Authorize comparison replacement at commit time

internal/comparison/publish.go

@@ -51,9 +51,17 @@ type DestinationPlan struct {
 	WorkingDirectory string
 	Target           string
 	Replace          bool
-	Exists           bool
+	state            destinationState
 }
 
+type destinationState uint8
+
+const (
+	destinationAbsent destinationState = iota
+	destinationEmpty
+	destinationBundle
+)
+
 // ErrUnrecognizedBundle marks a directory that is not a valid current-schema
 // Weatherreporter comparison bundle.
 var ErrUnrecognizedBundle = errors.New("unrecognized comparison bundle")
@@ -98,7 +106,7 @@ func PlanDestination(workingDirectory, target string, replace bool) (Destination
 		return DestinationPlan{}, newDestinationError(DestinationInspection, target, err)
 	}
 	if len(entries) == 0 {
-		plan.Exists = true
+		plan.state = destinationEmpty
 		return plan, nil
 	}
 	if !replace {
@@ -107,7 +115,7 @@ func PlanDestination(workingDirectory, target string, replace bool) (Destination
 	if _, err := RecognizeBundle(target); err != nil {
 		return DestinationPlan{}, newDestinationError(DestinationUnrecognized, target, err)
 	}
-	plan.Exists = true
+	plan.state = destinationBundle
 	return plan, nil
 }
 
@@ -312,7 +320,7 @@ func publish(ctx context.Context, plan DestinationPlan, bundle LogicalBundle, op
 	if err := ctx.Err(); err != nil {
 		return err
 	}
-	if !currentPlan.Exists {
+	if currentPlan.state == destinationAbsent {
 		if err := operations.rename(temporaryDirectory, currentPlan.Target); err != nil {
 			return fmt.Errorf("publish comparison bundle to %q: %w", currentPlan.Target, err)
 		}
@@ -327,15 +335,11 @@ func publish(ctx context.Context, plan DestinationPlan, bundle LogicalBundle, op
 	if err := operations.rename(currentPlan.Target, backupDirectory); err != nil {
 		return fmt.Errorf("back up comparison destination %q: %w", currentPlan.Target, err)
 	}
+	if err := authorizeMovedDestination(currentPlan, backupDirectory); err != nil {
+		return restoreMovedDestination(operations, backupDirectory, currentPlan.Target, err)
+	}
 	if err := operations.rename(temporaryDirectory, currentPlan.Target); err != nil {
-		restoreErr := operations.rename(backupDirectory, currentPlan.Target)
-		if restoreErr != nil {
-			return errors.Join(
-				fmt.Errorf("replace comparison destination %q: %w", currentPlan.Target, err),
-				fmt.Errorf("restore prior comparison destination from %q: %w", backupDirectory, restoreErr),
-			)
-		}
-		return fmt.Errorf("replace comparison destination %q: %w", currentPlan.Target, err)
+		return restoreMovedDestination(operations, backupDirectory, currentPlan.Target, fmt.Errorf("replace comparison destination %q: %w", currentPlan.Target, err))
 	}
 	temporaryDirectory = ""
 	if err := os.RemoveAll(backupDirectory); err != nil {
@@ -344,6 +348,38 @@ func publish(ctx context.Context, plan DestinationPlan, bundle LogicalBundle, op
 	return nil
 }
 
+func authorizeMovedDestination(plan DestinationPlan, backupDirectory string) error {
+	backupPlan, err := PlanDestination(plan.WorkingDirectory, backupDirectory, plan.Replace)
+	if err != nil {
+		return fmt.Errorf("authorize moved comparison destination %q: %w", backupDirectory, err)
+	}
+	if backupPlan.state != destinationEmpty && backupPlan.state != destinationBundle {
+		return fmt.Errorf("authorize moved comparison destination %q: destination disappeared", backupDirectory)
+	}
+	return nil
+}
+
+func restoreMovedDestination(operations publishOperations, backupDirectory, target string, cause error) error {
+	if _, err := os.Lstat(target); err == nil {
+		return errors.Join(
+			cause,
+			fmt.Errorf("restore prior comparison destination from %q: destination %q reappeared", backupDirectory, target),
+		)
+	} else if !errors.Is(err, os.ErrNotExist) {
+		return errors.Join(
+			cause,
+			fmt.Errorf("inspect comparison destination %q before restoring from %q: %w", target, backupDirectory, err),
+		)
+	}
+	if restoreErr := operations.rename(backupDirectory, target); restoreErr != nil {
+		return errors.Join(
+			cause,
+			fmt.Errorf("restore prior comparison destination from %q: %w", backupDirectory, restoreErr),
+		)
+	}
+	return cause
+}
+
 func writeLogicalBundle(ctx context.Context, directory string, bundle LogicalBundle, manifestData []byte) error {
 	if err := writeBundleFile(ctx, filepath.Join(directory, DataPackageFilename), bundle.DataPackage); err != nil {
 		return err