fix: incremental sync + unit tests for decide() logic

Sync change detection (3rd attempt — now correct):
- After UPLOAD: save null remote metadata (server mtime unknown until
  next listing); decide() treats null remoteModifiedAt as "not changed"
- After DOWNLOAD: read actual local mtime via accessor.lastModifiedMs()
  so the stored value matches what walkFiles() sees on next scan
- SKIP reconciliation: if known state has null timestamps and both sides
  exist, fill them in — stabilises state within 2 syncs after first transfer
- Extract syncDecide() as internal top-level function for testability

Unit tests (14 cases covering all key scenarios):
- First sync decisions (upload/download/conflict)
- Second sync after upload with null remote metadata → SKIP
- Second sync after download with recorded local mtime → SKIP
- Epoch-millis precision: same ms = SKIP, +1ms = change detected
- Regression: epoch-second stored value would have differed → now correct
- Delete behaviour (MIRROR vs KEEP)
- Direction filters (UPLOAD_ONLY, DOWNLOAD_ONLY)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/src/main/kotlin/com/syncflow/domain/sync/SyncEngine.kt

@@ -8,6 +8,7 @@ import com.syncflow.data.db.SyncFileStateDao
 import com.syncflow.data.db.SyncPairDao
 import com.syncflow.data.db.entities.SyncConflictEntity
 import com.syncflow.data.db.entities.SyncEventEntity
+import com.syncflow.data.db.entities.SyncFileStateEntity
 import com.syncflow.data.providers.CloudProvider
 import com.syncflow.domain.model.ConflictStrategy
 import com.syncflow.domain.model.DeleteBehavior
@@ -76,7 +77,7 @@ class SyncEngine @Inject constructor(
             val uploaded: Int = 0, val downloaded: Int = 0, val deleted: Int = 0,
             val skipped: Int = 0, val failed: Int = 0, val conflicts: Int = 0,
             val bytesTransferred: Long = 0L,
-            val newState: com.syncflow.data.db.entities.SyncFileStateEntity? = null,
+            val newState: SyncFileStateEntity? = null,
         )
 
         val outcomes: List<FileOutcome> = coroutineScope {
@@ -86,7 +87,7 @@ class SyncEngine @Inject constructor(
                         val local = localFiles[rel]
                         val remote = remoteFiles[rel]
                         val known = knownStates[rel]
-                        val decision = decide(pair.syncDirection, pair.conflictStrategy, pair.deleteBehavior, local, remote, known)
+                        val decision = syncDecide(pair.syncDirection, pair.conflictStrategy, pair.deleteBehavior, local, remote, known)
 
                         when (decision) {
                             SyncDecision.UPLOAD -> {
@@ -101,7 +102,10 @@ class SyncEngine @Inject constructor(
                                     return@withPermit FileOutcome(failed = 1)
                                 }
                                 logEvent(pair.id, SyncEventType.FILE_UPLOADED, rel, null, bytes)
-                                FileOutcome(uploaded = 1, bytesTransferred = bytes, newState = buildState(pair.id, rel, local!!, remote))
+                                // Remote metadata is unknown until the next listing; save null so
+                                // decide() treats it as "not changed" and reconciles on next SKIP.
+                                FileOutcome(uploaded = 1, bytesTransferred = bytes,
+                                    newState = buildState(pair.id, rel, local!!, remoteAfterTransfer = null))
                             }
                             SyncDecision.DOWNLOAD -> {
                                 val bytes = runCatching {
@@ -114,8 +118,14 @@ class SyncEngine @Inject constructor(
                                     logEvent(pair.id, SyncEventType.FILE_SKIPPED, rel, e.message, 0)
                                     return@withPermit FileOutcome(failed = 1)
                                 }
+                                // Read the actual local mtime written by the OS/SAF after download.
+                                val localMtime = runCatching { accessor.lastModifiedMs(rel) }
+                                    .getOrDefault(System.currentTimeMillis()).takeIf { it > 0L }
+                                    ?: System.currentTimeMillis()
                                 logEvent(pair.id, SyncEventType.FILE_DOWNLOADED, rel, null, bytes)
-                                FileOutcome(downloaded = 1, bytesTransferred = bytes, newState = buildState(pair.id, rel, null, remote))
+                                FileOutcome(downloaded = 1, bytesTransferred = bytes,
+                                    newState = buildState(pair.id, rel,
+                                        LocalFileInfo(rel, remote!!.sizeBytes, localMtime), remoteAfterTransfer = remote))
                             }
                             SyncDecision.DELETE_LOCAL -> {
                                 accessor.delete(rel)
@@ -143,7 +153,19 @@ class SyncEngine @Inject constructor(
                                 logEvent(pair.id, SyncEventType.CONFLICT_DETECTED, rel, null, 0)
                                 FileOutcome(conflicts = 1)
                             }
-                            SyncDecision.SKIP -> FileOutcome(skipped = 1)
+                            SyncDecision.SKIP -> {
+                                // Reconcile: if the known state is missing remote or local metadata
+                                // (saved right after an upload before we had the server's response),
+                                // fill it in now that we have the full listing.
+                                val needsReconcile = known != null &&
+                                    (known.remoteModifiedAt == null || known.localModifiedAt == null) &&
+                                    local != null && remote != null
+                                if (needsReconcile) {
+                                    FileOutcome(skipped = 1, newState = buildState(pair.id, rel, local, remoteAfterTransfer = remote))
+                                } else {
+                                    FileOutcome(skipped = 1)
+                                }
+                            }
                         }
                     }
                 }
@@ -162,86 +184,20 @@ class SyncEngine @Inject constructor(
         )
     }
 
