feat(migrate): add pending migrations function

migrate/migrate.go

@@ -7,6 +7,7 @@ package migrate
 import (
 	"bytes"
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"io/fs"
@@ -87,6 +88,49 @@ func List(ctx context.Context, session gocqlx.Session) ([]*Info, error) {
 	return v, nil
 }
 
+// Pending provides a listing of pending migrations.
+func Pending(ctx context.Context, session gocqlx.Session, f fs.FS) ([]*Info, error) {
+	applied, err := List(ctx, session)
+	if err != nil {
+		return nil, err
+	}
+
+	fm, err := fs.Glob(f, "*.cql")
+	if err != nil {
+		return nil, fmt.Errorf("list migrations: %w", err)
+	}
+
+	if len(applied) > len(fm) {
+		return nil, fmt.Errorf("database is ahead")
+	}
+
+	pending := make([]*Info, 0, len(fm)-len(applied))
+
+	for i := range applied {
+		if applied[i].Name != fm[i] {
+			fmt.Println(applied[i].Name, fm[i], i)
+			return nil, errors.New("inconsistent migrations")
+		}
+	}
+
+	for _, name := range fm[len(applied):] {
+		c, err := fileChecksum(f, name)
+		if err != nil {
+			return nil, fmt.Errorf("calculate checksum for %q: %w", name, err)
+		}
+
+		info := &Info{
+			Name:      filepath.Base(name),
+			StartTime: time.Now(),
+			Checksum:  c,
+		}
+
+		pending = append(pending, info)
+	}
+
+	return pending, nil
+}
+
 func ensureInfoTable(ctx context.Context, session gocqlx.Session) error {
 	return session.ContextQuery(ctx, infoSchema, nil).ExecRelease()
 }