Recurring tasks keep their rule with tasks.org and OpenTasks (#135) (#142)

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/AndroidTasksDataSource.kt

@@ -7,9 +7,12 @@ import android.content.ContentUris
 import android.content.ContentValues
 import android.content.Context
 import android.database.ContentObserver
+import android.database.Cursor
+import android.database.sqlite.SQLiteException
 import android.net.Uri
 import android.os.Handler
 import android.os.Looper
+import android.util.Log
 import dagger.hilt.android.qualifiers.ApplicationContext
 import de.jeanlucmakiola.agendula.data.tasks.TasksContract.Instances
 import de.jeanlucmakiola.agendula.data.tasks.TasksContract.Lists
@@ -73,14 +76,20 @@ class AndroidTasksDataSource @Inject constructor(
     }
 
     override fun task(taskId: Long): Task? {
-        val rows = queryInstances("${Instances.TASK_ID} = ?", arrayOf(taskId.toString()))
-        return rows.firstOrNull { it.distanceFromCurrent == 0 } ?: rows.firstOrNull()
+        roundLegacyAnchors()
+        val rows = instanceRows(resolver.query(TasksContract.instancesUri(authority()), null, "${Instances.TASK_ID} = ?", arrayOf(taskId.toString()), null))
+        return (rows.firstOrNull { it.task.distanceFromCurrent == 0 } ?: rows.firstOrNull())?.let { enrich(listOf(it)).single() }
     }
 
     override fun task(taskId: Long, occurrenceStart: Instant?): Task? {
         if (occurrenceStart == null) return task(taskId)
-        val rows = queryInstances("${Instances.TASK_ID} = ?", arrayOf(taskId.toString()))
-        return rows.firstOrNull { it.occurrenceStart == occurrenceStart } ?: task(taskId)
+        roundLegacyAnchors()
+        val rows = instanceRows(queryOccurrences(taskId))
+        val chosen = rows.firstOrNull { it.anchor == occurrenceStart }
+            ?: rows.firstOrNull { it.anchor?.let { a -> sameSecond(a, occurrenceStart) } == true }
+            ?: rows.firstOrNull { it.task.taskId == taskId && it.task.distanceFromCurrent == 0 }
+            ?: rows.firstOrNull { it.task.taskId == taskId }
+        return chosen?.let { enrich(listOf(it)).single() }
     }
 
     override fun subtasks(parentTaskId: Long): List<Task> =
@@ -107,23 +116,74 @@ class AndroidTasksDataSource @Inject constructor(
         }
     } ?: emptyList()
 
+    /**
+     * All columns and no sort: the instances view differs across provider
+     * versions (tasks.org's has no `is_recurring`), the by-name mapper nulls what
+     * is missing, and the repository re-sorts.
+     */
     private fun queryInstances(selection: String?, args: Array<String>?): List<Task> {
-        val uri = TasksContract.instancesUri(authority())
-        // projection = null (all columns): the instances view differs across
-        // provider versions (e.g. tasks.org's bundled OpenTasks has no
-        // `is_recurring` / `distance_from_current`). The by-name mapper reads
-        // whatever is present and nulls the rest; the repository re-sorts, so we
-        // don't depend on a provider sort column either.
-        val tasks = resolver.query(uri, null, selection, args, null)
-            ?.use { c ->
-                val reader = CursorColumnReader(c)
-                buildList { while (c.moveToNext()) add(TaskMapper.task(reader)) }
-            } ?: emptyList()
-        if (tasks.isEmpty()) return tasks
+        roundLegacyAnchors()
+        return instanceTasks(resolver.query(TasksContract.instancesUri(authority()), null, selection, args, null))
+    }
+
+    /** Maps [cursor]'s instance rows, with the rule each series keeps on its `tasks` row, and their tags. */
+    private fun instanceTasks(cursor: Cursor?): List<Task> = enrich(instanceRows(cursor))
+
+    /** [cursor]'s instance rows, before their rules and tags are looked up. */
+    private fun instanceRows(cursor: Cursor?): List<InstanceRow> {
+        var flagged = false
+        val rows = cursor?.use { c ->
+            flagged = c.getColumnIndex(Instances.IS_RECURRING) >= 0
+            val reader = CursorColumnReader(c)
+            buildList { while (c.moveToNext()) add(TaskMapper.instance(reader)) }
+        }.orEmpty()
+        // Where the provider flags recurrence, only flagged rows need their rule looked up.
+        return if (flagged) rows.map { if (it.task.isRecurring) it else it.copy(ruleOwner = NO_RULE) } else rows
+    }
+
+    /** [rows] completed with the rule each series keeps on its `tasks` row, and their tags. */
+    private fun enrich(rows: List<InstanceRow>): List<Task> {
+        if (rows.isEmpty()) return emptyList()
+        val rules = seriesRules(rows.map { it.ruleOwner }.filter { it != NO_RULE })
+        val tasks = rows.map { TaskMapper.withRule(it, rules[it.ruleOwner]) }
         val tags = categories(tasks.map { it.taskId })
         return tasks.map { task -> tags[task.taskId]?.let { task.copy(tags = it) } ?: task }
     }
 