-    private fun decide(
-        direction: SyncDirection,
-        conflictStrategy: ConflictStrategy,
-        deleteBehavior: DeleteBehavior,
-        local: LocalFileInfo?,
-        remote: RemoteFile?,
-        known: com.syncflow.data.db.entities.SyncFileStateEntity?,
-    ): SyncDecision {
-        val localExists = local != null
-        val remoteExists = remote != null
-
-        val localChanged = known == null || (localExists && local!!.lastModifiedMs != known.localModifiedAt?.toEpochMilli())
-        val remoteChanged = known == null || (remoteExists && (remote!!.etag != known.remoteEtag || remote.modifiedAt != known.remoteModifiedAt))
-
-        return when {
-            !localExists && !remoteExists -> SyncDecision.SKIP
-
-            localExists && !remoteExists -> when {
-                known == null -> when (direction) {
-                    SyncDirection.UPLOAD_ONLY, SyncDirection.TWO_WAY -> SyncDecision.UPLOAD
-                    else -> SyncDecision.SKIP
-                }
-                else -> when {
-                    deleteBehavior == DeleteBehavior.KEEP -> SyncDecision.SKIP
-                    direction == SyncDirection.DOWNLOAD_ONLY || direction == SyncDirection.TWO_WAY -> SyncDecision.DELETE_LOCAL
-                    else -> SyncDecision.SKIP
-                }
-            }
-
-            !localExists && remoteExists -> when {
-                known == null -> when (direction) {
-                    SyncDirection.DOWNLOAD_ONLY, SyncDirection.TWO_WAY -> SyncDecision.DOWNLOAD
-                    else -> SyncDecision.SKIP
-                }
-                else -> when {
-                    deleteBehavior == DeleteBehavior.KEEP -> SyncDecision.SKIP
-                    direction == SyncDirection.UPLOAD_ONLY || direction == SyncDirection.TWO_WAY -> SyncDecision.DELETE_REMOTE
-                    else -> SyncDecision.SKIP
-                }
-            }
-
-            localChanged && remoteChanged -> when (direction) {
-                SyncDirection.UPLOAD_ONLY -> SyncDecision.UPLOAD
-                SyncDirection.DOWNLOAD_ONLY -> SyncDecision.DOWNLOAD
-                SyncDirection.TWO_WAY -> when (conflictStrategy) {
-                    ConflictStrategy.KEEP_LOCAL -> SyncDecision.UPLOAD
-                    ConflictStrategy.KEEP_REMOTE -> SyncDecision.DOWNLOAD
-                    ConflictStrategy.KEEP_NEWEST -> if ((local?.lastModifiedMs ?: 0L) >= (remote?.modifiedAt?.toEpochMilli() ?: 0L)) SyncDecision.UPLOAD else SyncDecision.DOWNLOAD
-                    ConflictStrategy.KEEP_LARGEST -> if ((local?.sizeBytes ?: 0L) >= (remote?.sizeBytes ?: 0L)) SyncDecision.UPLOAD else SyncDecision.DOWNLOAD
-                    ConflictStrategy.KEEP_BOTH -> SyncDecision.CONFLICT
-                    ConflictStrategy.ASK -> SyncDecision.CONFLICT
-                }
-            }
-
-            localChanged -> when (direction) {
-                SyncDirection.UPLOAD_ONLY, SyncDirection.TWO_WAY -> SyncDecision.UPLOAD
-                else -> SyncDecision.SKIP
-            }
-            remoteChanged -> when (direction) {
-                SyncDirection.DOWNLOAD_ONLY, SyncDirection.TWO_WAY -> SyncDecision.DOWNLOAD
-                else -> SyncDecision.SKIP
-            }
-            else -> SyncDecision.SKIP
-        }
-    }
-
     private fun buildState(
         pairId: Long,
         rel: String,
         local: LocalFileInfo?,
-        remote: RemoteFile?,
-    ) = com.syncflow.data.db.entities.SyncFileStateEntity(
+        remoteAfterTransfer: RemoteFile?,
+    ) = SyncFileStateEntity(
         syncPairId = pairId,
         relativePath = rel,
         localModifiedAt = local?.lastModifiedMs?.let { Instant.ofEpochMilli(it) },
         localSizeBytes = local?.sizeBytes ?: 0L,
         localHash = null,
-        remoteModifiedAt = remote?.modifiedAt,
-        remoteSizeBytes = remote?.sizeBytes ?: 0L,
-        remoteEtag = remote?.etag,
+        remoteModifiedAt = remoteAfterTransfer?.modifiedAt,
+        remoteSizeBytes = remoteAfterTransfer?.sizeBytes ?: 0L,
+        remoteEtag = remoteAfterTransfer?.etag,
         lastSyncedAt = Instant.now(),
         syncedHash = null,
     )
@@ -251,6 +207,79 @@ class SyncEngine @Inject constructor(
     }
 }
 
