Extract generic migration logic into datastores/sql/migrate package

Move database-agnostic migration logic (file discovery, version tracking,
bootstrap detection) into a shared migrate package behind a Dialect
interface, leaving postgres as a thin wrapper.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

datastores/sql/migrate/migrate.go

@@ -0,0 +1,197 @@
+package migrate
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+	"time"
+)
+
+// Dialect abstracts the few SQL differences between database engines.
+type Dialect interface {
+	// Placeholder returns the bind parameter for position n (1-indexed).
+	// Postgres: "$1", "$2". MySQL/SQLite: "?".
+	Placeholder(n int) string
+
+	// TableExistsQuery returns SQL that checks whether a table exists.
+	// The query must accept a single bind parameter (the table name) and
+	// return a single boolean column.
+	TableExistsQuery() string
+}
+
+// Options configures the migration runner.
+type Options struct {
+	// MigrationsDir is the directory containing *.up.sql migration files.
+	// If empty, it defaults to "migrations" relative to the working directory.
+	MigrationsDir string
+
+	// BootstrapTable is the name of a table whose existence signals that the
+	// database already has a schema applied before the schema_migrations
+	// tracking table was introduced. When non-empty and this table exists but
+	// schema_migrations does not, all discovered migrations are recorded as
+	// already applied (bootstrapped). When empty, no bootstrapping is performed.
+	BootstrapTable string
+}
+
+// Run executes forward migrations against db using the given dialect.
+// It discovers *.up.sql files in the configured directory, tracks applied
+// versions in a schema_migrations table, and applies any pending migrations
+// in filename-sorted order.
+func Run(ctx context.Context, db *sql.DB, dialect Dialect, opts *Options) error {
+	if opts == nil {
+		opts = &Options{}
+	}
+
+	migrationsTableExists := tableExists(ctx, db, dialect, "schema_migrations")
+
+	if err := ensureMigrationsTable(ctx, db); err != nil {
+		return fmt.Errorf("failed to create migrations table: %w", err)
+	}
+
+	migrationsPath := opts.MigrationsDir
+	if migrationsPath == "" {
+		wd, err := os.Getwd()
+		if err != nil {
+			return fmt.Errorf("failed to get working directory: %w", err)
+		}
+		migrationsPath = filepath.Join(wd, "migrations")
+	}
+
+	migrations, err := discoverMigrations(migrationsPath)
+	if err != nil {
+		return fmt.Errorf("failed to discover migrations: %w", err)
+	}
+
+	if !migrationsTableExists && opts.BootstrapTable != "" {
+		if err := bootstrapMigrationState(ctx, db, dialect, migrations, opts.BootstrapTable); err != nil {
+			return fmt.Errorf("failed to bootstrap migration state: %w", err)
+		}
+	}
+
+	applied, err := getAppliedMigrations(ctx, db)
+	if err != nil {
+		return fmt.Errorf("failed to get applied migrations: %w", err)
+	}
+
+	for _, migration := range migrations {
+		if applied[migration] {
+			continue
+		}
+
+		migrationSQL, err := os.ReadFile(filepath.Join(migrationsPath, migration))
+		if err != nil {
+			return fmt.Errorf("failed to read migration file %s: %w", migration, err)
+		}
+
+		if _, err := db.ExecContext(ctx, string(migrationSQL)); err != nil {
+			return fmt.Errorf("failed to execute migration %s: %w", migration, err)
+		}
+
+		if err := recordMigration(ctx, db, dialect, migration); err != nil {
+			return fmt.Errorf("failed to record migration %s: %w", migration, err)
+		}
+
+		fmt.Printf("Applied migration: %s\n", migration)
+	}
+
+	return nil
+}
+
+// ensureMigrationsTable creates the schema_migrations table if it doesn't exist.
+func ensureMigrationsTable(ctx context.Context, db *sql.DB) error {
+	_, err := db.ExecContext(ctx, `
+		CREATE TABLE IF NOT EXISTS schema_migrations (
+			version VARCHAR(255) PRIMARY KEY,
+			applied_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP
+		)
+	`)
+	return err
+}
+
+// tableExists checks if a table exists in the database.
+func tableExists(ctx context.Context, db *sql.DB, dialect Dialect, tableName string) bool {
+	var exists bool
+	err := db.QueryRowContext(ctx,
+		dialect.TableExistsQuery(),
+		tableName,
+	).Scan(&exists)
+	return err == nil && exists
+}
+
+// bootstrapMigrationState marks all discovered migrations as applied when the
+// database already contains a schema (detected via sentinelTable) but has no
+// schema_migrations table yet.
+func bootstrapMigrationState(ctx context.Context, db *sql.DB, dialect Dialect, migrations []string, sentinelTable string) error {
+	if !tableExists(ctx, db, dialect, sentinelTable) {
+		return nil
+	}
+
+	for _, migration := range migrations {
+		if err := recordMigration(ctx, db, dialect, migration); err != nil {
+			return fmt.Errorf("failed to record bootstrapped migration %s: %w", migration, err)
+		}
+		fmt.Printf("Bootstrapped migration (already applied): %s\n", migration)
+	}
+
+	return nil
+}
+
+// getAppliedMigrations returns a set of already-applied migration filenames.
+func getAppliedMigrations(ctx context.Context, db *sql.DB) (map[string]bool, error) {
+	applied := make(map[string]bool)
+
+	rows, err := db.QueryContext(ctx, "SELECT version FROM schema_migrations")
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	for rows.Next() {
+		var version string
+		if err := rows.Scan(&version); err != nil {
+			return nil, err
+		}
+		applied[version] = true
+	}
+
+	return applied, rows.Err()
+}
+
+// recordMigration records a migration version as applied.
+func recordMigration(ctx context.Context, db *sql.DB, dialect Dialect, version string) error {
+	_, err := db.ExecContext(ctx,
+		fmt.Sprintf("INSERT INTO schema_migrations (version, applied_at) VALUES (%s, %s)",
+			dialect.Placeholder(1), dialect.Placeholder(2)),
+		version, time.Now(),
+	)
+	return err
+}
+
+// discoverMigrations finds all *.up.sql files in the given directory and
+// returns them sorted by filename (relies on numeric prefixes like
+// 000001_, 000002_, etc.).
+func discoverMigrations(migrationsPath string) ([]string, error) {
+	entries, err := os.ReadDir(migrationsPath)
+	if err != nil {
+		return nil, err
+	}
+
+	var migrations []string
+	for _, entry := range entries {
+		if entry.IsDir() {
+			continue
+		}
+		name := entry.Name()
+		if strings.HasSuffix(name, ".up.sql") {
+			migrations = append(migrations, name)
+		}
+	}
+
+	sort.Strings(migrations)
+
+	return migrations, nil
+}