+    /** The recurrence of each of [taskIds] that is a series, keyed by task id. */
+    private fun seriesRules(taskIds: Collection<Long>): Map<Long, SeriesRule> = queryByIds(
+        TasksContract.tasksUri(authority()),
+        arrayOf(Tasks.ID, Tasks.RRULE, Tasks.RDATE),
+        Tasks.ID,
+        taskIds,
+        filter = "${Tasks.RRULE} IS NOT NULL OR ${Tasks.RDATE} IS NOT NULL",
+    ) { r -> r.getLong(Tasks.ID)?.let { id -> TaskMapper.seriesRule(r)?.let { id to it } } }.toMap()
+
+    /**
+     * Rows of [uri] whose [idColumn] is one of [ids], in chunks that stay under
+     * SQLite's bound-argument limit, each mapped by [read] (null rows dropped).
+     */
+    private fun <T : Any> queryByIds(
+        uri: Uri,
+        projection: Array<String>,
+        idColumn: String,
+        ids: Collection<Long>,
+        filter: String? = null,
+        filterArgs: List<String> = emptyList(),
+        sortOrder: String? = null,
+        read: (ColumnReader) -> T?,
+    ): List<T> = ids.distinct().chunked(MAX_SQL_ARGS - filterArgs.size).flatMap { chunk ->
+        val selection = buildString {
+            append("$idColumn IN (${chunk.joinToString(",") { "?" }})")
+            if (filter != null) append(" AND ($filter)")
+        }
+        val args = (chunk.map(Long::toString) + filterArgs).toTypedArray()
+        resolver.query(uri, projection, selection, args, sortOrder)?.use { c ->
+            val reader = CursorColumnReader(c)
+            buildList { while (c.moveToNext()) read(reader)?.let(::add) }
+        } ?: emptyList()
+    }
+
     override fun exportTasks(listId: Long): List<ExportTask> {
         // projection = null for the same reason queryInstances uses it: the tasks
         // table's shape varies across provider versions, and the by-name mapper
@@ -194,7 +254,7 @@ class AndroidTasksDataSource @Inject constructor(
     }
 
