
-              // TODO: better back-off
-              await new Promise((resolve) => setTimeout(resolve, 50));
-              return { retry: true };
-            } else if (!result.ready) {
-              // Checksums valid, but need more data for a consistent checkpoint.
-              // Continue waiting.
-              // landing here the whole time
-            } else {
+            const result = await this.applyCheckpoint(targetCheckpoint!, signal);
+            if (result.endIteration) {
+              return;
+            } else if (result.applied) {
               appliedCheckpoint = targetCheckpoint;
-              this.logger.debug('validated checkpoint', appliedCheckpoint);
-              this.updateSyncStatus({
-                connected: true,
-                lastSyncedAt: new Date(),
-                dataFlow: {
-                  downloading: false,
-                  downloadError: undefined
-                }
-              });
             }
-
             validatedCheckpoint = targetCheckpoint;
           } else if (isStreamingSyncCheckpointPartiallyComplete(line)) {
             const priority = line.partial_checkpoint_complete.priority;
@@ -615,9 +596,10 @@ The next upload iteration will be delayed.`);
               // This means checksums failed. Start again with a new checkpoint.
               // TODO: better back-off
               await new Promise((resolve) => setTimeout(resolve, 50));
-              return { retry: true };
+              return;
             } else if (!result.ready) {
-              // Need more data for a consistent partial sync within a priority - continue waiting.
+              // If we have pending uploads, we can't complete new checkpoints outside of priority 0.
+              // We'll resolve this for a complete checkpoint.
             } else {
               // We'll keep on downloading, but can report that this priority is synced now.
               this.logger.debug('partial checkpoint validation succeeded');
@@ -691,7 +673,7 @@ The next upload iteration will be delayed.`);
                * (uses the same one), this should have some delay.
                */
               await this.delayRetry();
-              return { retry: true };
+              return ;
             }
             this.triggerCrudUpload();
           } else {
@@ -707,37 +689,67 @@ The next upload iteration will be delayed.`);
                 }
               });
             } else if (validatedCheckpoint === targetCheckpoint) {
-              const result = await this.options.adapter.syncLocalDatabase(targetCheckpoint!);
-              if (!result.checkpointValid) {
-                // This means checksums failed. Start again with a new checkpoint.
-                // TODO: better back-off
-                await new Promise((resolve) => setTimeout(resolve, 50));
-                return { retry: false };
-              } else if (!result.ready) {
-                // Checksums valid, but need more data for a consistent checkpoint.
-                // Continue waiting.
-              } else {
+              const result = await this.applyCheckpoint(targetCheckpoint!, signal);
+              if (result.endIteration) {
+                return;
+              } else if (result.applied) {
                 appliedCheckpoint = targetCheckpoint;
-                this.updateSyncStatus({
-                  connected: true,
-                  lastSyncedAt: new Date(),
-                  priorityStatusEntries: [],
-                  dataFlow: {
-                    downloading: false,
-                    downloadError: undefined
-                  }
-                });
               }
             }
           }
         }
         this.logger.debug('Stream input empty');
         // Connection closed. Likely due to auth issue.
-        return { retry: true };
+        return;
       }
     });
   }
 
