feat(acp/handlers): fs/read_text_file + fs/write_text_file with path safety

internal/acp/handlers/server_fs_read.go

@@ -0,0 +1,108 @@
+package handlers
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// FsReadHandler handles fs/read_text_file (spec §5.4).
+type FsReadHandler struct{}
+
+// NewFsReadHandler constructs an FsReadHandler.
+func NewFsReadHandler() *FsReadHandler { return &FsReadHandler{} }
+
+// fsReadParams is the ACP-protocol input for fs/read_text_file.
+type fsReadParams struct {
+	SessionID string `json:"sessionId"`
+	Path      string `json:"path"`
+	Line      *int   `json:"line,omitempty"`
+	Limit     *int   `json:"limit,omitempty"`
+}
+
+type fsReadResult struct {
+	Content string `json:"content"`
+}
+
+// Handle reads a file inside sess.CwdPath, returning content (with optional
+// line/limit slicing). Returns FsError for path-escape or read failure.
+func (h *FsReadHandler) Handle(_ context.Context, sess SessionContext, paramsJSON json.RawMessage) FsResult {
+	var p fsReadParams
+	if err := json.Unmarshal(paramsJSON, &p); err != nil {
+		return FsResult{Err: &FsError{Code: -32602, Message: "invalid params: " + err.Error()}}
+	}
+	abs, err := resolveSafePath(sess.CwdPath, p.Path)
+	if err != nil {
+		return FsResult{Err: &FsError{Code: -32001, Message: err.Error()}}
+	}
+	data, err := os.ReadFile(abs)
+	if err != nil {
+		if errors.Is(err, fs.ErrNotExist) {
+			return FsResult{Err: &FsError{Code: -32603, Message: "file not found"}}
+		}
+		return FsResult{Err: &FsError{Code: -32603, Message: "read: " + err.Error()}}
+	}
+	content := string(data)
+	if p.Line != nil || p.Limit != nil {
+		content = sliceLines(content, p.Line, p.Limit)
+	}
+	out, _ := json.Marshal(fsReadResult{Content: content})
+	return FsResult{OK: out}
+}
+
+// sliceLines returns lines[line:line+limit] joined by "\n".
+// line is 0-based; limit caps the line count.
+func sliceLines(s string, line, limit *int) string {
+	lines := strings.Split(s, "\n")
+	start := 0
+	if line != nil && *line > 0 {
+		start = *line
+	}
+	if start > len(lines) {
+		start = len(lines)
+	}
+	end := len(lines)
+	if limit != nil && *limit >= 0 {
+		if start+*limit < end {
+			end = start + *limit
+		}
+	}
+	return strings.Join(lines[start:end], "\n")
+}
+
+// resolveSafePath duplicates the logic from internal/acp.ResolveSafePath to
+// avoid an import cycle (handlers is a sub-package and acp imports handlers).
+func resolveSafePath(cwd, requested string) (string, error) {
+	if !filepath.IsAbs(cwd) {
+		return "", fmt.Errorf("cwd must be absolute")
+	}
+	cleanCwd := filepath.Clean(cwd)
+	var abs string
+	if filepath.IsAbs(requested) {
+		abs = filepath.Clean(requested)
+	} else {
+		abs = filepath.Clean(filepath.Join(cleanCwd, requested))
+	}
+	if !hasPathPrefix(abs, cleanCwd) {
+		return "", fmt.Errorf("path outside worktree")
+	}
+	if real, err := filepath.EvalSymlinks(abs); err == nil {
+		if !hasPathPrefix(real, cleanCwd) {
+			return "", fmt.Errorf("path resolves outside worktree via symlink")
+		}
+	}
+	return abs, nil
+}
+
+func hasPathPrefix(abs, base string) bool {
+	if abs == base {
+		return true
+	}
+	sep := string(filepath.Separator)
+	return strings.HasPrefix(abs, base+sep)
+}