     override fun updateSeries(seriesId: Long, occurrenceStart: Instant, form: TaskForm, reminders: List<TaskReminder>?) {
-        val occurrence = task(seriesId, occurrenceStart)
+        val occurrence = occurrence(seriesId, occurrenceStart) { TaskMapper.instance(it).task } ?: task(seriesId)
         val anchors = resolver.query(
             taskUri(authority(), seriesId), arrayOf(Tasks.DTSTART, Tasks.DUE), null, null, null,
         )?.use { c ->
@@ -202,14 +262,14 @@ class AndroidTasksDataSource @Inject constructor(
             val r = CursorColumnReader(c)
             r.getLong(Tasks.DTSTART) to r.getLong(Tasks.DUE)
         }
-        fun shifted(anchor: Long?, was: Instant?, now: Instant?): Long? = when {
-            now == null -> null
-            anchor == null || was == null -> now.toEpochMilliseconds()
-            else -> anchor + (now - was).inWholeMilliseconds
-        }
-        val values = TaskWriteMapper.seriesValues(form, ZoneId.systemDefault().id) +
-            (Tasks.DTSTART to shifted(anchors?.first, occurrence?.start, form.start)) +
-            (Tasks.DUE to shifted(anchors?.second, occurrence?.due, form.due))
+        val base = TaskWriteMapper.seriesValues(form, ZoneId.systemDefault().id)
+        // Without an anchor to shift, the form's own value stands — already pinned for an all-day series.
+        fun shifted(column: String, anchor: Long?, was: Instant?, now: Instant?): Any? =
+            if (now == null || anchor == null || was == null) base[column]
+            else providerMillis(anchor + (now - was).inWholeMilliseconds)
+        val values = base +
+            (Tasks.DTSTART to shifted(Tasks.DTSTART, anchors?.first, occurrence?.start, form.start)) +
+            (Tasks.DUE to shifted(Tasks.DUE, anchors?.second, occurrence?.due, form.due))
         val rows = resolver.update(taskUri(authority(), seriesId), values.toContentValues(), null, null)
         if (rows == 0) throw TaskWriteFailedException("update series $seriesId")
         setCategories(seriesId, form.tags)
@@ -253,9 +313,10 @@ class AndroidTasksDataSource @Inject constructor(
         if (!c.moveToFirst()) return@use null
         val r = CursorColumnReader(c)
         val anchor = r.getLong(Tasks.DTSTART) ?: r.getLong(Tasks.DUE) ?: return@use null
+        val rule = TaskMapper.seriesRule(r)
         RecurrenceSpec(
-            rrule = r.getString(Tasks.RRULE)?.ifBlank { null },
-            rdate = r.getString(Tasks.RDATE),
+            rrule = rule?.rrule,
+            rdate = rule?.rdate,
             exdate = r.getString(Tasks.EXDATE),
             anchor = Instant.fromEpochMilliseconds(anchor),
             isAllDay = r.getBoolean(Tasks.IS_ALLDAY),
@@ -290,7 +351,7 @@ class AndroidTasksDataSource @Inject constructor(
         buildList {
             while (c.moveToNext()) {
                 val original = r.getLong(Tasks.ORIGINAL_INSTANCE_TIME) ?: continue
-                if (original >= from.toEpochMilliseconds()) r.getLong(Tasks.ID)?.let(::add)
+                if (providerMillis(original) >= providerMillis(from.toEpochMilliseconds())) r.getLong(Tasks.ID)?.let(::add)
             }
         }
     } ?: emptyList()
@@ -302,7 +363,10 @@ class AndroidTasksDataSource @Inject constructor(
     }
 
     override fun updateInstance(taskId: Long, occurrenceStart: Instant, form: TaskForm, reminders: List<TaskReminder>?) {
-        if (reminders == null) return writeInstance(taskId, occurrenceStart, form)
+        val hit = instanceFor(taskId, occurrenceStart)
+            ?: throw TaskWriteFailedException("update instance $taskId@$occurrenceStart: no such occurrence")
+        hit.overrideId?.let { return updateOverride(it, form, reminders) }
+        if (reminders == null) return writeInstance(taskId, hit.instanceId, form)
         // ⚠️ Written to the series, and then put back. Forking an occurrence
         // copies the task's *current* properties onto the new override row, so
         // setting the alarm beforehand is the only way to carry it across — and
@@ -311,15 +375,26 @@ class AndroidTasksDataSource @Inject constructor(
         val seriesReminders = alarmRows(taskId).map { it.second }
         writeReminders(taskId, reminders)
         try {
-            writeInstance(taskId, occurrenceStart, form)
+            writeInstance(taskId, hit.instanceId, form)
         } finally {
             setReminders(taskId, seriesReminders)
         }
     }
 
-    private fun writeInstance(taskId: Long, occurrenceStart: Instant, form: TaskForm) {
-        val instanceId = instanceIdFor(taskId, occurrenceStart)
-            ?: throw TaskWriteFailedException("update instance $taskId@$occurrenceStart: no such occurrence")
+    /**
+     * An occurrence that already has its own row is written there directly. The
+     * fork-and-restore dance in [updateInstance] would put its old tags and
+     * reminders back over the new ones.
+     */
+    private fun updateOverride(overrideId: Long, form: TaskForm, reminders: List<TaskReminder>?) {
+        val values = TaskWriteMapper.instanceValues(form, ZoneId.systemDefault().id)
+        val rows = resolver.update(taskUri(authority(), overrideId), values.toContentValues(), null, null)
+        if (rows == 0) throw TaskWriteFailedException("update override $overrideId")
+        setCategories(overrideId, form.tags)
+        writeReminders(overrideId, reminders)
+    }
+
+    private fun writeInstance(taskId: Long, instanceId: Long, form: TaskForm) {
         val values = TaskWriteMapper.instanceValues(form, ZoneId.systemDefault().id)
         val uri = TasksContract.instanceUri(authority(), instanceId)
         // The fork copies the series' properties onto the override, so the tags
@@ -346,28 +421,136 @@ class AndroidTasksDataSource @Inject constructor(
         }
     }
 
