Updated tracking / transitions of the task's version state.

app/lib/task/backend.dart

@@ -689,17 +689,9 @@ class TaskBackend {
 
       zone = versionState.zone!;
       instance = versionState.instance!;
-
-      // Remove instanceName, zone, secretToken, and set attempts = 0
-      state.versions![version] = PackageVersionStateInfo(
-        scheduled: versionState.scheduled,
+      state.versions![version] = versionState.complete(
         docs: hasDocIndexHtml,
         pana: summary != null,
-        finished: true,
-        attempts: 0,
-        instance: null, // version is no-longer running on this instance
-        secretToken: null, // TODO: Consider retaining this for idempotency
-        zone: null,
       );
 
       // Determine if something else was running on the instance
@@ -1002,13 +994,12 @@ class TaskBackend {
     await for (final state in _db.tasks.listAllForCurrentRuntime()) {
       final zone = taskWorkerCloudCompute.zones.first;
       // ignore: invalid_use_of_visible_for_testing_member
-      final updated = await updatePackageStateWithPendingVersions(
+      final payload = await updatePackageStateWithPendingVersions(
         _db,
         state.package,
         zone,
         taskWorkerCloudCompute.generateInstanceName(),
       );
-      final payload = updated?.$1;
       if (payload == null) continue;
       await processPayload(payload);
     }
@@ -1418,7 +1409,6 @@ final class _TaskDataAccess {
   Future<void> restorePreviousVersionsState(
     String packageName,
     String instanceName,
-    Map<String, PackageVersionStateInfo> previousVersionsMap,
   ) async {
     await withRetryTransaction(_db, (tx) async {
       final s = await tx.tasks.lookupOrNull(packageName);
@@ -1429,7 +1419,7 @@ final class _TaskDataAccess {
       s.versions!.addEntries(
         s.versions!.entries
             .where((e) => e.value.instance == instanceName)
-            .map((e) => MapEntry(e.key, previousVersionsMap[e.key]!)),
+            .map((e) => MapEntry(e.key, e.value.resetAfterFailedAttempt())),
       );
       s.pendingAt = derivePendingAt(
         versions: s.versions!,

app/lib/task/models.dart

@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:convert' show json;
+import 'dart:math';
 
 import 'package:clock/clock.dart';
 import 'package:json_annotation/json_annotation.dart';
@@ -249,7 +250,7 @@ List<String> derivePendingVersions({
 }
 
 /// State of a given `version` within a [PackageState].
-@JsonSerializable()
+@JsonSerializable(includeIfNull: false)
 class PackageVersionStateInfo {
   PackageVersionStatus get status {
     if (attempts == 0 && scheduled == initialTimestamp) {
@@ -319,6 +320,9 @@ class PackageVersionStateInfo {
   /// comparison. Please use [isAuthorized] for validating a request.
   final String? secretToken;
 
+  /// The previous scheduled timestamp (if we are currently in an active schedule).
+  final DateTime? previousScheduled;
+
   /// Return true, if [token] matches [secretToken] and it has not expired.
   ///
   /// This does a fixed-time comparison to mitigate timing attacks.
@@ -347,6 +351,7 @@ class PackageVersionStateInfo {
     this.docs = false,
     this.pana = false,
     this.finished = false,
+    this.previousScheduled,
   });
 
   factory PackageVersionStateInfo.fromJson(Map<String, dynamic> m) =>
@@ -364,6 +369,53 @@ class PackageVersionStateInfo {
         'secretToken: $secretToken',
       ].join(', ') +
       ')';
+
+  // Remove instanceName, zone, secretToken, and set attempts = 0
+  PackageVersionStateInfo complete({required bool pana, required bool docs}) {
+    return PackageVersionStateInfo(
+      scheduled: scheduled,
+      attempts: 0,
+      docs: docs,
+      pana: pana,
+      finished: true,
+      zone: null,
+      instance: null, // version is no-longer running on this instance
+      secretToken: null, // TODO: Consider retaining this for idempotency
+      previousScheduled: null,
+    );
+  }
+
+  /// Derives a new version state with scheduling information.
+  PackageVersionStateInfo scheduleNew({
+    required String zone,
+    required String instanceName,
+  }) {
+    return PackageVersionStateInfo(
+      scheduled: clock.now(),
+      attempts: attempts + 1,
+      zone: zone,
+      instance: instanceName,
+      secretToken: createUuid(),
+      finished: finished,
+      docs: docs,
+      pana: pana,
+      previousScheduled: scheduled,
+    );
+  }
+
+  /// Reverts the status of the last scheduling attempt, which has presumably failed.
+  PackageVersionStateInfo resetAfterFailedAttempt() {
+    return PackageVersionStateInfo(
+      scheduled: previousScheduled ?? initialTimestamp,
+      attempts: max(0, attempts - 1),
+      zone: null,
+      instance: null,
+      secretToken: null,
+      finished: finished,
+      docs: docs,
+      pana: pana,
+    );
+  }
 }
 
