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19-defer-cleanup-chain/README.md

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+# 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)