internal: add function ClearElements in surgeon package

Signed-off-by: Benjamin Wang <wachao@vmware.com>

internal/guts_cli/guts_cli.go

@@ -50,6 +50,10 @@ func ReadPage(path string, pageID uint64) (*common.Page, []byte, error) {
 		return nil, nil, fmt.Errorf("error: %w, Page claims to have %d overflow pages (>=hwm=%d). Interrupting to avoid risky OOM", ErrCorrupt, overflowN, hwm)
 	}
 
+	if overflowN == 0 {
+		return p, buf, nil
+	}
+
 	// Re-read entire Page (with overflow) into buffer.
 	buf = make([]byte, (uint64(overflowN)+1)*pageSize)
 	if n, err := f.ReadAt(buf, int64(pageID*pageSize)); err != nil {

internal/surgeon/surgeon.go

@@ -2,6 +2,7 @@ package surgeon
 
 import (
 	"fmt"
+
 	"go.etcd.io/bbolt/internal/common"
 	"go.etcd.io/bbolt/internal/guts_cli"
 )
@@ -16,19 +17,105 @@ func CopyPage(path string, srcPage common.Pgid, target common.Pgid) error {
 }
 
 func ClearPage(path string, pgId common.Pgid) error {
+	return ClearPageElements(path, pgId, 0, -1)
+}
+
+func ClearPageElements(path string, pgId common.Pgid, start, end int) error {
 	// Read the page
 	p, buf, err := guts_cli.ReadPage(path, uint64(pgId))
 	if err != nil {
 		return fmt.Errorf("ReadPage failed: %w", err)
 	}
 
-	// Update and rewrite the page
-	p.SetCount(0)
-	p.SetOverflow(0)
+	if !p.IsLeafPage() && !p.IsBranchPage() {
+		return fmt.Errorf("can't clear elements in %q page", p.Typ())
+	}
+
+	elementCnt := int(p.Count())
+
+	if elementCnt == 0 {
+		return nil
+	}
+
+	if start < 0 || start >= elementCnt {
+		return fmt.Errorf("the start index (%d) is out of range [0, %d)", start, elementCnt)
+	}
+
+	if (end < 0 || end > elementCnt) && end != -1 {
+		return fmt.Errorf("the end index (%d) is out of range [0, %d]", end, elementCnt)
+	}
+
+	if start > end && end != -1 {
+		return fmt.Errorf("the start index (%d) is bigger than the end index (%d)", start, end)
+	}
+
+	if start == end {
+		return fmt.Errorf("invalid: the start index (%d) is equal to the end index (%d)", start, end)
+	}
+
+	preOverflow := p.Overflow()
+
+	if end == int(p.Count()) || end == -1 {
+		p.SetCount(uint16(start))
+		p.SetOverflow(0)
+		if preOverflow != 0 || p.IsBranchPage() {
+			if err := clearFreelist(path); err != nil {
+				return err
+			}
+		}
+	} else {
+		inodes := common.ReadInodeFromPage(p)
+		inodes = append(inodes[:start], inodes[end:]...)
+
+		p.SetCount(uint16(len(inodes)))
+		dataWritten := common.WriteInodeToPage(inodes, p)
+
+		pageSize, _, err := guts_cli.ReadPageAndHWMSize(path)
+		if err != nil {
+			return fmt.Errorf("ReadPageAndHWMSize failed: %w", err)
+		}
+		if dataWritten%uint32(pageSize) == 0 {
+			p.SetOverflow(dataWritten/uint32(pageSize) - 1)
+		} else {
+			p.SetOverflow(dataWritten / uint32(pageSize))
+		}
+	}
+
 	if err := guts_cli.WritePage(path, buf); err != nil {
 		return fmt.Errorf("WritePage failed: %w", err)
 	}
 
+	if preOverflow != p.Overflow() || p.IsBranchPage() {
+		return clearFreelist(path)
+	}
+
+	return nil
+}
+
+func clearFreelist(path string) error {
+	if err := clearFreelistAt(path, 0); err != nil {
+		return fmt.Errorf("clearFreelist on meta page 0 failed: %w", err)
+	}
+	if err := clearFreelistAt(path, 1); err != nil {
+		return fmt.Errorf("clearFreelist on meta page 1 failed: %w", err)
+	}
+	return nil
+}
+
+func clearFreelistAt(path string, pageId uint64) error {
+	_, buf, err := guts_cli.ReadPage(path, pageId)
+	if err != nil {
+		return fmt.Errorf("ReadPage %d failed: %w", pageId, err)
+	}
+
+	meta := common.LoadPageMeta(buf)
+	meta.SetFreelist(common.PgidNoFreelist)
+	meta.SetChecksum(meta.Sum64())
+
+	if err := guts_cli.WritePage(path, buf); err != nil {
+		return fmt.Errorf("WritePage %d failed: %w", pageId, err)
+	}
+
 	return nil
 }