Implement press, and hold switch actions#2
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OwenMcGirr
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Jan 2, 2026
Fixed race condition in getInstance() where multiple threads could create
multiple database instances due to incomplete double-checked locking.
**Problem:**
The original code only checked INSTANCE before entering synchronized:
```kotlin
return INSTANCE ?: synchronized(this) {
val instance = Room.databaseBuilder(...).build()
INSTANCE = instance
instance
}
```
**Race Condition Scenario:**
1. Thread A checks INSTANCE == null
2. Thread B checks INSTANCE == null
3. Thread A enters synchronized, creates DB instance #1
4. Thread B waits for lock
5. Thread B enters synchronized, creates DB instance #2 (BUG!)
6. Two database instances exist, causing corruption
**Solution:**
Added second null-check inside synchronized block (double-checked locking):
```kotlin
return INSTANCE ?: synchronized(this) {
// Second check AFTER acquiring lock
INSTANCE ?: Room.databaseBuilder(...).build().also { INSTANCE = it }
}
```
**How It Works:**
1. Thread A checks INSTANCE == null (first check)
2. Thread B checks INSTANCE == null (first check)
3. Thread A enters synchronized, checks again (second check), creates instance
4. Thread B waits for lock
5. Thread B enters synchronized, checks again (second check), sees INSTANCE != null
6. Thread B returns existing instance - no duplicate!
**Thread Safety:**
- @volatile ensures INSTANCE writes are visible across threads
- synchronized prevents concurrent creation
- Double-check minimizes synchronization overhead (only on first access)
This is the standard double-checked locking pattern for thread-safe
singleton initialization.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
3 tasks
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