Singleton database creation is not thread-safe

[rhashimoto]

URL of codelab:
https://developer.android.com/codelabs/basic-android-kotlin-compose-persisting-data-room#0

In which task and step of the codelab can this issue be found?
Task 7 (Create a Database instance), step 20. The text says:

Multiple threads can potentially ask for a database instance at the same time, which results in two databases instead of one. This issue is known as a race condition. Wrapping the code to get the database inside a synchronized block means that only one thread of execution at a time can enter this block of code, which makes sure the database only gets initialized once. Use synchronized{} block to avoid the race condition.

Describe the problem
As written, InventoryDatabase.getDatabase() is actually not thread-safe. There is a race condition if another thread sets Instance after the test but before the current thread acquires the lock.

The proper pattern is double-checked locking which requires another test of the condition inside the synchronized block.

basic-android-kotlin-compose-training-inventory-app/app/src/main/java/com/example/inventory/data/InventoryDatabase.kt

Lines 36 to 39 in e0773b7

	fun getDatabase(context: Context): InventoryDatabase {
	// if the Instance is not null, return it, otherwise create a new database instance.
	return Instance ?: synchronized(this) {
	Room.databaseBuilder(context, InventoryDatabase::class.java, "item_database")

[rhashimoto]

Duplicate of #122.