IssueTracker

Hi Ed, Thank you so much for these suggestions. I've been reviewing them and merging them in. Hopefully it should be live. I've included a thank you note too in the article.

manually closing the FingerprintHelperFragment works as a workaround
```
   override fun onPause() {
        // WORKAROUND for https://issuetracker.google.com/issues/123857949
        activity?.let { activity ->
            activity.supportFragmentManager.findFragmentByTag("FingerprintHelperFragment")?.let {
                activity.supportFragmentManager.beginTransaction().remove(it).commitAllowingStateLoss()
            }
            super.onPause()
        }
    }
```

Issue is reproducible. More details are here - https://drive.google.com/corp/drive/folders/1jaiApoKzTxQc-Rg46D8qmP9MpKhkBo2D  (internal only link).

There's might be something slightly different in the way lifecycle is being managed between O and lower, and P+

Looking at how mAuthenticationCallback is being passed around here should give us some ideas

https://cs.corp.google.com/aosp-androidx/biometric/src/main/java/androidx/biometric/BiometricPrompt.java?q=biometricprompt.java&dr

Josh can you take a look when convenient? I think the callback isn't getting to the FingerprintHelperFragment for some reason. We can also chat offline when you get to this.

This bug is actually a duplicate of https://issuetracker.google.com/issues/121117380. This issue happens because the new AuthenticationCallback is not being supplied to the biometric prompt fragments if they already exist in the FragmentManager.

Until this is fixed, users of this library MUST manually check for and remove any biometric prompt fragments from the FragmentManager before attempting to call BiometricPrompt#authenticate().

#7
I think that:

1. BiometricPrompt#authenticateInternal should call mFingerprintHelperFragment.setCallback method even if mFingerprintHelperFragment was existed before.
2. FingerprintHelperFragment#cleanup has to set mClientAuthenticationCallback and mExecutor to null to prevent leaking.

My mistake in point one:
1. Biometric#mLifecycleObserver#onResume should call mFingerprintHelperFragment.setCallback(...) even if mFingerprintDialogFragment is null.

Here is my temporary fix. Just place it in androidx.biometric package and use it like BiometricPrompt:

package androidx.biometric

import android.annotation.SuppressLint
import androidx.fragment.app.FragmentActivity
import java.util.concurrent.Executor

/**
 * Fix for isssue https://issuetracker.google.com/issues/123857949
 */
@SuppressLint("RestrictedApi")
class FixedBiometricPrompt(
    private val fragmentActivity: FragmentActivity,
    executor: Executor,
    callback: BiometricPrompt.AuthenticationCallback
) {
    private val delegate: BiometricPrompt

    init {
        // set executor and callback to helper fragment
        (fragmentActivity.supportFragmentManager.findFragmentByTag("FingerprintHelperFragment") as? FingerprintHelperFragment)?.setCallback(
            executor,
            callback
        )

        delegate = BiometricPrompt(fragmentActivity, executor, callback)
    }

    fun authenticate(info: BiometricPrompt.PromptInfo, crypto: BiometricPrompt.CryptoObject) {
        delegate.authenticate(info, crypto)
    }

    fun authenticate(info: BiometricPrompt.PromptInfo) {
        delegate.authenticate(info)
    }

    fun cancelAuthentication() {
        delegate.cancelAuthentication()
    }

    fun cleanup() {
        // remove executor and callback from helper fragment to prevent memory leak
        (fragmentActivity.supportFragmentManager.findFragmentByTag("FingerprintHelperFragment") as? FingerprintHelperFragment)?.setCallback(
            null,
            null
        )
    }
}

...
Then call function cleanup() in onDestroyView...

    override fun onDestroyView() {
        super.onDestroyView()

        biometricPrompt.cleanup()
    }

the proper fix (on google's side) is pretty simple
on line 498
if (mFingerprintHelperFragment == null) {
                mFingerprintHelperFragment = FingerprintHelperFragment.newInstance();
                mFingerprintHelperFragment.setCallback(mExecutor, mAuthenticationCallback);   // <-- this line should be moved out side the if statement
            }

Hmm we already fixed this. Just need to cut a new build for alpha04 now.

The bug was referenced in this CL but somehow buganizer didn't close it

https://android-review.googlesource.com/c/platform/frameworks/support/+/917882/