+// Top-level so unit tests can call it directly without instantiating SyncEngine.
+internal fun syncDecide(
+    direction: SyncDirection,
+    conflictStrategy: ConflictStrategy,
+    deleteBehavior: DeleteBehavior,
+    local: LocalFileInfo?,
+    remote: RemoteFile?,
+    known: SyncFileStateEntity?,
+): SyncDecision {
+    val localExists = local != null
+    val remoteExists = remote != null
+
+    // Treat null known timestamps as "not yet recorded" — don't treat as changed.
+    // The SKIP reconciliation pass will fill them in on the next sync.
+    val localChanged = known == null ||
+        (localExists && known.localModifiedAt != null &&
+            local!!.lastModifiedMs != known.localModifiedAt.toEpochMilli())
+    val remoteChanged = known == null ||
+        (remoteExists && known.remoteModifiedAt != null &&
+            (remote!!.etag != known.remoteEtag || remote.modifiedAt != known.remoteModifiedAt))
+
+    return when {
+        !localExists && !remoteExists -> SyncDecision.SKIP
+
+        localExists && !remoteExists -> when {
+            known == null -> when (direction) {
+                SyncDirection.UPLOAD_ONLY, SyncDirection.TWO_WAY -> SyncDecision.UPLOAD
+                else -> SyncDecision.SKIP
+            }
+            else -> when {
+                deleteBehavior == DeleteBehavior.KEEP -> SyncDecision.SKIP
+                direction == SyncDirection.DOWNLOAD_ONLY || direction == SyncDirection.TWO_WAY -> SyncDecision.DELETE_LOCAL
+                else -> SyncDecision.SKIP
+            }
+        }
+
+        !localExists && remoteExists -> when {
+            known == null -> when (direction) {
+                SyncDirection.DOWNLOAD_ONLY, SyncDirection.TWO_WAY -> SyncDecision.DOWNLOAD
+                else -> SyncDecision.SKIP
+            }
+            else -> when {
+                deleteBehavior == DeleteBehavior.KEEP -> SyncDecision.SKIP
+                direction == SyncDirection.UPLOAD_ONLY || direction == SyncDirection.TWO_WAY -> SyncDecision.DELETE_REMOTE
+                else -> SyncDecision.SKIP
+            }
+        }
+
+        localChanged && remoteChanged -> when (direction) {
+            SyncDirection.UPLOAD_ONLY -> SyncDecision.UPLOAD
+            SyncDirection.DOWNLOAD_ONLY -> SyncDecision.DOWNLOAD
+            SyncDirection.TWO_WAY -> when (conflictStrategy) {
+                ConflictStrategy.KEEP_LOCAL -> SyncDecision.UPLOAD
+                ConflictStrategy.KEEP_REMOTE -> SyncDecision.DOWNLOAD
+                ConflictStrategy.KEEP_NEWEST -> if ((local?.lastModifiedMs ?: 0L) >= (remote?.modifiedAt?.toEpochMilli() ?: 0L)) SyncDecision.UPLOAD else SyncDecision.DOWNLOAD
+                ConflictStrategy.KEEP_LARGEST -> if ((local?.sizeBytes ?: 0L) >= (remote?.sizeBytes ?: 0L)) SyncDecision.UPLOAD else SyncDecision.DOWNLOAD
+                ConflictStrategy.KEEP_BOTH -> SyncDecision.CONFLICT
+                ConflictStrategy.ASK -> SyncDecision.CONFLICT
+            }
+        }
+
+        localChanged -> when (direction) {
+            SyncDirection.UPLOAD_ONLY, SyncDirection.TWO_WAY -> SyncDecision.UPLOAD
+            else -> SyncDecision.SKIP
+        }
+        remoteChanged -> when (direction) {
+            SyncDirection.DOWNLOAD_ONLY, SyncDirection.TWO_WAY -> SyncDecision.DOWNLOAD
+            else -> SyncDecision.SKIP
+        }
+        else -> SyncDecision.SKIP
+    }
+}
+
 enum class SyncDecision { UPLOAD, DOWNLOAD, DELETE_LOCAL, DELETE_REMOTE, CONFLICT, SKIP }
 
