Bind restore to committed remote snapshots

internal/previouscache/previouscache.go

@@ -36,6 +36,9 @@ type Record struct {
 	LocalRelativePath string
 	LocalPath         string
 	RemoteKey         string
+	SHA256            string
+	Size              int64
+	Generation        string
 	S3Bucket          string
 }
 
@@ -119,13 +122,23 @@ func BuildPlan(
 	if err != nil {
 		return nil, err
 	}
-	result.Records = append(result.Records, Record{
+	manifestRecord := Record{
 		Kind:              InputKindManifest,
 		LocalRelativePath: manifestRel,
 		LocalPath:         paths.PreviousManifestPath,
 		RemoteKey:         currentManifestKey,
 		S3Bucket:          bucket,
-	})
+	}
+	if current.Commit != nil {
+		committedManifest, ok := current.Commit.Artifact(artifacts.RemoteArtifactTypeSessionManifest)
+		if !ok || committedManifest.DestinationKey != currentManifestKey {
+			return nil, fmt.Errorf("previous-session remote commit does not declare its session manifest")
+		}
+		manifestRecord.SHA256 = committedManifest.SHA256
+		manifestRecord.Size = committedManifest.Size
+		manifestRecord.Generation = committedManifest.Generation
+	}
+	result.Records = append(result.Records, manifestRecord)
 
 	for _, requirement := range orderedRequirements {
 		candidates := artifactRelativePathCandidates(requirement.Name, previousManifest, cfg)
@@ -142,18 +155,28 @@ func BuildPlan(
 
 		selectedRel := ""
 		selectedKey := ""
+		var selectedArtifact *artifacts.RemoteArtifact
 		for _, candidate := range candidates {
+			if current.Commit != nil {
+				artifact, ok := committedPublishedArtifact(current.Commit, previousSessionPrefix, candidate)
+				if !ok {
+					continue
+				}
+				selectedRel = candidate
+				selectedKey = artifact.DestinationKey
+				selectedArtifact = &artifact
+				break
+			}
 			remoteKey := artifacts.S3PublishedOutputKey(previousSessionPrefix, candidate)
 			exists, err := store.Exists(ctx, remoteKey)
 			if err != nil {
 				return nil, fmt.Errorf("check previous-session artifact object %q: %w", remoteKey, err)
 			}
-			if !exists {
-				continue
+			if exists {
+				selectedRel = candidate
+				selectedKey = remoteKey
+				break
 			}
-			selectedRel = candidate
-			selectedKey = remoteKey
-			break
 		}
 		if selectedRel == "" {
 			if requirement.Required {
@@ -174,7 +197,7 @@ func BuildPlan(
 		if err != nil {
 			return nil, err
 		}
-		result.Records = append(result.Records, Record{
+		record := Record{
 			Kind:              InputKindArtifact,
 			RequirementName:   requirement.Name,
 			Required:          requirement.Required,
@@ -182,7 +205,13 @@ func BuildPlan(
 			LocalPath:         localPath,
 			RemoteKey:         selectedKey,
 			S3Bucket:          bucket,
-		})
+		}
+		if selectedArtifact != nil {
+			record.SHA256 = selectedArtifact.SHA256
+			record.Size = selectedArtifact.Size
+			record.Generation = selectedArtifact.Generation
+		}
+		result.Records = append(result.Records, record)
 	}
 
 	sort.Strings(result.SkippedMissing)
@@ -195,6 +224,19 @@ func BuildPlan(
 	return result, nil
 }
 
+func committedPublishedArtifact(commit *artifacts.RemoteCommitManifest, sessionPrefix, relativePath string) (artifacts.RemoteArtifact, bool) {
+	if commit == nil {
+		return artifacts.RemoteArtifact{}, false
+	}
+	want := artifacts.S3RunRelativeDestinationKey(artifacts.S3RunPrefix(sessionPrefix, commit.RunID), relativePath)
+	for _, artifact := range commit.Artifacts {
+		if artifact.Type == artifacts.RemoteArtifactTypePublishedOutput && artifact.DestinationKey == want {
+			return artifact, true
+		}
+	}
+	return artifacts.RemoteArtifact{}, false
+}
+
 func requiredPreviousArtifactNames(requirements []artifacts.PreviousArtifactRequirement) []string {
 	names := make([]string, 0, len(requirements))
 	for _, requirement := range requirements {