+    /** An occurrence's instance row, and the override row behind it if it has one. */
+    private class InstanceHit(val instanceId: Long, val overrideId: Long?)
+
     /**
-     * The provider's instance row id for one occurrence.
+     * The provider's instance row for one occurrence.
      *
      * The seam addresses occurrences by `(taskId, occurrenceStart)`; writing
      * through the instances URI still needs the row id, so it is looked up here
-     * rather than carried around above the data layer. Selection is on `task_id`
-     * only — the anchor is matched in Kotlin because the column that holds it
-     * (`instance_original_time`) is missing on older provider schemas, where a
-     * WHERE clause naming it would throw instead of falling back.
+     * rather than carried around above the data layer.
      */
-    private fun instanceIdFor(taskId: Long, occurrenceStart: Instant): Long? {
-        val uri = TasksContract.instancesUri(authority())
-        val selection = "${Instances.TASK_ID} = ?"
-        return resolver.query(uri, null, selection, arrayOf(taskId.toString()), null)?.use { c ->
+    private fun instanceFor(taskId: Long, occurrenceStart: Instant): InstanceHit? {
+        roundLegacyAnchors()
+        return occurrence(taskId, occurrenceStart) { r ->
+            r.getLong(Tasks.ID)?.let { id ->
+                InstanceHit(id, r.getLong(Instances.TASK_ID)?.takeIf { r.getLong(Tasks.ORIGINAL_INSTANCE_ID) != null })
+            }
+        }
+    }
+
+    private fun instanceIdFor(taskId: Long, occurrenceStart: Instant): Long? = instanceFor(taskId, occurrenceStart)?.instanceId
+
+    /**
+     * [read] applied to the instance row of [taskId]'s occurrence at
+     * [occurrenceStart] — its own, or an override's that replaced it — matched in
+     * whole seconds, as the provider expands them. The anchor is matched in
+     * Kotlin because `instance_original_time` is missing on older schemas.
+     */
+    private fun <T> occurrence(taskId: Long, occurrenceStart: Instant, read: (ColumnReader) -> T): T? =
+        queryOccurrences(taskId)?.use { c ->
             val reader = CursorColumnReader(c)
+            // An exact match first: a row of the same second is a fallback, not a peer.
+            var near: Int? = null
             while (c.moveToNext()) {
-                if (TaskMapper.occurrenceAnchor(reader) == occurrenceStart) {
-                    return@use reader.getLong(Tasks.ID)
+                val anchor = TaskMapper.occurrenceAnchor(reader) ?: continue
+                if (anchor == occurrenceStart) return@use read(reader)
+                if (near == null && sameSecond(anchor, occurrenceStart)) near = c.position
+            }
+            near?.let { c.moveToPosition(it); read(reader) }
+        }
+
+    /**
+     * Instance rows of [taskId] and of the overrides standing in for its
+     * occurrences. A provider whose view lacks `original_instance_id` gets the
+     * task's own rows only, rather than a failed query.
+     */
+    private fun queryOccurrences(taskId: Long): Cursor? {
+        val authority = authority()
+        val uri = TasksContract.instancesUri(authority)
+        val id = taskId.toString()
+        if (ownRowsOnlyFor != authority) {
+            try {
+                return resolver.query(uri, null, OCCURRENCE_OF, arrayOf(id, id), null)
+            } catch (_: SQLiteException) {
+                ownRowsOnlyFor = authority
+            } catch (_: IllegalArgumentException) {
+                ownRowsOnlyFor = authority
+            }
+        }
+        return resolver.query(uri, null, "${Instances.TASK_ID} = ?", arrayOf(id), null)
+    }
+
+    /** The provider whose instances view turned out to lack `original_instance_id`. */
+    @Volatile
+    private var ownRowsOnlyFor: String? = null
+
+    @Volatile
+    private var anchorsRoundedFor: String? = null
+
+    private val roundingLock = Any()
+
+    /**
+     * Rounds the DTSTART/DUE a build before [providerMillis] stored with
+     * milliseconds, which the provider cannot move past a completed occurrence.
+     * Once per provider, before the first read: concurrent readers wait for it.
+     * Written as the rows' sync adapter, since the server only ever had whole
+     * seconds, so nothing is dirtied or re-uploaded. Overrides need nothing: the
+     * provider forks them at its own whole-second instance times.
+     *
+     * Never fails a read. A row the provider refuses is left as it was and tried
+     * again on the next start; only a pass that rounded everything is remembered.
+     */
+    private fun roundLegacyAnchors() {
+        val authority = authority()
+        if (anchorsRoundedFor == authority) return
+        synchronized(roundingLock) {
+            if (anchorsRoundedFor == authority) return
+            val prefs = context.getSharedPreferences(MIGRATIONS, Context.MODE_PRIVATE)
+            val key = "anchors_rounded:$authority"
+            val done = prefs.getBoolean(key, false) || runCatching { roundAll(authority) }
+                .onFailure { Log.w(TAG, "Rounding legacy anchors failed", it) }
+                .getOrDefault(false)
+            if (done) prefs.edit().putBoolean(key, true).apply()
+            anchorsRoundedFor = authority
+        }
+    }
+
+    /** True when every stale row was rounded. */
+    private fun roundAll(authority: String): Boolean {
+        val columns = listOf(Tasks.DTSTART, Tasks.DUE)
+        val ops = resolver.query(
+            TasksContract.tasksUri(authority),
+            arrayOf(Tasks.ID, Tasks.ACCOUNT_NAME, Tasks.ACCOUNT_TYPE) + columns,
+            columns.joinToString(" OR ") { "$it % 1000 != 0" },
+            null,
+            null,
+        )?.use { c ->
+            val r = CursorColumnReader(c)
+            buildList<ContentProviderOperation> {
+                while (c.moveToNext()) {
+                    val id = r.getLong(Tasks.ID) ?: continue
+                    val values = buildMap<String, Any?> {
+                        for (column in columns) {
+                            val millis = r.getLong(column) ?: continue
+                            if (providerMillis(millis) != millis) put(column, providerMillis(millis))
+                        }
+                    }
+                    val name = r.getString(Tasks.ACCOUNT_NAME)
+                    val type = r.getString(Tasks.ACCOUNT_TYPE)
+                    val uri = if (name.isNullOrEmpty() || type.isNullOrEmpty()) taskUri(authority, id)
+                    else TasksContract.asSyncAdapter(taskUri(authority, id), name, type)
+                    add(ContentProviderOperation.newUpdate(uri).withValues(values.toContentValues()).build())
                 }
             }
-            null
-        }
+        } ?: return false
+        if (ops.isEmpty()) return true
+        val batch = runCatching { resolver.applyBatch(authority, ArrayList(ops)) }.getOrNull()
+        if (batch != null) return batch.all { (it.count ?: 0) > 0 }
+        // One refused row fails the whole batch; row by row, the rest still go.
+        return ops.map { op -> runCatching { resolver.applyBatch(authority, arrayListOf(op)).single().count ?: 0 }.getOrDefault(0) > 0 }
+            .all { it }
     }
 
