feat: integrate DB with Open/Close/Put/Delete/Get

- Add db.go with DB struct: Open/Close/Put/Delete/Get/GetDurableSequence/IsWriteStopped
- Add db_test.go with lifecycle, put/get, delete, recovery, and corruption tests
- Extract sentinel errors to errkit/ leaf package to break import cycle (wal -> go_kv)
- Update wal/sequence.go, wal/writer.go to import errkit instead of root package
- Root errors.go re-exports from errkit for backward compatibility

Phase 1 Wave 4d complete (T19).

.omo/boulder.json

@@ -8,7 +8,7 @@
       "plan_name": "phase1-wal",
       "status": "active",
       "started_at": "2026-06-12T05:09:32.588Z",
-      "updated_at": "2026-06-12T06:02:11.867Z",
+      "updated_at": "2026-06-12T06:09:22.858Z",
       "session_ids": [
         "opencode:ses_145c3bae9ffeTB2zbsTym0Cev8"
       ],
@@ -120,6 +120,19 @@
           "status": "completed",
           "ended_at": "2026-06-12T06:02:11.867Z",
           "elapsed_ms": 16713
+        },
+        "todo:19": {
+          "task_key": "todo:19",
+          "task_label": "19",
+          "task_title": "DB integration: Open/Close/Put/Delete/Get",
+          "session_id": "opencode:ses_1459293beffeLZq5Ac22xWZfBy",
+          "agent": "Sisyphus-Junior",
+          "category": "deep",
+          "updated_at": "2026-06-12T06:09:22.858Z",
+          "started_at": "2026-06-12T06:07:14.120Z",
+          "status": "completed",
+          "ended_at": "2026-06-12T06:09:22.858Z",
+          "elapsed_ms": 128738
         }
       }
     }
@@ -127,7 +140,7 @@
   "active_plan": "/home/dailz/workspace/src/go-kv/.omo/plans/phase1-wal.md",
   "started_at": "2026-06-12T05:09:32.588Z",
   "status": "active",
-  "updated_at": "2026-06-12T06:02:11.867Z",
+  "updated_at": "2026-06-12T06:09:22.858Z",
   "session_ids": [
     "opencode:ses_145c3bae9ffeTB2zbsTym0Cev8"
   ],
@@ -239,6 +252,19 @@
       "status": "completed",
       "ended_at": "2026-06-12T06:02:11.867Z",
       "elapsed_ms": 16713
+    },
+    "todo:19": {
+      "task_key": "todo:19",
+      "task_label": "19",
+      "task_title": "DB integration: Open/Close/Put/Delete/Get",
+      "session_id": "opencode:ses_1459293beffeLZq5Ac22xWZfBy",
+      "agent": "Sisyphus-Junior",
+      "category": "deep",
+      "updated_at": "2026-06-12T06:09:22.858Z",
+      "started_at": "2026-06-12T06:07:14.120Z",
+      "status": "completed",
+      "ended_at": "2026-06-12T06:09:22.858Z",
+      "elapsed_ms": 128738
     }
   },
   "agent": "atlas"

.omo/plans/phase1-wal.md

@@ -1412,7 +1412,7 @@ Max Concurrent: 6 (Wave 1b)
   **Commit**: YES
   - Message: `feat: implement MANIFEST stub and CURRENT file`
 
-- [ ] 19. DB integration: Open/Close/Put/Delete/Get
+- [x] 19. DB integration: Open/Close/Put/Delete/Get
 