 /// A [db.Property] encoding a Map from version to [PackageVersionStateInfo] as JSON.

app/lib/task/scheduler.dart

@@ -10,7 +10,6 @@ import 'package:meta/meta.dart';
 import 'package:pub_dev/package/backend.dart';
 import 'package:pub_dev/shared/configuration.dart';
 import 'package:pub_dev/shared/datastore.dart';
-import 'package:pub_dev/shared/utils.dart';
 import 'package:pub_dev/task/backend.dart';
 import 'package:pub_dev/task/cloudcompute/cloudcompute.dart';
 import 'package:pub_dev/task/models.dart';
@@ -101,13 +100,12 @@ Future<(CreateInstancesState, Duration)> runOneCreateInstancesCycle(
     final instanceName = compute.generateInstanceName();
     final zone = pickZone();
 
-    final updated = await updatePackageStateWithPendingVersions(
+    final payload = await updatePackageStateWithPendingVersions(
       db,
       selected.package,
       zone,
       instanceName,
     );
-    final payload = updated?.$1;
     if (payload == null) {
       return;
     }
@@ -174,15 +172,13 @@ Future<(CreateInstancesState, Duration)> runOneCreateInstancesCycle(
       banZone(zone, minutes: 15);
     }
     if (rollbackPackageState) {
-      final oldVersionsMap = updated?.$2 ?? const {};
-      // Restore the state of the PackageState for versions that were
+      // Restire the state of the PackageState for versions that were
       // suppose to run on the instance we just failed to create.
       // If this doesn't work, we'll eventually retry. Hence, correctness
       // does not hinge on this transaction being successful.
       await db.tasks.restorePreviousVersionsState(
         selected.package,
         instanceName,
-        oldVersionsMap,
       );
     }
   }
@@ -221,11 +217,8 @@ Future<(CreateInstancesState, Duration)> runOneCreateInstancesCycle(
 
 /// Updates the package state with versions that are already pending or
 /// will be pending soon.
-///
-/// Returns the payload and the old status of the state info version map
 @visibleForTesting
-Future<(Payload, Map<String, PackageVersionStateInfo>)?>
-updatePackageStateWithPendingVersions(
+Future<Payload?> updatePackageStateWithPendingVersions(
   DatastoreDB db,
   String package,
   String zone,
@@ -237,7 +230,6 @@ updatePackageStateWithPendingVersions(
       // presumably the package was deleted.
       return null;
     }
-    final oldVersionsMap = {...?s.versions};
 
     final now = clock.now();
     final pendingVersions = derivePendingVersions(
@@ -253,14 +245,7 @@ updatePackageStateWithPendingVersions(
     // Update PackageState
     s.versions!.addAll({
       for (final v in pendingVersions.map((v) => v.toString()))
-        v: PackageVersionStateInfo(
-          scheduled: now,
-          attempts: s.versions![v]!.attempts + 1,
-          zone: zone,
-          instance: instanceName,
-          secretToken: createUuid(),
-          finished: s.versions![v]!.finished,
-        ),
+        v: s.versions![v]!.scheduleNew(zone: zone, instanceName: instanceName),
     });
     s.pendingAt = derivePendingAt(
       versions: s.versions!,
@@ -279,6 +264,6 @@ updatePackageStateWithPendingVersions(
         ),
       ),
     );
-    return (payload, oldVersionsMap);
+    return payload;
   });
 }