 data class SyncResult(

app/src/test/kotlin/com/syncflow/domain/sync/SyncDecideTest.kt

@@ -0,0 +1,128 @@
+package com.syncflow.domain.sync
+
+import com.syncflow.data.db.entities.SyncFileStateEntity
+import com.syncflow.domain.model.*
+import org.junit.Assert.assertEquals
+import org.junit.Test
+import java.time.Instant
+
+class SyncDecideTest {
+
+    private val MS  = 1_716_000_000_000L
+    private val MS2 = MS + 5_000L
+
+    private fun local(ms: Long = MS, size: Long = 100L) = LocalFileInfo("test.txt", size, ms)
+
+    private fun remote(ms: Long = MS, etag: String? = "abc", size: Long = 100L) =
+        RemoteFile(
+            path = "path/test.txt", name = "test.txt", isDirectory = false,
+            sizeBytes = size, modifiedAt = Instant.ofEpochMilli(ms),
+            etag = etag, mimeType = null,
+        )
+
+    private fun state(localMs: Long? = MS, remoteMs: Long? = MS, etag: String? = "abc") =
+        SyncFileStateEntity(
+            syncPairId = 1L, relativePath = "test.txt",
+            localModifiedAt  = localMs?.let  { Instant.ofEpochMilli(it) },
+            localSizeBytes   = 100L, localHash = null,
+            remoteModifiedAt = remoteMs?.let { Instant.ofEpochMilli(it) },
+            remoteSizeBytes  = 100L, remoteEtag = etag,
+            lastSyncedAt = Instant.now(), syncedHash = null,
+        )
+
+    private fun decide(
+        local: LocalFileInfo?, remote: RemoteFile?, known: SyncFileStateEntity? = null,
+        dir: SyncDirection = SyncDirection.TWO_WAY,
+        conflict: ConflictStrategy = ConflictStrategy.KEEP_NEWEST,
+        delete: DeleteBehavior = DeleteBehavior.MIRROR,
+    ) = syncDecide(dir, conflict, delete, local, remote, known)
+
+    // ── first sync (no known state) ───────────────────────────────────────────
+
+    @Test fun `first sync both exist local newer uploads`() =
+        assertEquals(SyncDecision.UPLOAD, decide(local(MS2), remote(MS)))
+
+    @Test fun `first sync both exist remote newer downloads`() =
+        assertEquals(SyncDecision.DOWNLOAD, decide(local(MS), remote(MS2)))
+
+    @Test fun `first sync local only TWO_WAY uploads`() =
+        assertEquals(SyncDecision.UPLOAD, decide(local(), null))
+
+    @Test fun `first sync remote only TWO_WAY downloads`() =
+        assertEquals(SyncDecision.DOWNLOAD, decide(null, remote()))
+
+    // ── after upload: remote metadata null in state ───────────────────────────
+
+    @Test fun `second sync after upload remote metadata null skips`() {
+        // State saved after upload: local mtime known, remote unknown (null).
+        val known = state(localMs = MS, remoteMs = null, etag = null)
+        // Remote listing shows a new mtime (server assigned), but we treat null as "no change".
+        assertEquals(SyncDecision.SKIP, decide(local(MS), remote(MS2), known))
+    }
+
+    @Test fun `after upload local changed again re-uploads`() {
+        val known = state(localMs = MS, remoteMs = null, etag = null)
+        assertEquals(SyncDecision.UPLOAD, decide(local(MS2), remote(MS2), known))
+    }
+
