fix(GODT-2224): Properly handle context cancellation during sync

There was an issue where new attachment download requests would hang
forever due to not checking whether the context was cancelled. At this
point there were no more workers to consume to channel messages.

internal/user/sync.go

@@ -344,7 +344,7 @@ func syncMessages(
 
 	flushUpdateCh := make(chan flushUpdate)
 
-	errorCh := make(chan error, maxParallelDownloads+2)
+	errorCh := make(chan error, maxParallelDownloads*4)
 
 	// Go routine in charge of downloading message metadata
 	logging.GoAnnotated(ctx, func(ctx context.Context) {
@@ -360,13 +360,15 @@ func syncMessages(
 			metadata, err := client.GetMessageMetadataPage(ctx, 0, len(metadataChunk), proton.MessageFilter{ID: metadataChunk})
 			if err != nil {
 				downloadReq.err = err
-				downloadCh <- downloadReq
+				select {
+				case downloadCh <- downloadReq:
+				case <-ctx.Done():
+					return
+				}
 				return
 			}
 
 			if ctx.Err() != nil {
-				downloadReq.err = err
-				downloadCh <- downloadReq
 				return
 			}
 
@@ -789,7 +791,11 @@ func (a *attachmentDownloader) getAttachments(ctx context.Context, attachments [
 	resultChs := make([]chan attachmentResult, len(attachments))
 	for i, id := range attachments {
 		resultChs[i] = make(chan attachmentResult, 1)
-		a.workerCh <- attachmentJob{id: id.ID, result: resultChs[i], size: id.Size}
+		select {
+		case a.workerCh <- attachmentJob{id: id.ID, result: resultChs[i], size: id.Size}:
+		case <-ctx.Done():
+			return nil, ctx.Err()
+		}
 	}
 
 	result := make([][]byte, len(attachments))