     override fun setReminders(taskId: Long, reminders: List<TaskReminder>) {
@@ -430,29 +613,19 @@ class AndroidTasksDataSource @Inject constructor(
     }
 
     /** Tags by task id for [taskIds], in stored order. */
-    private fun categories(taskIds: Collection<Long>): Map<Long, List<String>> {
-        val rows = taskIds.distinct().chunked(MAX_SQL_ARGS - 1).flatMap { chunk ->
-            val selection = "${Properties.MIMETYPE} = ? AND ${Properties.TASK_ID} IN (${chunk.joinToString(",") { "?" }})"
-            val args = (listOf(TasksContract.Category.MIMETYPE) + chunk.map(Long::toString)).toTypedArray()
-            resolver.query(
-                TasksContract.propertiesUri(authority()),
-                arrayOf(Properties.TASK_ID, TasksContract.Category.NAME),
-                selection,
-                args,
-                Properties.PROPERTY_ID,
-            )?.use { c ->
-                val reader = CursorColumnReader(c)
-                buildList {
-                    while (c.moveToNext()) {
-                        val id = reader.getLong(Properties.TASK_ID) ?: continue
-                        val name = reader.getString(TasksContract.Category.NAME) ?: continue
-                        add(id to name)
-                    }
-                }
-            } ?: emptyList()
-        }
-        return rows.groupBy({ it.first }, { it.second }).mapValues { normalizeTags(it.value) }
-    }
+    private fun categories(taskIds: Collection<Long>): Map<Long, List<String>> = queryByIds(
+        TasksContract.propertiesUri(authority()),
+        arrayOf(Properties.TASK_ID, TasksContract.Category.NAME),
+        Properties.TASK_ID,
+        taskIds,
+        filter = "${Properties.MIMETYPE} = ?",
+        filterArgs = listOf(TasksContract.Category.MIMETYPE),
+        sortOrder = Properties.PROPERTY_ID,
+    ) { r ->
+        val id = r.getLong(Properties.TASK_ID)
+        val name = r.getString(TasksContract.Category.NAME)
+        if (id == null || name == null) null else id to name
+    }.groupBy({ it.first }, { it.second }).mapValues { normalizeTags(it.value) }
 
