chore: include complete dc refactor implementation and audits

internal/executor/edit.go

@@ -0,0 +1,130 @@
+package executor
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sync"
+)
+
+var ErrBlockNotUnique = errors.New("old_text must occur exactly once")
+var ErrEditConflict = errors.New("file changed during edit")
+
+var editMu sync.Mutex
+
+type EditBlockResult struct {
+	BytesWritten int64
+}
+
+func EditBlock(path, oldText, newText string, expectedHash ...string) (EditBlockResult, error) {
+	editMu.Lock()
+	defer editMu.Unlock()
+
+	if path == "" {
+		return EditBlockResult{}, errors.New("path required")
+	}
+	if oldText == "" {
+		return EditBlockResult{}, errors.New("old_text required")
+	}
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return EditBlockResult{}, err
+	}
+	initial := sha256.Sum256(data)
+	initialHash := hex.EncodeToString(initial[:])
+	if len(expectedHash) > 0 && expectedHash[0] != "" && expectedHash[0] != initialHash {
+		return EditBlockResult{}, ErrEditConflict
+	}
+
+	src := string(data)
+	count := countNonOverlapping(src, oldText)
+	if count != 1 {
+		if count == 0 {
+			return EditBlockResult{}, fmt.Errorf("%w: found 0 matches", ErrBlockNotUnique)
+		}
+		return EditBlockResult{}, fmt.Errorf("%w: found %d matches", ErrBlockNotUnique, count)
+	}
+	updated := replaceOnce(src, oldText, newText)
+
+	info, err := os.Stat(path)
+	if err != nil {
+		return EditBlockResult{}, err
+	}
+
+	// Re-read immediately before commit. This is the CAS check against
+	// writers outside this process; editMu serializes our own writers.
+	latest, err := os.ReadFile(path)
+	if err != nil {
+		return EditBlockResult{}, err
+	}
+	latestSum := sha256.Sum256(latest)
+	if hex.EncodeToString(latestSum[:]) != initialHash {
+		return EditBlockResult{}, ErrEditConflict
+	}
+
+	dir := filepath.Dir(path)
+	tmp, err := os.CreateTemp(dir, ".vps-commander-edit-*")
+	if err != nil {
+		return EditBlockResult{}, err
+	}
+	tmpName := tmp.Name()
+	defer os.Remove(tmpName)
+	if err := tmp.Chmod(info.Mode().Perm()); err != nil {
+		_ = tmp.Close()
+		return EditBlockResult{}, err
+	}
+	if _, err := tmp.WriteString(updated); err != nil {
+		_ = tmp.Close()
+		return EditBlockResult{}, err
+	}
+	if err := tmp.Sync(); err != nil {
+		_ = tmp.Close()
+		return EditBlockResult{}, err
+	}
+	if err := tmp.Close(); err != nil {
+		return EditBlockResult{}, err
+	}
+	if err := os.Rename(tmpName, path); err != nil {
+		return EditBlockResult{}, err
+	}
+	if d, err := os.Open(dir); err == nil {
+		_ = d.Sync()
+		_ = d.Close()
+	}
+	return EditBlockResult{BytesWritten: int64(len(updated))}, nil
+}
+
+func countNonOverlapping(src, old string) int {
+	n, count := 0, 0
+	for {
+		i := indexString(src[n:], old)
+		if i < 0 {
+			return count
+		}
+		count++
+		n += i + len(old)
+		if n > len(src) {
+			return count
+		}
+	}
+}
+
+func replaceOnce(src, old, new string) string {
+	i := indexString(src, old)
+	return src[:i] + new + src[i+len(old):]
+}
+
+func indexString(src, needle string) int {
+	if len(needle) == 0 {
+		return 0
+	}
+	for i := 0; i+len(needle) <= len(src); i++ {
+		if src[i:i+len(needle)] == needle {
+			return i
+		}
+	}
+	return -1
+}