   **What to do**:
   - 创建 `db.go`：`DB` 结构体和核心 API

db.go

@@ -0,0 +1,160 @@
+package go_kv
+
+import (
+	"fmt"
+	"os"
+	"sync"
+	"sync/atomic"
+
+	"github.com/dailz/go-kv/config"
+	"github.com/dailz/go-kv/memtable"
+	"github.com/dailz/go-kv/wal"
+)
+
+// DB is the public database handle for the storage engine.
+type DB struct {
+	cfg         *config.WalConfig
+	dir         string
+	writer      *wal.WalWriter
+	recoveryMem *memtable.MemTable
+	closed      atomic.Bool
+}
+
+// Open opens a database directory, recovers durable WAL entries, and starts a
+// fresh WAL writer at the next recovered segment and sequence.
+func Open(dir string, cfg *config.WalConfig) (*DB, error) {
+	if cfg == nil {
+		defaults := config.Defaults()
+		cfg = &defaults
+	}
+	if err := cfg.Validate(); err != nil {
+		return nil, fmt.Errorf("db: validate config: %w", err)
+	}
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		return nil, fmt.Errorf("db: create directory: %w", err)
+	}
+
+	recoveryMem := memtable.NewMemTable(cfg.MemTableSize)
+	replayer := &memTableReplayer{mem: recoveryMem}
+	result, err := wal.Recover(dir, replayer)
+	if err != nil {
+		return nil, fmt.Errorf("db: recover WAL: %w", err)
+	}
+	if err := replayer.Err(); err != nil {
+		return nil, fmt.Errorf("db: replay WAL: %w", err)
+	}
+	recoveryMem.Publish(^uint64(0))
+
+	writer, err := wal.NewWalWriter(cfg, dir, result.NextSegmentID, result.NextSequence)
+	if err != nil {
+		return nil, fmt.Errorf("db: create WAL writer: %w", err)
+	}
+
+	return &DB{
+		cfg:         cfg,
+		dir:         dir,
+		writer:      writer,
+		recoveryMem: recoveryMem,
+	}, nil
+}
+
+// Close drains outstanding writes and releases database resources.
+func (db *DB) Close() error {
+	if db.closed.Swap(true) {
+		return nil
+	}
+	if err := db.writer.Close(); err != nil {
+		return fmt.Errorf("db: close WAL writer: %w", err)
+	}
+	return nil
+}
+
+// Put stores key with value.
+func (db *DB) Put(key, value []byte) error {
+	if db.closed.Load() {
+		return ErrWriteStopped
+	}
+	if err := db.writer.Put(key, value); err != nil {
+		return fmt.Errorf("db: put: %w", err)
+	}
+	return nil
+}
+
+// Delete removes key.
+func (db *DB) Delete(key []byte) error {
+	if db.closed.Load() {
+		return ErrWriteStopped
+	}
+	if err := db.writer.Delete(key); err != nil {
+		return fmt.Errorf("db: delete: %w", err)
+	}
+	return nil
+}
+
+// Get returns the newest visible value for key across recovered and new writes.
+func (db *DB) Get(key []byte) *memtable.GetResult {
+	recovered := db.recoveryMem.Get(key)
+	current := db.writer.Get(key)
+
+	if current.Found || current.Sequence > recovered.Sequence {
+		return current
+	}
+	if !recovered.Found {
+		return &memtable.GetResult{Found: false}
+	}
+	if recovered.Value == nil {
+		return &memtable.GetResult{Found: false, Sequence: recovered.Sequence}
+	}
+	return recovered
+}
+
+// GetDurableSequence returns the highest sequence durably fsynced to WAL.
+func (db *DB) GetDurableSequence() uint64 {
+	return db.writer.GetDurableSequence()
+}
+
+// IsWriteStopped reports whether the DB rejects new writes.
+func (db *DB) IsWriteStopped() bool {
+	return db.closed.Load() || db.writer.IsWriteStopped()
+}
+
+type memTableReplayer struct {
+	mem *memtable.MemTable
+	mu  sync.Mutex
+	err error
+}
+
+func (r *memTableReplayer) ReplayPut(key, value []byte, sequence uint64) {
+	if err := r.mem.PutPending(cloneBytes(key), cloneBytes(value), sequence); err != nil {
+		r.setErr(fmt.Errorf("put pending: %w", err))
+	}
+}
+
+func (r *memTableReplayer) ReplayDelete(key []byte, sequence uint64) {
+	if err := r.mem.DeletePending(cloneBytes(key), sequence); err != nil {
+		r.setErr(fmt.Errorf("delete pending: %w", err))
+	}
+}
+
+func (r *memTableReplayer) Err() error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	return r.err
+}
+
+func (r *memTableReplayer) setErr(err error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if r.err == nil {
+		r.err = err
+	}
+}
+
+func cloneBytes(src []byte) []byte {
+	if src == nil {
+		return nil
+	}
+	dst := make([]byte, len(src))
+	copy(dst, src)
+	return dst
+}

