CBL-4428 : Fix crash when starting multiple live queries concurrently #3209
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* Background: Based on LiteCore’s thread safety doc, c4queryobs_* (except c4queryobs_create which is thread-safe) functions should be called under database-exclusive lock. Without doing that, when creating and starting multiple live queries concurrently, there could be a race condition as each live queries try to get the background database at the same time. The result of the race is that multiple background databases could be created but only the lastone stays and the rest will be freed. Therefore, some live queries will hold on to the bad background database which will cause the crash when the database is used. * Fixed CBLQueryObserver to follow thread-safety instruction by calling c4queryobs_* functions under the database exclusive lock. * Fixed Swift Query’s ListenerToken’s remove() function. After calling the objective-c CBLListenerToken’s remove() function, it will need to do some additional cleanup as well. * Removed CBLQuery and CBLQueryObserver’s description function to use the default description. Currently they are used for some logging but those excessive information is redundant with what LiteCore is currently logging. More importantly, they could cause some crashes as they don’t handle null objects properly.
jianminzhao
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Dec 18, 2023
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Background: Based on LiteCore’s thread safety doc, c4queryobs_* (except c4queryobs_create which is thread-safe) functions should be called under database-exclusive lock. Without doing that, when creating and starting multiple live queries concurrently, there could be a race condition as each live queries try to get the background database at the same time. The result of the race is that multiple background databases could be created but only the lastone stays and the rest will be freed. Therefore, some live queries will hold on to the bad background database which will cause the crash when the database is used.
Fixed CBLQueryObserver to follow thread-safety instruction by calling c4queryobs_* functions under the database exclusive lock.
Fixed Swift Query’s ListenerToken’s remove() function. After calling the objective-c CBLListenerToken’s remove() function, it will need to do some additional cleanup as well.
Removed CBLQuery and CBLQueryObserver’s description function to use the default description. Currently they are used for some logging but those excessive information is redundant with what LiteCore is currently logging. More importantly, they could cause some crashes as they don’t handle null objects properly.