SyncManager: don't create worker threads when not necessary; shut Executor down after using it

import at.bitfire.ical4android.TaskProvider

import at.bitfire.ical4android.TaskProvider

import at.bitfire.ical4android.UsesThreadContextClassLoader

import at.bitfire.ical4android.UsesThreadContextClassLoader

import at.bitfire.vcard4android.ContactsStorageException

import at.bitfire.vcard4android.ContactsStorageException

import kotlinx.coroutines.asCoroutineDispatcher

import kotlinx.coroutines.*

import kotlinx.coroutines.coroutineScope

import kotlinx.coroutines.launch

import kotlinx.coroutines.runBlocking

import okhttp3.HttpUrl

import okhttp3.HttpUrl

import okhttp3.RequestBody

import okhttp3.RequestBody

import org.apache.commons.lang3.exception.ContextedException

import org.apache.commons.lang3.exception.ContextedException

     * We use our own dispatcher to make sure that all threads have [Thread.getContextClassLoader] set,

     * We use our own dispatcher to make sure that all threads have [Thread.getContextClassLoader] set,

     * which is required for dav4jvm and ical4j (because they rely on [ServiceLoader]).

     * which is required for dav4jvm and ical4j (because they rely on [ServiceLoader]).

     */

     */

    private val workDispatcher = Executors.newFixedThreadPool(

    private val _workDispatcher = lazy {

                // number of threads = number of CPUs, but max. 4

        Executors     // number of threads = number of CPUs, but max. 4

                min(Runtime.getRuntime().availableProcessors(), 4)

            .newFixedThreadPool(min(Runtime.getRuntime().availableProcessors(), 4))

            ).asCoroutineDispatcher()

            .asCoroutineDispatcher()

    }

    private val workDispatcher by _workDispatcher

    private val mainAccount = if (localCollection is LocalAddressBook)

    private val mainAccount = if (localCollection is LocalAddressBook)

        localCollection.mainAccount

        localCollection.mainAccount

    override fun close() {

    override fun close() {

        httpClient.close()

        httpClient.close()

        // shut down worker threads (only if the work dispatcher has been actually created)

        if (_workDispatcher.isInitialized())

            workDispatcher.close()

    }

    }