+  private async applyCheckpoint(checkpoint: Checkpoint, abort: AbortSignal) {
+    let result = await this.options.adapter.syncLocalDatabase(checkpoint);
+    const pending = this.pendingCrudUpload;
+
+    if (!result.checkpointValid) {
+      this.logger.debug('Checksum mismatch in checkpoint, will reconnect');
+      // This means checksums failed. Start again with a new checkpoint.
+      // TODO: better back-off
+      await new Promise((resolve) => setTimeout(resolve, 50));
+      return { applied: false, endIteration: true };
+    } else if (!result.ready && pending != null) {
+      // We have pending entries in the local upload queue or are waiting to confirm a write
+      // checkpoint, which prevented this checkpoint from applying. Wait for that to complete and
+      // try again.
+      this.logger.debug(
+        'Could not apply checkpoint due to local data. Waiting for in-progress upload before retrying.'
+      );
+      await Promise.race([pending, onAbortPromise(abort)]);
+
+      if (abort.aborted) {
+        return { applied: false, endIteration: true };
+      }
+
+      // Try again now that uploads have completed.
+      result = await this.options.adapter.syncLocalDatabase(checkpoint);
+    }
+
+    if (result.checkpointValid && result.ready) {
+      this.logger.debug('validated checkpoint', checkpoint);
+      this.updateSyncStatus({
+        connected: true,
+        lastSyncedAt: new Date(),
+        dataFlow: {
+          downloading: false,
+          downloadError: undefined
+        }
+      });
+
+      return { applied: true, endIteration: false };
+    } else {
+      this.logger.debug('Could not apply checkpoint. Waiting for next sync complete line.');
+      return { applied: false, endIteration: false };
+    }
+  }
+
   protected updateSyncStatus(options: SyncStatusOptions) {
     const updatedStatus = new SyncStatus({
       connected: options.connected ?? this.syncStatus.connected,
diff --git a/packages/common/src/utils/async.ts b/packages/common/src/utils/async.ts
@@ -48,3 +48,13 @@ export function throttleLeadingTrailing(func: () => void, wait: number) {
     }
   };
 }
+
+export function onAbortPromise(signal: AbortSignal): Promise<void> {
+  return new Promise<void>((resolve) => {
+    if (signal.aborted) {
+      resolve();
+    } else {
+      signal.onabort = () => resolve();
+    }
+  });
+}
diff --git a/packages/web/tests/stream.test.ts b/packages/web/tests/stream.test.ts
@@ -46,6 +46,101 @@ describe('Streaming', { sequential: true }, () => {
       })
     )
   );
+
+  it('Should handle checkpoints during the upload process', async () => {
+    const { powersync, remote, uploadSpy } = await generateConnectedDatabase();
+    expect(powersync.connected).toBe(true);
+
+    let resolveUploadPromise: () => void;
+    let resolveUploadStartedPromise: () => void;
+    const completeUploadPromise = new Promise<void>((resolve) => {
+      resolveUploadPromise = resolve;
+    });
+    const uploadStartedPromise = new Promise<void>((resolve) => {
+      resolveUploadStartedPromise = resolve;
+    });
+
+    async function expectUserRows(amount: number) {
+      const row = await powersync.get<{ r: number }>('SELECT COUNT(*) AS r FROM users');
+      expect(row.r).toBe(amount);
+    }
+
+    uploadSpy.mockImplementation(async (db) => {
+      const batch = await db.getCrudBatch();
+      if (!batch) return;
+
+      resolveUploadStartedPromise();
+      await completeUploadPromise;
+      await batch?.complete();
+    });
+
+    // trigger an upload
+    await powersync.execute('INSERT INTO users (id, name) VALUES (uuid(), ?)', ['from local']);
+    await expectUserRows(1);
+    await uploadStartedPromise;
+
+    // A connector could have uploaded data (triggering a checkpoint) before finishing
+    remote.enqueueLine({
+      checkpoint: {
+        write_checkpoint: '1',
+        last_op_id: '2',
+        buckets: [{ bucket: 'a', priority: 3, checksum: 0 }]
+      }
+    });
+    remote.generateCheckpoint.mockImplementation(() => {
+      return {
+        data: {
+          write_checkpoint: '1'
+        }
+      };
+    });
+
+    remote.enqueueLine({
+      data: {
+        bucket: 'a',
+        data: [
+          {
+            checksum: 0,
+            op_id: '1',
+            op: 'PUT',
+            object_id: '1',
+            object_type: 'users',
+            data: '{"id": "test1", "name": "from local"}'
+          },
+          {
+            checksum: 0,
+            op_id: '2',
+            op: 'PUT',
+            object_id: '2',
+            object_type: 'users',
+            data: '{"id": "test1", "name": "additional entry"}'
+          }
+        ]
+      }
+    });
+    remote.enqueueLine({
+      checkpoint_complete: {
+        last_op_id: '2'
+      }
+    });
+
+    // Give the sync client some time to process these
+    await new Promise<void>((resolve) => setTimeout(resolve, 500));
+
+    // Despite receiving a valid checkpoint with two rows, it should not be visible because we have local data.
+    await expectUserRows(1);
+
+    // Mark the upload as completed. This should trigger a write_checkpoint.json request
+    resolveUploadPromise!();
+    await vi.waitFor(() => {
+      expect(remote.generateCheckpoint.mock.calls.length).equals(1);
+    });
+
+    // Completing the upload should also make the checkpoint visible without it being sent again.
+    await vi.waitFor(async () => {
+      await expectUserRows(2);
+    });
+  });
 });
 
 function describeStreamingTests(createConnectedDatabase: () => Promise<ConnectedDatabaseUtils>) {
diff --git a/packages/web/tests/utils/MockStreamOpenFactory.ts b/packages/web/tests/utils/MockStreamOpenFactory.ts
@@ -18,6 +18,7 @@ import {
   WebPowerSyncOpenFactoryOptions,
   WebStreamingSyncImplementation
 } from '@powersync/web';
+import { MockedFunction, vi } from 'vitest';
 
 export class TestConnector implements PowerSyncBackendConnector {
   async fetchCredentials(): Promise<PowerSyncCredentials> {
@@ -35,12 +36,21 @@ export class TestConnector implements PowerSyncBackendConnector {
 export class MockRemote extends AbstractRemote {
   streamController: ReadableStreamDefaultController<StreamingSyncLine> | null;
   errorOnStreamStart = false;
+  generateCheckpoint: MockedFunction<() => any>;
+
   constructor(
     connector: RemoteConnector,
     protected onStreamRequested: () => void
   ) {
     super(connector);
     this.streamController = null;
+    this.generateCheckpoint = vi.fn(() => {
+      return {
+        data: {
+          write_checkpoint: '1000'
+        }
+      };
+    });
   }
 
   async getBSON(): Promise<BSONImplementation> {
@@ -53,11 +63,7 @@ export class MockRemote extends AbstractRemote {
   async get(path: string, headers?: Record<string, string> | undefined): Promise<any> {
     // mock a response for write checkpoint API
     if (path.includes('checkpoint')) {
-      return {
-        data: {
-          write_checkpoint: '1000'
-        }
-      };
+      return this.generateCheckpoint();
     }
     throw new Error('Not implemented');
   }

Original file line number	Diff line number	Diff line change
`@@ -48,3 +48,13 @@ export function throttleLeadingTrailing(func: () => void, wait: number) {`
`48`	`48`	`}`
`49`	`49`	`};`
`50`	`50`	`}`
	`51`	`+`
	`52`	`+export function onAbortPromise(signal: AbortSignal): Promise<void> {`
	`53`	`+ return new Promise<void>((resolve) => {`
	`54`	`+ if (signal.aborted) {`
	`55`	`+ resolve();`
	`56`	`+ } else {`
	`57`	`+ signal.onabort = () => resolve();`
	`58`	`+ }`
	`59`	`+ });`
	`60`	`+}`