go-kata/19-defer-cleanup-chain/README.md

Kata 19: The Cleanup Chain (defer + LIFO + Error Preservation)

Target Idioms: defer Discipline, Named Returns, Error Composition (errors.Join), Close/Rollback Ordering
Difficulty: 🟡 Intermediate

🧠 The "Why"

defer is easy to misuse:

• deferring in loops (resource spikes),
• ignoring Close() / Rollback() errors,
• losing the original failure when cleanup also fails,
• wrong cleanup ordering (commit then rollback nonsense).

Idiomatic Go keeps cleanup local, ordered, and preserves important errors.

🎯 The Scenario

You implement BackupDatabase:

• open output file
• connect DB
• begin transaction
• stream rows to file
• commit If anything fails, you must close/rollback what was already acquired.

🛠 The Challenge

Implement:

• func BackupDatabase(ctx context.Context, dbURL, filename string) (err error)

Use mock interfaces for DB + Tx + Rows if you want (recommended).

1. Functional Requirements

• Open file for writing.
• Connect to DB.
• Begin Tx.
• Write data (simulate streaming).
• Commit on success.
• On failure: rollback + close resources in correct order.

2. The "Idiomatic" Constraints (Pass/Fail Criteria)

• Defer cleanup immediately after acquisition.
• No manual cleanup paths except by controlling flags (e.g., committed bool) used by deferred funcs.
• Preserve both errors: if main operation fails and cleanup fails too, return a combined error (errors.Join).
• Named return err so defers can amend it safely.
• No defer-in-loop for per-row resources: if your mock has per-row closers, show the correct pattern.

🧪 Self-Correction (Test Yourself)

1. Tx Begin Fails

   • Make Begin() error.
   • Pass: file + db connection still close.

2. Commit Fails + Close Fails

   • Make Commit() return error and also make file.Close() return error.
   • Pass: returned error clearly contains both (use errors.Join).

3. No FD Leak

   • Run 1000 times.
   • Pass: file descriptors don’t grow.

📚 Resources

• https://go.dev/blog/defer-panic-and-recover
• https://go.dev/doc/go1.20 (errors.Join)