Fix back-reference index in AndroidCalendar.addEvent (#59)

* Make `BackReference` a data class to see the back reference value

* Add test for problem

* Fix back-reference index in AndroidCalendar event addition

- Modify `addEvent` method in `AndroidCalendar` to use the correct back-reference index for event rows.
- Update `withValueBackReference` function to include documentation about proper usage.

Author: rfc2822

lib/src/androidTest/kotlin/at/bitfire/synctools/storage/calendar/AndroidCalendarTest.kt

@@ -8,6 +8,7 @@ package at.bitfire.synctools.storage.calendar
 
 import android.accounts.Account
 import android.content.ContentProviderClient
+import android.content.ContentUris
 import android.content.ContentValues
 import android.content.Entity
 import android.provider.CalendarContract
@@ -17,6 +18,7 @@ import androidx.core.content.contentValuesOf
 import androidx.test.platform.app.InstrumentationRegistry
 import at.bitfire.ical4android.impl.TestCalendar
 import at.bitfire.ical4android.util.MiscUtils.closeCompat
+import at.bitfire.synctools.storage.BatchOperation
 import at.bitfire.synctools.test.InitCalendarProviderRule
 import at.bitfire.synctools.test.assertContentValuesEqual
 import org.junit.After
@@ -84,6 +86,44 @@ class AndroidCalendarTest {
         assertEquals(123, result.reminders.first().getAsInteger(Reminders.MINUTES))
     }
 
+    @Test
+    fun testAddEvent_toBatch_AsSecondOperation() {
+        val batch = CalendarBatchOperation(client)
+
+        // first operation: no-op
+        batch += BatchOperation.CpoBuilder
+            .newUpdate(calendar.eventsUri)
+            .withValue(Events._SYNC_ID, "won't happen")
+            .withSelection("${Events._SYNC_ID}=?", arrayOf("testAddEvent_toBatch_AsSecondOperation"))
+
+        // second operation (event row index > 0)
+        val values = contentValuesOf(
+            Events.CALENDAR_ID to calendar.id,
+            Events.DTSTART to now,
+            Events.DTEND to now + 3600000,
+            Events.TITLE to "Some Event"
+        )
+        val entity = Entity(values)
+        val reminder = contentValuesOf(
+            Reminders.MINUTES to 123
+        )
+        entity.subValues.add(Entity.NamedContentValues(Reminders.CONTENT_URI, reminder))
+        val idx = batch.nextBackrefIdx()
+        calendar.addEvent(entity, batch)
+
+        batch.commit()
+        val id = ContentUris.parseId(batch.getResult(idx)!!.uri!!)
+
+        // verify that event has been inserted
+        val result = calendar.getEvent(id)!!
+        assertEquals(id, result.id)
+        assertEquals(now, result.dtStart)
+        assertEquals(now + 3600000, result.dtEnd)
+        assertEquals("Some Event", result.title)
+        assertEquals(1, result.reminders.size)
+        assertEquals(123, result.reminders.first().getAsInteger(Reminders.MINUTES))
+    }
+
     @Test
     fun testFindEvents() {
         calendar.addEvent(Entity(contentValuesOf(

lib/src/main/kotlin/at/bitfire/synctools/storage/BatchOperation.kt

@@ -249,6 +249,13 @@ open class BatchOperation internal constructor(
             return this
         }
 
+        /**
+         * Creates a back-reference to the result of a previous INSERT in the batch.
+         *
+         * - Only reference INSERT operations (not UPDATE)!
+         * - Don't assume an index value, especially if it's possible that previous operations could
+         *   have added operations to the batch. Instead, get the index from [at.bitfire.synctools.storage.BatchOperation.nextBackrefIdx].
+         */
         fun withValueBackReference(key: String, index: Int): CpoBuilder {
             valueBackrefs[key] = BackReference(index)
             return this

lib/src/main/kotlin/at/bitfire/synctools/storage/calendar/AndroidCalendar.kt

@@ -96,14 +96,15 @@ class AndroidCalendar(
     }
 
     internal fun addEvent(entity: Entity, batch: CalendarBatchOperation) {
-        // insert main row
+        // insert event row
+        val eventRowIdx = batch.nextBackrefIdx()
         batch += CpoBuilder.newInsert(eventsUri).withValues(entity.entityValues)
 
-        // insert data rows (with reference to main row ID)
+        // insert data rows (with reference to event row ID)
         for (row in entity.subValues)
             batch += CpoBuilder.newInsert(row.uri.asSyncAdapter(account))
                 .withValues(row.values)
-                .withValueBackReference(AndroidEvent2.DATA_ROW_EVENT_ID, /* result of first operation with index = */ 0)
+                .withValueBackReference(AndroidEvent2.DATA_ROW_EVENT_ID, eventRowIdx)
     }
 
     /**