[#1298] writecache: Add shrink flag for Seal command

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

pkg/local_object_storage/writecache/mode.go

@@ -2,16 +2,25 @@ package writecache
 
 import (
 	"context"
+	"errors"
 	"fmt"
+	"os"
+	"path/filepath"
 	"time"
 
 	"git.frostfs.info/TrueCloudLab/frostfs-node/internal/logs"
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/shard/mode"
 	"git.frostfs.info/TrueCloudLab/frostfs-observability/tracing"
+	"go.etcd.io/bbolt"
 	"go.opentelemetry.io/otel/attribute"
 	"go.opentelemetry.io/otel/trace"
 )
 
+type setModePrm struct {
+	ignoreErrors bool
+	shrink       bool
+}
+
 // SetMode sets write-cache mode of operation.
 // When shard is put in read-only mode all objects in memory are flushed to disk
 // and all background jobs are suspended.
@@ -25,7 +34,7 @@ func (c *cache) SetMode(m mode.Mode) error {
 	c.modeMtx.Lock()
 	defer c.modeMtx.Unlock()
 
-	err := c.setMode(ctx, m, true)
+	err := c.setMode(ctx, m, setModePrm{ignoreErrors: true})
 	if err == nil {
 		c.metrics.SetMode(mode.ConvertToComponentModeDegraded(m))
 	}
@@ -33,21 +42,19 @@ func (c *cache) SetMode(m mode.Mode) error {
 }
 
 // setMode applies new mode. Must be called with cache.modeMtx lock taken.
-func (c *cache) setMode(ctx context.Context, m mode.Mode, ignoreErrors bool) error {
+func (c *cache) setMode(ctx context.Context, m mode.Mode, prm setModePrm) error {
 	var err error
 	turnOffMeta := m.NoMetabase()
 
 	if turnOffMeta && !c.mode.NoMetabase() {
-		err = c.flush(ctx, ignoreErrors)
+		err = c.flush(ctx, prm.ignoreErrors)
 		if err != nil {
 			return err
 		}
 	}
 
-	if c.db != nil {
-		if err = c.db.Close(); err != nil {
-			return fmt.Errorf("can't close write-cache database: %w", err)
-		}
+	if err := c.closeDB(prm.shrink); err != nil {
+		return err
 	}
 
 	// Suspend producers to ensure there are channel send operations in fly.
@@ -71,6 +78,40 @@ func (c *cache) setMode(ctx context.Context, m mode.Mode, ignoreErrors bool) err
 	return nil
 }
 
+func (c *cache) closeDB(shrink bool) error {
+	if c.db == nil {
+		return nil
+	}
+	if !shrink {
+		if err := c.db.Close(); err != nil {
+			return fmt.Errorf("can't close write-cache database: %w", err)
+		}
+		return nil
+	}
+
+	var empty bool
+	err := c.db.View(func(tx *bbolt.Tx) error {
+		b := tx.Bucket(defaultBucket)
+		empty = b == nil || b.Stats().KeyN == 0
+		return nil
+	})
+	if err != nil && !errors.Is(err, bbolt.ErrDatabaseNotOpen) {
+		return fmt.Errorf("failed to check DB items: %w", err)
+	}
+	if err := c.db.Close(); err != nil {
+		return fmt.Errorf("can't close write-cache database: %w", err)
+	}
+	if empty {
+		err := os.Remove(filepath.Join(c.path, dbName))
+		if err != nil && !os.IsNotExist(err) {
+			return fmt.Errorf("failed to remove DB file: %w", err)
+		}
+	} else {
+		c.log.Info(logs.WritecacheShrinkSkippedNotEmpty)
+	}
+	return nil
+}
+
 // readOnly returns true if current mode is read-only.
 // `c.modeMtx` must be taken.
 func (c *cache) readOnly() bool {