db_test.go

@@ -0,0 +1,176 @@
+package go_kv
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestDBOpenClose(t *testing.T) {
+	db, err := Open(t.TempDir(), nil)
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+	if err := db.Put([]byte("k"), []byte("v")); err != nil {
+		t.Fatalf("Put: %v", err)
+	}
+	if err := db.Close(); err != nil {
+		t.Fatalf("Close: %v", err)
+	}
+}
+
+func TestDBPutGet(t *testing.T) {
+	db := openTestDB(t, t.TempDir())
+
+	if err := db.Put([]byte("k1"), []byte("v1")); err != nil {
+		t.Fatalf("Put: %v", err)
+	}
+
+	got := db.Get([]byte("k1"))
+	if !got.Found {
+		t.Fatal("Get(k1) not found")
+	}
+	if string(got.Value) != "v1" {
+		t.Fatalf("Get(k1) value = %q, want %q", got.Value, "v1")
+	}
+}
+
+func TestDBDelete(t *testing.T) {
+	db := openTestDB(t, t.TempDir())
+
+	if err := db.Put([]byte("k"), []byte("v")); err != nil {
+		t.Fatalf("Put: %v", err)
+	}
+	if err := db.Delete([]byte("k")); err != nil {
+		t.Fatalf("Delete: %v", err)
+	}
+
+	got := db.Get([]byte("k"))
+	if got.Found {
+		t.Fatalf("Get(k) found deleted key with value %q", got.Value)
+	}
+}
+
+func TestDBRecovery(t *testing.T) {
+	dir := t.TempDir()
+	db := openTestDB(t, dir)
+
+	entries := map[string]string{
+		"k1": "v1",
+		"k2": "v2",
+		"k3": "v3",
+	}
+	for key, value := range entries {
+		if err := db.Put([]byte(key), []byte(value)); err != nil {
+			t.Fatalf("Put(%q): %v", key, err)
+		}
+	}
+	closeTestDB(t, db)
+
+	reopened := openTestDB(t, dir)
+	for key, want := range entries {
+		got := reopened.Get([]byte(key))
+		if !got.Found {
+			t.Fatalf("Get(%q) not found after recovery", key)
+		}
+		if string(got.Value) != want {
+			t.Fatalf("Get(%q) value = %q, want %q", key, got.Value, want)
+		}
+	}
+}
+
+func TestDBRecoveryAfterCorruption(t *testing.T) {
+	dir := t.TempDir()
+	db := openTestDB(t, dir)
+
+	if err := db.Put([]byte("good1"), []byte("v1")); err != nil {
+		t.Fatalf("Put good1: %v", err)
+	}
+	if err := db.Put([]byte("good2"), []byte("v2")); err != nil {
+		t.Fatalf("Put good2: %v", err)
+	}
+	closeTestDB(t, db)
+
+	appendWalTail(t, dir, []byte{0xde, 0xad, 0xbe, 0xef})
+
+	reopened := openTestDB(t, dir)
+	for key, want := range map[string]string{"good1": "v1", "good2": "v2"} {
+		got := reopened.Get([]byte(key))
+		if !got.Found {
+			t.Fatalf("Get(%q) not found after corrupted-tail recovery", key)
+		}
+		if string(got.Value) != want {
+			t.Fatalf("Get(%q) value = %q, want %q", key, got.Value, want)
+		}
+	}
+}
+
+func TestDBMultiplePuts(t *testing.T) {
+	dir := t.TempDir()
+	db := openTestDB(t, dir)
+
+	for i := range 50 {
+		key := fmt.Appendf(nil, "k%d", i)
+		value := fmt.Appendf(nil, "v%d", i)
+		if err := db.Put(key, value); err != nil {
+			t.Fatalf("Put(%q): %v", key, err)
+		}
+	}
+	closeTestDB(t, db)
+
+	reopened := openTestDB(t, dir)
+	for i := range 50 {
+		key := fmt.Appendf(nil, "k%d", i)
+		want := fmt.Sprintf("v%d", i)
+		got := reopened.Get(key)
+		if !got.Found {
+			t.Fatalf("Get(%q) not found after recovery", key)
+		}
+		if string(got.Value) != want {
+			t.Fatalf("Get(%q) value = %q, want %q", key, got.Value, want)
+		}
+	}
+}
+
+func openTestDB(t *testing.T, dir string) *DB {
+	t.Helper()
+
+	db, err := Open(dir, nil)
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+	t.Cleanup(func() {
+		if !db.closed.Load() {
+			if err := db.Close(); err != nil {
+				t.Fatalf("Close cleanup: %v", err)
+			}
+		}
+	})
+	return db
+}
+
+func closeTestDB(t *testing.T, db *DB) {
+	t.Helper()
+	if err := db.Close(); err != nil {
+		t.Fatalf("Close: %v", err)
+	}
+}
+
+func appendWalTail(t *testing.T, dir string, tail []byte) {
+	t.Helper()
+
+	path := filepath.Join(dir, "segment-0.wal")
+	f, err := os.OpenFile(path, os.O_APPEND|os.O_WRONLY, 0)
+	if err != nil {
+		t.Fatalf("OpenFile(%s): %v", path, err)
+	}
+	defer func() {
+		if err := f.Close(); err != nil {
+			t.Fatalf("Close corrupted WAL file: %v", err)
+		}
+	}()
+	if _, err := f.Write(tail); err != nil {
+		t.Fatalf("Write corrupted WAL tail: %v", err)
+	}
+}

