Create backup.go

04-errors-semantics/19-defer-cleanup-chain/backup.go

@@ -0,0 +1,146 @@
+package defer_cleanup_chain
+
+import (
+	"bufio"
+	"context"
+	"errors"
+	"fmt"
+	"os"
+)
+
+// Tx models the transaction lifecycle we need for this kata.
+// Keeping this small makes it easy to provide fake implementations in tests.
+type Tx interface {
+	Commit() error
+	Rollback() error
+}
+
+// Rows models a streaming result set.
+// Values returns the current row payload after Next() moved to a valid item.
+type Rows interface {
+	Next() bool
+	Values() (id int, payload string, err error)
+	Err() error
+	Close() error
+}
+
+// DB is the minimal database contract needed by BackupDatabase.
+// The implementation can be real SQL, in-memory, or fully mocked in tests.
+type DB interface {
+	Begin(ctx context.Context) (Tx, error)
+	QueryBackupRows(ctx context.Context, tx Tx) (Rows, error)
+	Close() error
+}
+
+// Connector abstracts DB creation so tests can inject controlled failures.
+type Connector func(ctx context.Context, dbURL string) (DB, error)
+
+// defaultConnector is intentionally unconfigured.
+// In real code this would be wired to an actual driver in init/bootstrap code.
+var defaultConnector Connector = func(ctx context.Context, dbURL string) (DB, error) {
+	return nil, errors.New("no database connector configured")
+}
+
+// BackupDatabase performs the backup flow while preserving both operation and
+// cleanup errors. It uses a named return error so deferred cleanups can append
+// their own failures via errors.Join.
+func BackupDatabase(ctx context.Context, dbURL, filename string) (err error) {
+	return backupDatabase(ctx, dbURL, filename, defaultConnector, os.Create)
+}
+
+// backupDatabase is split from BackupDatabase to keep the public API clean while
+// allowing tests to inject open/connect behavior and force edge cases.
+func backupDatabase(
+	ctx context.Context,
+	dbURL, filename string,
+	connect Connector,
+	openFile func(name string) (*os.File, error),
+) (err error) {
+	// Acquire #1: output file.
+	// Defer is registered immediately so every return path closes the file.
+	file, err := openFile(filename)
+	if err != nil {
+		return fmt.Errorf("open output file: %w", err)
+	}
+	defer func() {
+		if closeErr := file.Close(); closeErr != nil {
+			err = errors.Join(err, fmt.Errorf("close output file: %w", closeErr))
+		}
+	}()
+
+	// Wrap the file with a buffered writer. This is another acquired resource:
+	// data can remain in memory until Flush runs, so we must defer Flush as well.
+	writer := bufio.NewWriter(file)
+	defer func() {
+		if flushErr := writer.Flush(); flushErr != nil {
+			err = errors.Join(err, fmt.Errorf("flush writer: %w", flushErr))
+		}
+	}()
+
+	// Acquire #2: DB connection.
+	db, err := connect(ctx, dbURL)
+	if err != nil {
+		return fmt.Errorf("connect DB: %w", err)
+	}
+	defer func() {
+		if closeErr := db.Close(); closeErr != nil {
+			err = errors.Join(err, fmt.Errorf("close DB connection: %w", closeErr))
+		}
+	}()
+
+	// Acquire #3: transaction.
+	// We avoid manual rollback branches by using a committed flag consumed by defer.
+	tx, err := db.Begin(ctx)
+	if err != nil {
+		return fmt.Errorf("begin transaction: %w", err)
+	}
+	committed := false
+	defer func() {
+		if committed {
+			return
+		}
+		if rollbackErr := tx.Rollback(); rollbackErr != nil {
+			err = errors.Join(err, fmt.Errorf("rollback transaction: %w", rollbackErr))
+		}
+	}()
+
+	// Acquire #4: row stream (logical cursor/network resource).
+	rows, err := db.QueryBackupRows(ctx, tx)
+	if err != nil {
+		return fmt.Errorf("query backup rows: %w", err)
+	}
+	defer func() {
+		if closeErr := rows.Close(); closeErr != nil {
+			err = errors.Join(err, fmt.Errorf("close rows: %w", closeErr))
+		}
+	}()
+
+	// Stream each row to the backup file.
+	// We intentionally do not defer anything inside this loop.
+	for rows.Next() {
+		if ctxErr := ctx.Err(); ctxErr != nil {
+			return fmt.Errorf("backup canceled: %w", ctxErr)
+		}
+
+		id, payload, rowErr := rows.Values()
+		if rowErr != nil {
+			return fmt.Errorf("read row values: %w", rowErr)
+		}
+
+		if _, writeErr := fmt.Fprintf(writer, "%d,%s\n", id, payload); writeErr != nil {
+			return fmt.Errorf("write backup row: %w", writeErr)
+		}
+	}
+	if rowsErr := rows.Err(); rowsErr != nil {
+		return fmt.Errorf("iterate backup rows: %w", rowsErr)
+	}
+
+	// Commit is the success boundary.
+	// If commit fails, committed remains false and deferred rollback still runs.
+	if commitErr := tx.Commit(); commitErr != nil {
+		return fmt.Errorf("commit transaction: %w", commitErr)
+	}
+	committed = true
+
+	return nil
+}