feat: 修复下载bug

app/src/main/java/io/legado/app/service/DownloadService.kt

@@ -88,17 +88,25 @@ class DownloadService : BaseService() {
 
     private fun getBook(bookUrl: String): Book? {
         var book = bookMap[bookUrl]
+        if (book == null) {
+            synchronized(this) {
+                book = bookMap[bookUrl]
                 if (book == null) {
                     book = App.db.bookDao().getBook(bookUrl)
                 }
                 if (book == null) {
                     removeDownload(bookUrl)
                 }
+            }
+        }
         return book
     }
 
     private fun getWebBook(bookUrl: String, origin: String): WebBook? {
         var webBook = webBookMap[origin]
+        if (webBook == null) {
+            synchronized(this) {
+                webBook = webBookMap[origin]
                 if (webBook == null) {
                     App.db.bookSourceDao().getBookSource(origin)?.let {
                         webBook = WebBook(it)
@@ -107,6 +115,8 @@ class DownloadService : BaseService() {
                 if (webBook == null) {
                     removeDownload(bookUrl)
                 }
+            }
+        }
         return webBook
     }