DB Error: database is locked

[cbenhagen]

When one download finishes while others are being created it can happen that the DB is locked.

[hnvn]

I think this case only happens on the Android side because I already queued all write to DB actions on iOS. My main idea is: lock for write and allow read always. By the way, I forgot running a transaction when I insert a new task into DB. That would account for this bug.

[cbenhagen]

Sorry I forgot to mention that I only tried this on iOS so far. This is where the bug occurs.

[hnvn]

Is there any log for this case?

[cbenhagen]

methodCallHandler: enqueue
execute query: INSERT INTO task (task_id,url,status,progress,file_name,saved_dir,headers,resumable,show_notification,open_file_from_notification,time_created) VALUES ("com.example.test.download.background.1536665611.774887.14","https://example.com/A001C015_180901_A4R4-thumb.jpg?mtime=1536384218.54",1,0,"A001C015_180901_A4R4-thumb.jpg","/Users/ben/Library/Developer/CoreSimulator/Devices/2370C450-CC60-41A5-912A-E0E583C20889/data/Containers/Data/Application/8800C18D-EAFB-4670-B856-A5E9EF543B36/Documents","{authorization: Basic aabbcc=}",0,0,0,1536665630531)
open DB successfully
Query was executed successfully. Affected rows = 1
methodCallHandler: enqueue
execute query: INSERT INTO task (task_id,url,status,progress,file_name,saved_dir,headers,resumable,show_notification,open_file_from_notification,time_created) VALUES ("com.example.test.download.background.1536665611.774887.15","https://example.com/A001C016_180901_A4R4-thumb.jpg?mtime=1536384218.57",1,0,"A001C016_180901_A4R4-thumb.jpg","/Users/ben/Library/Developer/CoreSimulator/Devices/2370C450-CC60-41A5-912A-E0E583C20889/data/Containers/Data/Application/8800C18D-EAFB-4670-B856-A5E9EF543B36/Documents","{authorization: Basic aabbcc=}",0,0,0,1536665630542)
open DB successfully
Query was executed successfully. Affected rows = 1
methodCallHandler: enqueue
execute query: INSERT INTO task (task_id,url,status,progress,file_name,saved_dir,headers,resumable,show_notification,open_file_from_notification,time_created) VALUES ("com.example.test.download.background.1536665611.774887.16","https://example.com/A001C017_180901_A4R4-thumb.jpg?mtime=1536384218.59",1,0,"A001C017_180901_A4R4-thumb.jpg","/Users/ben/Library/Developer/CoreSimulator/Devices/2370C450-CC60-41A5-912A-E0E583C20889/data/Containers/Data/Application/8800C18D-EAFB-4670-B856-A5E9EF543B36/Documents","{authorization: Basic aabbcc=}",0,0,0,1536665630550)
open DB successfully
Query was executed successfully. Affected rows = 1
methodCallHandler: enqueue
execute query: INSERT INTO task (task_id,url,status,progress,file_name,saved_dir,headers,resumable,show_notification,open_file_from_notification,time_created) VALUES ("com.example.test.download.background.1536665611.774887.17","https://example.com/A001C018_180901_A4R4-thumb.jpg?mtime=1536384218.61",1,0,"A001C018_180901_A4R4-thumb.jpg","/Users/ben/Library/Developer/CoreSimulator/Devices/2370C450-CC60-41A5-912A-E0E583C20889/data/Containers/Data/Application/8800C18D-EAFB-4670-B856-A5E9EF543B36/Documents","{authorization: Basic aabbcc=}",0,0,0,1536665630560)
execute query: SELECT * FROM task WHERE task_id = "com.example.test.download.background.1536665611.774887.1" ORDER BY id DESC LIMIT 1
open DB successfully
open DB successfully
Load task successfully
Query was executed successfully. Affected rows = 1
methodCallHandler: enqueue
execute query: UPDATE task SET status=3, progress=100 WHERE task_id="com.example.test.download.background.1536665611.774887.1"
open DB successfully
Query was executed successfully. Affected rows = 1
execute query: INSERT INTO task (task_id,url,status,progress,file_name,saved_dir,headers,resumable,show_notification,open_file_from_notification,time_created) VALUES ("com.example.test.download.background.1536665611.774887.18","https://example.com/A001C019_180901_A4R4-thumb.jpg?mtime=1536384218.72",1,0,"A001C019_180901_A4R4-thumb.jpg","/Users/ben/Library/Developer/CoreSimulator/Devices/2370C450-CC60-41A5-912A-E0E583C20889/data/Containers/Data/Application/8800C18D-EAFB-4670-B856-A5E9EF543B36/Documents","{authorization: Basic aabbcc=}",0,0,0,1536665630572)
execute query: SELECT * FROM task WHERE task_id = "com.example.test.download.background.1536665611.774887.2" ORDER BY id DESC LIMIT 1
open DB successfully
open DB successfully
database is locked
Load task successfully
Query was executed successfully. Affected rows = 1
methodCallHandler: enqueue
Lost connection to device.
*** First throw call stack:
(
	0   CoreFoundation                      0x0000000109dbb7f6 __exceptionPreprocess + 294
	1   libobjc.A.dylib                     0x0000000108f35035 objc_exception_throw + 48
	2   CoreFoundation                      0x0000000109dbb675 +[NSException raise:format:] + 197
	3   Foundation                          0x00000001089f6422 -[NSURL(NSURL) initFileURLWithPath:] + 127
	4   Foundation                          0x00000001089f6c8e +[NSURL(NSURL) fileURLWithPath:] + 45
	5   Runner                              0x0000000106f48ff7 -[FlutterDownloaderPlugin URLSession:downloadTask:didFinishDownloadingToURL:] + 503
	6   CFNetwork                           0x000000010ebcf5f8 __82-[NSURLSession delegate_downloadTask:didFinishDownloadingToURL:completionHandler:]_block_invoke + 38
	7   Foundation                          0x00000001089a32b7 __NS<…>

[hnvn]

That would be the problem of multiple threads. Two threads access to DB at the moment and as far as the logs run, the later action was failed by DB locked then it returns an empty object, the saved file path can't be loaded, so application crashed because the function [NSURL initFileURLWithPath:] doesn't accept a nil value. That's the case I read from your logs.
Multiple threads in objective-c is quite dark for me. I think I should keep the information of running task in memory instead of DB, so I can get rid of loading tasks from DB too frequently and prevent this matter.

[hnvn]

By the way, I am quite busy during the week. There can be time at the weekend. So I am willing if you can contribute to this repository.

[cbenhagen]

I tried implementing a retry loop but it seemed quite ugly and didn't fully work. But I'd give it another shot. Objc is pretty new for me though. We could also have the DB handler in one thread and give it the data through a queue.