Retry failed attachment downloads

SignalServiceKit/src/Messages/Attachments/OWSAttachmentDownloads.m

@@ -378,6 +378,10 @@ typedef void (^AttachmentDownloadFailure)(NSError *error);
         });
     };
 
+    __block NSUInteger retryCount = 0;
+    NSUInteger maxRetryCount = 4;
+    __block void (^attempt)();
+    attempt = ^() {
         dispatch_async([OWSDispatch attachmentsQueue], ^{
             [self downloadFromLocation:attachmentPointer.downloadURL
                 job:job
@@ -387,10 +391,17 @@ typedef void (^AttachmentDownloadFailure)(NSError *error);
                                         success:markAndHandleSuccess
                                         failure:markAndHandleFailure];
                 }
-            failure:^(NSURLSessionTask *_Nullable task, NSError *error) {
+                failure:^(NSURLSessionTask *task, NSError *error) {
+                    if (retryCount == maxRetryCount) {
                         markAndHandleFailure(error);
+                    } else {
+                        retryCount += 1;
+                        attempt();
+                    }
                 }];
         });
+    };
+    attempt();
 }
 
 - (void)decryptAttachmentPath:(NSString *)encryptedDataFilePath
@@ -630,6 +641,7 @@ typedef void (^AttachmentDownloadFailure)(NSError *error);
             }
             successHandler(tempFilePath);
         }];
+
     [task resume];
 }