errkit/errors.go

@@ -0,0 +1,27 @@
+// Package errkit defines sentinel errors shared across packages in the go-kv module.
+//
+// These errors live in a leaf package to avoid import cycles: both the root
+// package and sub-packages (wal, memtable, etc.) can import errkit without
+// creating circular dependencies.
+package errkit
+
+import "errors"
+
+// ErrCommitUnknown indicates the commit result is indeterminate: the WAL write
+// was attempted but the caller cannot assume the write definitely failed or
+// succeeded. The caller must query the commit state before retrying.
+var ErrCommitUnknown = errors.New("go-kv: commit result unknown")
+
+// ErrWriteStopped indicates the engine has entered a write-stopped state and
+// rejects all subsequent writes.
+var ErrWriteStopped = errors.New("go-kv: write stopped")
+
+// ErrSequenceExhausted indicates the sequence number space is exhausted. The
+// engine enters a terminal state requiring database migration or rebuild.
+var ErrSequenceExhausted = errors.New("go-kv: sequence exhausted")
+
+// ErrWALCorrupted indicates WAL data corruption was detected.
+var ErrWALCorrupted = errors.New("go-kv: WAL corrupted")
+
+// ErrInvalidConfig indicates an invalid configuration was provided.
+var ErrInvalidConfig = errors.New("go-kv: invalid config")

errors.go

@@ -1,22 +1,11 @@
 package go_kv
 
-import "errors"
+import "github.com/dailz/go-kv/errkit"
 
-// ErrCommitUnknown indicates the commit result is indeterminate: the WAL write
+var (
-// was attempted but the caller cannot assume the write definitely failed or
+	ErrCommitUnknown     = errkit.ErrCommitUnknown
-// succeeded. The caller must query the commit state before retrying.
+	ErrWriteStopped      = errkit.ErrWriteStopped
-var ErrCommitUnknown = errors.New("go-kv: commit result unknown")
+	ErrSequenceExhausted = errkit.ErrSequenceExhausted
-
+	ErrWALCorrupted      = errkit.ErrWALCorrupted
-// ErrWriteStopped indicates the engine has entered a write-stopped state and
+	ErrInvalidConfig     = errkit.ErrInvalidConfig
-// rejects all subsequent writes.
+)
-var ErrWriteStopped = errors.New("go-kv: write stopped")
-
-// ErrSequenceExhausted indicates the sequence number space is exhausted. The
-// engine enters a terminal state requiring database migration or rebuild.
-var ErrSequenceExhausted = errors.New("go-kv: sequence exhausted")
-
-// ErrWALCorrupted indicates WAL data corruption was detected.
-var ErrWALCorrupted = errors.New("go-kv: WAL corrupted")
-
-// ErrInvalidConfig indicates an invalid configuration was provided.
-var ErrInvalidConfig = errors.New("go-kv: invalid config")

wal/sequence.go

@@ -3,7 +3,7 @@ package wal
 import (
 	"sync/atomic"
 
-	"github.com/dailz/go-kv"
+	"github.com/dailz/go-kv/errkit"
 )
 
 // SequenceManager manages monotonic sequence number allocation for the WAL.
@@ -29,16 +29,16 @@ func NewSequenceManager(recoveredSequence uint64) *SequenceManager {
 // Returns ErrSequenceExhausted if count is 0 or the allocation would overflow uint64.
 func (sm *SequenceManager) AllocateBatch(count uint32) (baseSequence uint64, err error) {
 	if count == 0 {
-		return 0, go_kv.ErrSequenceExhausted
+		return 0, errkit.ErrSequenceExhausted
 	}
 	for {
 		if sm.exhausted.Load() {
-			return 0, go_kv.ErrSequenceExhausted
+			return 0, errkit.ErrSequenceExhausted
 		}
 		base := sm.nextSequence.Load()
 		last := base + uint64(count) - 1
 		if last < base {
-			return 0, go_kv.ErrSequenceExhausted
+			return 0, errkit.ErrSequenceExhausted
 		}
 		newNext := last + 1
 		if !sm.nextSequence.CompareAndSwap(base, newNext) {

wal/sequence_test.go

@@ -7,7 +7,7 @@ import (
 	"sync/atomic"
 	"testing"
 
