Add a lock when pushing changes.

This is not needed for proper deployments where the server properly locks
the database tables. But in some older etesync server versions and possibly
bad databases, this is not the case.
OpenTasks causes EteSync to concurrently try to sync, so this at least makes
sure this race condition doesn't happen for these rare cases.

app/src/main/java/com/etesync/syncadapter/syncadapter/SyncManager.kt

@@ -33,8 +33,10 @@ import java.io.Closeable
 import java.io.FileNotFoundException
 import java.io.IOException
 import java.util.*
+import java.util.concurrent.locks.ReentrantLock
 import java.util.logging.Level
 import javax.net.ssl.SSLHandshakeException
+import kotlin.concurrent.withLock
 
 abstract class SyncManager<T: LocalResource<*>> @Throws(Exceptions.IntegrityException::class, Exceptions.GenericCryptoException::class)
 constructor(protected val context: Context, protected val account: Account, protected val settings: AccountSettings, protected val extras: Bundle, protected val authority: String, protected val syncResult: SyncResult, journalUid: String, protected val serviceType: CollectionInfo.Type, accountName: String): Closeable {
@@ -393,8 +395,11 @@ constructor(protected val context: Context, protected val account: Account, prot
         // FIXME: Deal with failure (someone else uploaded before we go here)
         try {
             if (!localEntries!!.isEmpty()) {
-                val entries = localEntries
-                journal!!.create(entries!!, remoteCTag)
+                val entries = localEntries!!
+                pushLock.withLock {
+                    journal!!.create(entries, remoteCTag)
+                }
+
                 // Persist the entries after they've been pushed
                 for (entry in entries) {
                     val cEntry = SyncEntry.fromJournalEntry(crypto, entry)
@@ -577,5 +582,7 @@ constructor(protected val context: Context, protected val account: Account, prot
     companion object {
         private val MAX_FETCH = 50
         private val MAX_PUSH = 30
+
+        private val pushLock = ReentrantLock()
     }
 }