Disallow multiple sync accounts

app/src/main/java/org/koitharu/kotatsu/sync/ui/SyncAuthActivity.kt

@@ -8,6 +8,7 @@ import android.text.Editable
 import android.text.TextWatcher
 import android.view.View
 import android.widget.Button
+import android.widget.Toast
 import androidx.activity.OnBackPressedCallback
 import androidx.activity.viewModels
 import androidx.core.graphics.Insets
@@ -54,6 +55,9 @@ class SyncAuthActivity : BaseActivity<ActivitySyncAuthBinding>(), View.OnClickLi
 		viewModel.onTokenObtained.observe(this, ::onTokenReceived)
 		viewModel.onError.observe(this, ::onError)
 		viewModel.isLoading.observe(this, ::onLoadingStateChanged)
+		viewModel.onAccountAlreadyExists.observe(this) {
+			onAccountAlreadyExists()
+		}
 
 		supportFragmentManager.setFragmentResultListener(SyncHostDialogFragment.REQUEST_KEY, this, this)
 		pageBackCallback.update()
@@ -151,6 +155,16 @@ class SyncAuthActivity : BaseActivity<ActivitySyncAuthBinding>(), View.OnClickLi
 		finish()
 	}
 
+	private fun onAccountAlreadyExists() {
+		Toast.makeText(this, R.string.account_already_exists, Toast.LENGTH_SHORT)
+			.show()
+		accountAuthenticatorResponse?.onError(
+			AccountManager.ERROR_CODE_UNSUPPORTED_OPERATION,
+			getString(R.string.account_already_exists),
+		)
+		super.finishAfterTransition()
+	}
+
 	private class EmailTextWatcher(
 		private val button: Button,
 	) : TextWatcher {

app/src/main/java/org/koitharu/kotatsu/sync/ui/SyncAuthViewModel.kt

@@ -1,5 +1,6 @@
 package org.koitharu.kotatsu.sync.ui
 
+import android.accounts.AccountManager
 import android.content.Context
 import androidx.lifecycle.MutableLiveData
 import dagger.hilt.android.lifecycle.HiltViewModel
@@ -19,11 +20,22 @@ class SyncAuthViewModel @Inject constructor(
 	private val api: SyncAuthApi,
 ) : BaseViewModel() {
 
+	val onAccountAlreadyExists = SingleLiveEvent<Unit>()
 	val onTokenObtained = SingleLiveEvent<SyncAuthResult>()
 	val host = MutableLiveData("")
 
 	private val defaultHost = context.getString(R.string.sync_host_default)
 
+	init {
+		launchJob(Dispatchers.Default) {
+			val am = AccountManager.get(context)
+			val accounts = am.getAccountsByType(context.getString(R.string.account_type_sync))
+			if (accounts.isNotEmpty()) {
+				onAccountAlreadyExists.emitCall(Unit)
+			}
+		}
+	}
+
 	fun obtainToken(email: String, password: String) {
 		val hostValue = host.value.ifNullOrEmpty { defaultHost }
 		launchLoadingJob(Dispatchers.Default) {