Possible locking issues in PollingWildCardChangeToken?

[omajid]

runtime/src/libraries/Microsoft.Extensions.FileProviders.Physical/src/PollingWildCardChangeToken.cs

Lines 87 to 106 in c5e8f83

	public bool HasChanged
	{
	get
	{
	if (_changed)
	{
	return _changed;
	}
	if (Clock.UtcNow - _lastScanTimeUtc >= PollingInterval)
	{
	lock (_enumerationLock)
	{
	_changed = CalculateChanges();
	}
	}
	return _changed;
	}
	}

This code checks _changed but not under locks. It doesn't double-check it either. It seems like this race might be possible:

0. Thread 1 and 2 enter this method
1. Thread 1 sees _changed is false
2. Thread 2 sees _changed is false
3. Thread 1 obtains lock, and scans files and updates _previousHash and sets _changed = true
4. Thread 2 obtains lock, scans files, comparing the just-updated-by-Thread-1 value of _previousHash with the new hash value, and concludes _changed is false.

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