     /**
      * Replaces [taskId]'s category rows in one batch, so a refused insert cannot
@@ -489,7 +662,7 @@ class AndroidTasksDataSource @Inject constructor(
     /** Through the instances URI, like [setCompletedInstance]. */
     override fun setCancelledInstance(taskId: Long, occurrenceStart: Instant, cancelled: Boolean) {
         val instanceId = instanceIdFor(taskId, occurrenceStart)
-            ?: return setCancelled(taskId, cancelled)
+            ?: return plainUnlessSeries(taskId, occurrenceStart) { setCancelled(taskId, cancelled) }
         val uri = TasksContract.instanceUri(authority(), instanceId)
         val rows = resolver.update(uri, cancelValues(cancelled).toContentValues(), null, null)
         if (rows == 0) throw TaskWriteFailedException("cancel instance $instanceId")
@@ -508,22 +681,42 @@ class AndroidTasksDataSource @Inject constructor(
 
     /**
      * Through the instances URI, which is what makes the provider fork an override
-     * rather than close the series. No instance row for the anchor means the task
-     * is not a series after all — the plain write is then the right one.
+     * rather than close the series. No instance row for the anchor on a task that
+     * is not a series means the plain write is the right one.
      */
     override fun setCompletedInstance(taskId: Long, occurrenceStart: Instant, completed: Boolean) {
         val instanceId = instanceIdFor(taskId, occurrenceStart)
-            ?: return setCompleted(taskId, completed)
+            ?: return plainUnlessSeries(taskId, occurrenceStart) { setCompleted(taskId, completed) }
         val values = TaskWriteMapper.completionValues(completed, System.currentTimeMillis())
         val uri = TasksContract.instanceUri(authority(), instanceId)
         val rows = resolver.update(uri, values.toContentValues(), null, null)
         if (rows == 0) throw TaskWriteFailedException("complete instance $instanceId")
     }
 
+    /**
+     * [write] for a task that turned out not to be a series. A series whose
+     * occurrence was not found fails instead: the plain write would close all of it.
+     */
+    private fun plainUnlessSeries(taskId: Long, occurrenceStart: Instant, write: () -> Unit) {
+        if (seriesRules(listOf(taskId)).isNotEmpty()) {
+            throw TaskWriteFailedException("instance $taskId@$occurrenceStart: no such occurrence")
+        }
+        write()
+    }
+
     override fun deleteTask(taskId: Long) {
         resolver.delete(taskUri(authority(), taskId), null, null)
     }
 
+    /**
+     * Through its instance, which the provider turns into an EXDATE on the master,
+     * as the Room store does. Deleting the row alone leaves the master without one.
+     */
+    override fun deleteOverride(taskId: Long, occurrenceStart: Instant) {
+        val instanceId = instanceIdFor(taskId, occurrenceStart) ?: return deleteTask(taskId)
+        resolver.delete(TasksContract.instanceUri(authority(), instanceId), null, null)
+    }
+
     override fun createLocalList(name: String, color: Int): Long {
         val uri = TasksContract.asSyncAdapter(
             TasksContract.listsUri(authority()),
@@ -594,5 +787,18 @@ class AndroidTasksDataSource @Inject constructor(
     }
 }
 
+/** An instance row of the task, or of an override standing in for one of its occurrences. */
+private const val OCCURRENCE_OF = "${Instances.TASK_ID} = ? OR ${Tasks.ORIGINAL_INSTANCE_ID} = ?"
+
+private fun sameSecond(a: Instant, b: Instant): Boolean =
+    providerMillis(a.toEpochMilliseconds()) == providerMillis(b.toEpochMilliseconds())
+
+private const val MIGRATIONS = "provider_migrations"
+
+private const val TAG = "AndroidTasksDataSource"
+
+/** No rule to look up: the provider says the row does not recur. */
+private const val NO_RULE = -1L
+
 /** SQLite's default bound-argument limit on older Android releases. */
 private const val MAX_SQL_ARGS = 999

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/ModeRoutingTasksDataSource.kt

