Artifact used (ex. androidx.biometric:biometric:1.0.0-alpha02):
Devices/Android versions reproduced on: Android P, and also Android M-O

Sometimes, when the Biometric dialogs are displayed, and then the app is exited (with back button), the code inside the various fragments, BiometricFragment, FingerprintHelperFragment, and FingerprintDialogFragment doesn't properly clean up.

For example this code in FingerprintHelperFragment:

/**
* Remove the fragment so that resources can be freed.
*/
void cleanup() {
if (getActivity() != null) {
getActivity().getSupportFragmentManager().beginTransaction().remove(this).commit();
}
}

can be instrumented with a breakpoint, and shown that sometimes getActivity() is null, and so the fragment transaction to remove the fragment never happens. So, the next time the app tries to authenticate with the BiometricPrompt, the app will crash because the fragment has already been added.

The exception on Android M-O looks like:

java.lang.IllegalStateException: Fragment already added: FingerprintDialogFragment{8d6c61c #3 FingerprintDialogFragment}
at androidx.fragment.app.FragmentManagerImpl.addFragment(FragmentManager.java:1916)
at androidx.fragment.app.BackStackRecord.executeOps(BackStackRecord.java:765)
at androidx.fragment.app.FragmentManagerImpl.executeOps(FragmentManager.java:2625)
at androidx.fragment.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2411)
at androidx.fragment.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2366)
at androidx.fragment.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2273)
at androidx.fragment.app.FragmentManagerImpl$1.run(FragmentManager.java:733)
at android.os.Handler.handleCallback(Handler.java:790)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6494)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

and on Android P it will reference BiometricFragment instead.

Attempts to search for and remove fragments at runtime, before a Biometric prompt operation were attempted:

val fm = (context as FragmentActivity).supportFragmentManager

fm.fragments.find {
it is BiometricFragment || it is FingerprintDialogFragment
}.also { fragment ->
fragment?.let {
fm.beginTransaction().remove(it).commitAllowingStateLoss()
}
}

However, then the exception on Android M-O mentions FingerprintHelperFragment instead.

Which led to an attempt to mitigate this exception:

fm.fragments.find {
it is FingerprintHelperFragment
}.also { fragment ->
fragment?.let { fm.beginTransaction().remove(it).commitAllowingStateLoss() }
}

Which caused the fingerprint operations to exit with an error 5, that the operation had been cancelled.

Here is examples of how we are using the API:

fun enroll(context: Activity, username: String, refreshToken: String): Completable =
Completable.create { emitter ->

// AN-939
cleanupBiometricsFragment(context)

val callback = makeAuthenticationCallback {
try {
storeCredentials(username, refreshToken)

preferenceManager.putBoolean(PREF_BIOMETRICS_ENROLLED, true)
preferenceManager.putString(PREF_ENROLLED_EMAIL, username)

// This is what the Settings switch uses
preferenceManager.biometricsEnabled.set(true)

emitter.onComplete()
} catch (ex: Throwable) {
emitter.onError(ex)
}
}

val promptInfo = BiometricPrompt.PromptInfo.Builder()
.setTitle(context.getString(R.string.biometrics_enroll_title))
.setDescription(context.getString(R.string.biometrics_enroll_subtitle))
.setNegativeButtonText(context.getString(R.string.biometrics_cancel))
.build()

try {
BiometricPrompt(context as FragmentActivity, MAIN_THREAD, callback)
.authenticate(promptInfo)
} catch (ex: Throwable) {
preferenceManager.putBoolean(PREF_BIOMETRICS_ENROLLED, false)
emitter.onError(ex)
}
}

Not sure, from the API if there's some way to ensure that everything is cleaned up properly at the end of the callback. I tried calling biometricPrompt.cancelAuthentication() but that also does not fix this issue.

If required, will try to make a reproduction in a small test app to demonstrate the problem.