remove corrupted snapshots when pruning

before a corrupted snapshot would DoS pruning

app/src/main/java/com/stevesoltys/seedvault/repo/Loader.kt

@@ -96,6 +96,7 @@ internal class Loader(
             }
         } catch (e: Exception) {
             log.error(e) { "Error writing cache file $cacheFile: " }
+            cacheFile?.delete()
         }
         // get associated data for version, used for authenticated decryption
         val ad = crypto.getAdForVersion(version)

app/src/main/java/com/stevesoltys/seedvault/repo/Pruner.kt

@@ -11,6 +11,7 @@ import com.stevesoltys.seedvault.proto.Snapshot
 import io.github.oshai.kotlinlogging.KotlinLogging
 import org.calyxos.seedvault.core.backends.AppBackupFileType
 import org.calyxos.seedvault.core.backends.TopLevelFolder
+import java.security.GeneralSecurityException
 import java.time.LocalDate
 import java.time.temporal.ChronoField
 import java.time.temporal.TemporalAdjuster
@@ -42,9 +43,14 @@ internal class Pruner(
         val snapshotMap = mutableMapOf<Long, AppBackupFileType.Snapshot>()
         val snapshots = mutableListOf<Snapshot>()
         snapshotHandles.forEach { handle ->
-            val snapshot = snapshotManager.loadSnapshot(handle) // exception is allowed to bubble up
+            try {
+                val snapshot = snapshotManager.loadSnapshot(handle)
                 snapshotMap[snapshot.token] = handle
                 snapshots.add(snapshot)
+            } catch (e: GeneralSecurityException) {
+                log.error(e) { "Error loading snapshot $handle, will remove: " }
+                snapshotManager.removeSnapshot(handle)
+            } // other exceptions (like IOException) are allowed to bubble up, so we try again
         }
         // find out which snapshots to keep
         val toKeep = getTokenToKeep(snapshotMap.keys)

app/src/main/java/com/stevesoltys/seedvault/worker/ApkBackup.kt

@@ -113,6 +113,7 @@ internal class ApkBackup(
             val blobMap = chunkIds.associateWith { chunkId ->
                 latestSnapshot.blobsMap[chunkId] ?: error("Missing blob for $chunkId")
             }
+            // TODO could also check if all blobs are (still) available in BlobCache
             // important: add old APK to snapshot or it wouldn't be part of backup
             snapshotCreator.onApkBackedUp(packageInfo, oldApk, blobMap)
             return

app/src/test/java/com/stevesoltys/seedvault/repo/PrunerTest.kt

@@ -23,6 +23,7 @@ import org.calyxos.seedvault.core.backends.FileInfo
 import org.calyxos.seedvault.core.backends.TopLevelFolder
 import org.calyxos.seedvault.core.toHexString
 import org.junit.jupiter.api.Test
+import java.security.GeneralSecurityException
 import java.time.LocalDateTime
 import java.time.ZoneOffset.UTC
 import java.util.concurrent.TimeUnit
@@ -61,6 +62,32 @@ internal class PrunerTest : TransportTest() {
         pruner.removeOldSnapshotsAndPruneUnusedBlobs()
     }
 
+    @Test
+    fun `corrupted snapshot gets removed`() = runBlocking {
+        val snapshot = snapshot.copy { token = System.currentTimeMillis() }
+        every { crypto.repoId } returns repoId
+        every { backendManager.backend } returns backend
+        coEvery {
+            backend.list(folder, AppBackupFileType.Snapshot::class, callback = captureLambda())
+        } answers {
+            val fileInfo = FileInfo(snapshotHandle1, Random.nextLong(Long.MAX_VALUE))
+            lambda<(FileInfo) -> Unit>().captured.invoke(fileInfo)
+        }
+        coEvery { snapshotManager.loadSnapshot(snapshotHandle1) } throws GeneralSecurityException()
+        coEvery { snapshotManager.removeSnapshot(snapshotHandle1) } just Runs
+
+        // we only find blobs that are in snapshots
+        val blobIds = snapshot.blobsMap.values.map { it.id.hexFromProto() }
+        expectLoadingBlobs(blobIds)
+
+        // but since all snapshots got removed, we remove all blobs :(
+        blobIds.forEach {
+            coEvery { backend.remove(AppBackupFileType.Blob(repoId, it)) } just Runs
+        }
+
+        pruner.removeOldSnapshotsAndPruneUnusedBlobs()
+    }
+
     @Test
     fun `three snapshots from same day don't remove blobs`() = runBlocking {
         val snapshotMap = mapOf(