@@ -67,6 +67,7 @@ class ModeRoutingTasksDataSource(
         active().setCancelledInstance(taskId, occurrenceStart, cancelled)
 
     override fun deleteTask(taskId: Long) = active().deleteTask(taskId)
+    override fun deleteOverride(taskId: Long, occurrenceStart: Instant) = active().deleteOverride(taskId, occurrenceStart)
     override fun createLocalList(name: String, color: Int): Long = active().createLocalList(name, color)
     override fun updateList(listId: Long, name: String, color: Int) = active().updateList(listId, name, color)
     override fun deleteList(listId: Long) = active().deleteList(listId)

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/TaskMapper.kt

@@ -13,20 +13,23 @@ import kotlin.time.Instant
 /** Maps a provider row (via [ColumnReader]) to a domain model. Pure + testable. */
 object TaskMapper {
 
-    fun task(r: ColumnReader): Task {
+    /**
+     * Maps an `instances` row, without its series' rule: the dmfs instances view
+     * (DB 22 and 23) masks `rrule`/`rdate` with `NULL`, so the row alone cannot
+     * tell a series from a one-off on tasks.org's bundled provider (DB 22), which
+     * also lacks `is_recurring`. [withRule] completes it with the rule read from
+     * the master's `tasks` row. An override names its master in
+     * `original_instance_id`.
+     */
+    fun instance(r: ColumnReader): InstanceRow {
         fun instant(name: String): Instant? =
             r.getLong(name)?.let { Instant.fromEpochMilliseconds(it) }
 
         val rowId = r.getLong(Tasks.ID) ?: 0L
-        // Derived from the rule columns rather than the `is_recurring` column
-        // alone: that column only exists from OpenTasks 1.4.0 (DB 23) and is
-        // absent on tasks.org's bundled provider (DB 22), where reading it
-        // would silently report every recurring task as one-off — and route
-        // its edits onto the series anchor.
-        val recurring = r.getString(Tasks.RRULE) != null ||
-            r.getString(Tasks.RDATE) != null ||
-            r.getBoolean(Instances.IS_RECURRING)
-        return Task(
+        val masterId = r.getLong(Tasks.ORIGINAL_INSTANCE_ID)
+        val recurring = masterId != null || r.getBoolean(Instances.IS_RECURRING)
+        val anchor = occurrenceAnchor(r)
+        val task = Task(
             taskId = r.getLong(Instances.TASK_ID) ?: rowId,
             listId = r.getLong(Tasks.LIST_ID) ?: 0L,
             title = r.getString(Tasks.TITLE).orEmpty(),
@@ -47,13 +50,31 @@ object TaskMapper {
             accountName = r.getString(Tasks.ACCOUNT_NAME),
             parentId = r.getLong(Tasks.PARENT_ID),
             isRecurring = recurring,
-            occurrenceStart = if (recurring) occurrenceAnchor(r) else null,
+            occurrenceStart = if (recurring) anchor else null,
             distanceFromCurrent = r.getInt(Instances.DISTANCE_FROM_CURRENT),
             created = instant(Tasks.CREATED),
             lastModified = instant(Tasks.LAST_MODIFIED),
-            seriesId = if (recurring) r.getLong(Instances.TASK_ID) ?: rowId else null,
-            recurrenceRule = r.getString(Tasks.RRULE),
+            seriesId = if (recurring) masterId ?: r.getLong(Instances.TASK_ID) ?: rowId else null,
         )
+        return InstanceRow(task, ruleOwner = masterId ?: task.taskId, anchor = anchor)
+    }
+
+    /** [row]'s task as an occurrence of the series [rule] belongs to, or as it is without one. */
+    fun withRule(row: InstanceRow, rule: SeriesRule?): Task {
+        if (rule == null) return row.task
+        return row.task.copy(
+            isRecurring = true,
+            occurrenceStart = row.task.occurrenceStart ?: row.anchor,
+            seriesId = row.task.seriesId ?: row.ruleOwner,
+            recurrenceRule = rule.rrule,
+        )
+    }
+
+    /** The rule columns of a `tasks` row, or null when it does not recur. */
+    fun seriesRule(r: ColumnReader): SeriesRule? {
+        val rrule = r.getString(Tasks.RRULE)?.ifBlank { null }
+        val rdate = r.getString(Tasks.RDATE)?.ifBlank { null }
+        return if (rrule == null && rdate == null) null else SeriesRule(rrule, rdate)
     }
 
     /**
@@ -119,3 +140,12 @@ object TaskMapper {
         owner = r.getString(Lists.OWNER),
     )
 }
+
+/** A series' recurrence as stored on its master row. */
+data class SeriesRule(val rrule: String?, val rdate: String?)
+
+/**
+ * An instance row's task before its series' rule is applied. [ruleOwner] is the
+ * task whose `tasks` row carries that rule: the master, for an override.
+ */
+data class InstanceRow(val task: Task, val ruleOwner: Long, val anchor: Instant?)