+    // ── after download: local mtime recorded ─────────────────────────────────
+
+    @Test fun `second sync fully recorded skips`() {
+        val known = state(localMs = MS, remoteMs = MS, etag = "abc")
+        assertEquals(SyncDecision.SKIP, decide(local(MS), remote(MS, etag = "abc"), known))
+    }
+
+    @Test fun `remote changed after download downloads`() {
+        val known = state(localMs = MS, remoteMs = MS, etag = "abc")
+        assertEquals(SyncDecision.DOWNLOAD, decide(local(MS), remote(MS2, etag = "xyz"), known))
+    }
+
+    @Test fun `local changed after download uploads`() {
+        val known = state(localMs = MS, remoteMs = MS, etag = "abc")
+        assertEquals(SyncDecision.UPLOAD, decide(local(MS2), remote(MS, etag = "abc"), known))
+    }
+
+    // ── epoch-millis precision ────────────────────────────────────────────────
+
+    @Test fun `same millisecond timestamp treated as unchanged`() {
+        val ts = 1_716_393_136_789L
+        assertEquals(SyncDecision.SKIP,
+            decide(local(ts), remote(ts, etag = "e"), state(localMs = ts, remoteMs = ts, etag = "e")))
+    }
+
+    @Test fun `1ms difference detected as local change`() {
+        val ts = 1_716_393_136_789L
+        assertEquals(SyncDecision.UPLOAD,
+            decide(local(ts + 1), remote(ts, etag = "e"), state(localMs = ts, remoteMs = ts, etag = "e")))
+    }
+
+    @Test fun `epoch-second stored value differs from millis comparison`() {
+        // If we stored 1716393136 (seconds) and compare to 1716393136000 (millis) they differ →
+        // This was the original bug — now we store millis so they should match.
+        val ms = 1_716_393_136_000L   // exact second boundary, no sub-second component
+        assertEquals(SyncDecision.SKIP,
+            decide(local(ms), remote(ms, etag = "e"), state(localMs = ms, remoteMs = ms, etag = "e")))
+    }
+
+    // ── delete behaviour ──────────────────────────────────────────────────────
+
+    @Test fun `local exists remote deleted TWO_WAY MIRROR deletes local`() =
+        assertEquals(SyncDecision.DELETE_LOCAL, decide(local(), null, state(), delete = DeleteBehavior.MIRROR))
+
+    @Test fun `local exists remote deleted KEEP skips`() =
+        assertEquals(SyncDecision.SKIP, decide(local(), null, state(), delete = DeleteBehavior.KEEP))
+
+    @Test fun `remote deleted UPLOAD_ONLY skips local deletion`() =
+        assertEquals(SyncDecision.SKIP,
+            decide(local(), null, state(), dir = SyncDirection.UPLOAD_ONLY))
+
+    // ── directions ────────────────────────────────────────────────────────────
+
+    @Test fun `UPLOAD_ONLY ignores remote changes`() =
+        assertEquals(SyncDecision.SKIP,
+            decide(local(MS), remote(MS2, etag = "new"), state(), dir = SyncDirection.UPLOAD_ONLY))
+
+    @Test fun `DOWNLOAD_ONLY ignores local changes`() =
+        assertEquals(SyncDecision.SKIP,
+            decide(local(MS2), remote(MS, etag = "abc"), state(), dir = SyncDirection.DOWNLOAD_ONLY))
+}

version.properties

@@ -1,2 +1,2 @@
-VERSION_NAME=1.0.5
-VERSION_CODE=6
+VERSION_NAME=1.0.6
+VERSION_CODE=7