Centralize remote current-state loading and preserve caller policy

internal/previouscache/previouscache.go

@@ -2,8 +2,8 @@ package previouscache
 
 import (
 	"context"
+	"errors"
 	"fmt"
-	"os"
 	"path"
 	"path/filepath"
 	"sort"
@@ -92,86 +92,28 @@ func BuildPlan(
 		campaign,
 		previousSessionID,
 	)
-	currentManifestKey, currentRunIDKey := artifacts.ResolveCurrentStateKeys(previousSessionPrefix)
 
 	result := &Plan{}
-
-	runPointerExists, err := store.Exists(ctx, currentRunIDKey)
+	current, err := artifacts.LoadCurrentState(ctx, store, previousSessionPrefix, artifacts.CurrentStateValidation{
+		ExpectedSessionID: previousSessionID,
+		ExpectedCampaign:  campaign,
+		ValidateRunID:     true,
+	})
 	if err != nil {
-		return nil, fmt.Errorf("check previous-session current run pointer %q: %w", currentRunIDKey, err)
-	}
-	if !runPointerExists {
-		if len(requiredNames) > 0 {
-			return nil, fmt.Errorf("required previous-session artifacts unavailable: remote current run pointer missing: %q", currentRunIDKey)
+		var runPointerMissing *artifacts.CurrentRunPointerMissingError
+		var manifestMissing *artifacts.CurrentManifestMissingError
+		if errors.As(err, &runPointerMissing) || errors.As(err, &manifestMissing) {
+			if len(requiredNames) > 0 {
+				return nil, fmt.Errorf("required previous-session artifacts unavailable: remote %w", err)
+			}
+			result.SkippedMissing = optionalNames
+			return result, nil
 		}
-		result.SkippedMissing = optionalNames
-		return result, nil
-	}
-
-	runIDTemp, err := storage.DownloadObjectToTemp(ctx, store, currentRunIDKey, "narratio-previous-run-id-*.txt")
-	if err != nil {
-		return nil, fmt.Errorf("download previous-session current run pointer %q: %w", currentRunIDKey, err)
-	}
-	defer func() { _ = os.Remove(runIDTemp) }()
-
-	runIDBytes, err := os.ReadFile(runIDTemp)
-	if err != nil {
-		return nil, fmt.Errorf("read previous-session current run pointer %q: %w", currentRunIDKey, err)
-	}
-	previousRunID := strings.TrimSpace(string(runIDBytes))
-	if previousRunID == "" {
-		return nil, fmt.Errorf("previous-session current run pointer %q is empty", currentRunIDKey)
-	}
-	result.PreviousRunID = previousRunID
-
-	manifestExists, err := store.Exists(ctx, currentManifestKey)
-	if err != nil {
-		return nil, fmt.Errorf("check previous-session current manifest %q: %w", currentManifestKey, err)
-	}
-	if !manifestExists {
-		if len(requiredNames) > 0 {
-			return nil, fmt.Errorf("required previous-session artifacts unavailable: remote current manifest missing: %q", currentManifestKey)
-		}
-		result.SkippedMissing = optionalNames
-		return result, nil
-	}
-
-	manifestTemp, err := storage.DownloadObjectToTemp(ctx, store, currentManifestKey, "narratio-previous-manifest-*.json")
-	if err != nil {
-		return nil, fmt.Errorf("download previous-session current manifest %q: %w", currentManifestKey, err)
-	}
-	defer func() { _ = os.Remove(manifestTemp) }()
-
-	manifestStore := &manifest.LocalStore{}
-	previousManifest, err := manifestStore.Load(ctx, manifestTemp)
-	if err != nil {
-		return nil, fmt.Errorf("decode downloaded previous-session manifest %q: %w", currentManifestKey, err)
-	}
-	if strings.TrimSpace(previousManifest.SessionID) != previousSessionID {
-		return nil, fmt.Errorf(
-			"previous-session manifest session_id %q does not match configured previous_session_id %q",
-			strings.TrimSpace(previousManifest.SessionID),
-			previousSessionID,
-		)
-	}
-	if strings.TrimSpace(previousManifest.Campaign) != campaign {
-		return nil, fmt.Errorf(
-			"previous-session manifest campaign %q does not match current campaign %q",
-			strings.TrimSpace(previousManifest.Campaign),
-			campaign,
-		)
-	}
-	if strings.TrimSpace(previousManifest.RunID) == "" {
-		return nil, fmt.Errorf("previous-session manifest run_id is required")
-	}
-	if strings.TrimSpace(previousManifest.RunID) != previousRunID {
-		return nil, fmt.Errorf(
-			"previous-session current run pointer %q references run %q but current manifest run_id is %q",
-			currentRunIDKey,
-			previousRunID,
-			strings.TrimSpace(previousManifest.RunID),
-		)
+		return nil, fmt.Errorf("load previous-session current state: %w", err)
 	}
+	result.PreviousRunID = strings.TrimSpace(current.RunID)
+	currentManifestKey := current.CurrentManifestKey
+	previousManifest := current.Manifest
 
 	manifestRel, err := relativeToSession(paths, paths.PreviousManifestPath)
 	if err != nil {