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/TaskWriteMapper.kt

@@ -17,6 +17,14 @@ private fun Instant.forAllDay(allDay: Boolean): Instant =
         Math.floorDiv(toEpochMilliseconds(), MILLIS_PER_DAY) * MILLIS_PER_DAY,
     )
 
+/**
+ * Whole seconds, as iCalendar stores times. The provider expands a series from
+ * second-precision instants, so a DTSTART carrying milliseconds never matches its
+ * own first occurrence: completing it detaches a copy but leaves the series where
+ * it was, and the same occurrence comes straight back.
+ */
+fun providerMillis(epochMillis: Long): Long = Math.floorDiv(epochMillis, 1000L) * 1000L
+
 /**
  * Turns a [TaskForm] / mutation into a name→value map. Pure (no ContentValues),
  * so it unit-tests on the JVM; [AndroidTasksDataSource] converts the map to
@@ -57,8 +65,8 @@ object TaskWriteMapper {
         // through DateTime.toAllDay(), which drops the time-of-day and resolves the
         // remaining date against UTC — so a local-midnight instant lands on the
         // previous day for anyone west of UTC. Pin all-day values to UTC midnight.
-        put(Tasks.DTSTART, form.start?.forAllDay(form.isAllDay)?.toEpochMilliseconds())
-        put(Tasks.DUE, form.due?.forAllDay(form.isAllDay)?.toEpochMilliseconds())
+        put(Tasks.DTSTART, form.start?.forAllDay(form.isAllDay)?.toEpochMilliseconds()?.let(::providerMillis))
+        put(Tasks.DUE, form.due?.forAllDay(form.isAllDay)?.toEpochMilliseconds()?.let(::providerMillis))
         // DUE and DURATION are mutually exclusive. The provider's Validating
         // processor evaluates the *merged* row (supplied values over the stored
         // ones), so writing DUE onto a task that already carries a DURATION throws

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/TasksDataSource.kt

@@ -124,13 +124,17 @@ interface TasksDataSource {
      *
      * Implementations fall back to [setCompleted] when the row turns out not to
      * be a series master — an override, or a plain task the caller happened to
-     * hand an anchor for — so the routing above cannot get this wrong.
+     * hand an anchor for. A series whose occurrence cannot be found fails with
+     * [TaskWriteFailedException] instead: the plain write would close all of it.
      */
     fun setCompletedInstance(taskId: Long, occurrenceStart: Instant, completed: Boolean)
 
     /** Cancel (or restore) one occurrence of a series; the counterpart of [setCompletedInstance]. */
     fun setCancelledInstance(taskId: Long, occurrenceStart: Instant, cancelled: Boolean)
     fun deleteTask(taskId: Long)
+
+    /** Delete the override [taskId] standing for one occurrence, excluding that occurrence from its series. */
+    fun deleteOverride(taskId: Long, occurrenceStart: Instant) = deleteTask(taskId)
     fun createLocalList(name: String, color: Int): Long
 
     /** Rename and recolour [listId]. */

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/TasksRepositoryImpl.kt

@@ -241,7 +241,7 @@ class TasksRepositoryImpl @Inject constructor(
                 RecurringScope.ThisOccurrence ->
                     // An override row deletes itself into an EXDATE; a generated
                     // occurrence has no row to delete.
-                    if (current.taskId != seriesId) dataSource.deleteTask(current.taskId)
+                    if (current.taskId != seriesId) dataSource.deleteOverride(current.taskId, occurrence)
                     else dataSource.deleteOccurrence(seriesId, occurrence)
                 RecurringScope.ThisAndFollowing -> dataSource.deleteFollowing(seriesId, occurrence)
                 RecurringScope.AllOccurrences -> dataSource.deleteWithSubtasks(seriesId)

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/provider/ProviderWriter.kt

@@ -189,7 +189,8 @@ class ProviderWriter internal constructor(
         val current = database.tasks().entity(taskId)?.takeUnless { it.isDeleted } ?: return false
         requireWritableList(current.listId)
         // An override deletes itself into an EXDATE on its master.
-        if (current.masterId != null) store.deleteTask(taskId) else store.deleteWithSubtasks(taskId)
+        val occurrence = current.recurrenceId
+        if (current.masterId != null && occurrence != null) store.deleteOverride(taskId, occurrence) else store.deleteWithSubtasks(taskId)
         return true
     }