-	"github.com/dailz/go-kv"
+	"github.com/dailz/go-kv/errkit"
 )
 
 func TestSequenceAllocation(t *testing.T) {
@@ -49,7 +49,7 @@ func TestSequenceOverflow(t *testing.T) {
 	// Remaining: MaxUint64-2, MaxUint64-1, MaxUint64 = 3 slots.
 	// Asking for 5 should overflow.
 	_, err := sm.AllocateBatch(5)
-	if !errors.Is(err, go_kv.ErrSequenceExhausted) {
+	if !errors.Is(err, errkit.ErrSequenceExhausted) {
 		t.Fatalf("expected ErrSequenceExhausted, got %v", err)
 	}
 
@@ -64,7 +64,7 @@ func TestSequenceOverflow(t *testing.T) {
 
 	// Now any further allocation should fail.
 	_, err = sm.AllocateBatch(1)
-	if !errors.Is(err, go_kv.ErrSequenceExhausted) {
+	if !errors.Is(err, errkit.ErrSequenceExhausted) {
 		t.Fatalf("expected ErrSequenceExhausted after exhaustion, got %v", err)
 	}
 }
@@ -108,7 +108,7 @@ func TestZeroCountRejected(t *testing.T) {
 	sm := NewSequenceManager(0)
 
 	_, err := sm.AllocateBatch(0)
-	if !errors.Is(err, go_kv.ErrSequenceExhausted) {
+	if !errors.Is(err, errkit.ErrSequenceExhausted) {
 		t.Fatalf("expected ErrSequenceExhausted for count=0, got %v", err)
 	}
 }

wal/writer.go

@@ -6,7 +6,7 @@ import (
 	"sync/atomic"
 	"time"
 
-	go_kv "github.com/dailz/go-kv"
+	"github.com/dailz/go-kv/errkit"
 	"github.com/dailz/go-kv/config"
 	"github.com/dailz/go-kv/memtable"
 )
@@ -131,7 +131,7 @@ func (ww *WalWriter) processRemaining() {
 
 func (ww *WalWriter) processBatch(requests []*CommitRequest) {
 	if ww.writeStopped.Load() {
-		ww.sendError(requests, go_kv.ErrWriteStopped)
+		ww.sendError(requests, errkit.ErrWriteStopped)
 		return
 	}
 
@@ -159,20 +159,20 @@ func (ww *WalWriter) processBatch(requests []*CommitRequest) {
 	}
 
 	if err := ww.segManager.AppendBatch(encoded); err != nil {
-		ww.stopWithError(requests, go_kv.ErrCommitUnknown)
+		ww.stopWithError(requests, errkit.ErrCommitUnknown)
 		return
 	}
 
 	active := ww.memTables.GetActive()
 	if err := writePending(active, entries, baseSequence); err != nil {
 		abortRange(active, baseSequence, lastSequence)
-		ww.stopWithError(requests, go_kv.ErrCommitUnknown)
+		ww.stopWithError(requests, errkit.ErrCommitUnknown)
 		return
 	}
 
 	if err := ww.segManager.Sync(); err != nil {
 		abortRange(active, baseSequence, lastSequence)
-		ww.stopWithError(requests, go_kv.ErrCommitUnknown)
+		ww.stopWithError(requests, errkit.ErrCommitUnknown)
 		return
 	}
 
@@ -263,7 +263,7 @@ func (ww *WalWriter) sendSuccess(requests []*CommitRequest, baseSequence uint64,
 // Put stores key with value.
 func (ww *WalWriter) Put(key, value []byte) error {
 	if ww.writeStopped.Load() {
-		return go_kv.ErrWriteStopped
+		return errkit.ErrWriteStopped
 	}
 
 	req := ww.queue.Submit([]*WalEntry{{
@@ -279,7 +279,7 @@ func (ww *WalWriter) Put(key, value []byte) error {
 // Delete removes key.
 func (ww *WalWriter) Delete(key []byte) error {
 	if ww.writeStopped.Load() {
-		return go_kv.ErrWriteStopped
+		return errkit.ErrWriteStopped
 	}
 
 	req := ww.queue.Submit([]*WalEntry{{