fix(agent/agentfiles): follow symlinks in write_file and edit_files (#23478)

Both write_file and edit_files use atomic writes (write to temp
file, then rename). Since rename operates on directory entries, it
replaces symlinks with regular files instead of writing through
the link to the target.

Add resolveSymlink() that uses afero.Lstater/LinkReader to resolve
symlink chains (up to 10 levels) before the atomic write. Both
writeFile and editFile resolve the path before any filesystem
operations, matching the behavior of 'echo content > symlink'.

Gracefully no-ops on filesystems that don't support symlinks (e.g.
MemMapFs used in existing tests).

agent/agentfiles/files.go

@@ -14,6 +14,7 @@ import (
 	"syscall"
 
 	"github.com/google/uuid"
+	"github.com/spf13/afero"
 	"golang.org/x/xerrors"
 
 	"cdr.dev/slog/v3"
@@ -319,8 +320,14 @@ func (api *API) writeFile(ctx context.Context, r *http.Request, path string) (HT
 		return http.StatusBadRequest, xerrors.Errorf("file path must be absolute: %q", path)
 	}
 
+	resolved, err := api.resolveSymlink(path)
+	if err != nil {
+		return http.StatusInternalServerError, xerrors.Errorf("resolve symlink %q: %w", path, err)
+	}
+	path = resolved
+
 	dir := filepath.Dir(path)
-	err := api.filesystem.MkdirAll(dir, 0o755)
+	err = api.filesystem.MkdirAll(dir, 0o755)
 	if err != nil {
 		status := http.StatusInternalServerError
 		switch {
@@ -410,6 +417,12 @@ func (api *API) editFile(ctx context.Context, path string, edits []workspacesdk.
 		return http.StatusBadRequest, xerrors.New("must specify at least one edit")
 	}
 
+	resolved, err := api.resolveSymlink(path)
+	if err != nil {
+		return http.StatusInternalServerError, xerrors.Errorf("resolve symlink %q: %w", path, err)
+	}
+	path = resolved
+
 	f, err := api.filesystem.Open(path)
 	if err != nil {
 		status := http.StatusInternalServerError
@@ -510,6 +523,52 @@ func (api *API) atomicWrite(ctx context.Context, path string, mode *os.FileMode,
 	return 0, nil
 }
 
+// resolveSymlink resolves a path through any symlinks so that
+// subsequent operations (such as atomic rename) target the real
+// file instead of replacing the symlink itself.
+//
+// The filesystem must implement afero.Lstater and afero.LinkReader
+// for resolution to occur; if it does not (e.g. MemMapFs), the
+// path is returned unchanged.
+func (api *API) resolveSymlink(path string) (string, error) {
+	const maxDepth = 10
+
+	lstater, hasLstat := api.filesystem.(afero.Lstater)
+	if !hasLstat {
+		return path, nil
+	}
+	reader, hasReadlink := api.filesystem.(afero.LinkReader)
+	if !hasReadlink {
+		return path, nil
+	}
+
+	for range maxDepth {
+		info, _, err := lstater.LstatIfPossible(path)
+		if err != nil {
+			// If the file does not exist yet (new file write),
+			// there is nothing to resolve.
+			if errors.Is(err, os.ErrNotExist) {
+				return path, nil
+			}
+			return "", err
+		}
+		if info.Mode()&os.ModeSymlink == 0 {
+			return path, nil
+		}
+
+		target, err := reader.ReadlinkIfPossible(path)
+		if err != nil {
+			return "", err
+		}
+		if !filepath.IsAbs(target) {
+			target = filepath.Join(filepath.Dir(path), target)
+		}
+		path = target
+	}
+
+	return "", xerrors.Errorf("too many levels of symlinks resolving %q", path)
+}
+
 // fuzzyReplace attempts to find `search` inside `content` and replace it
 // with `replace`. It uses a cascading match strategy inspired by
 // openai/codex's apply_patch:

agent/agentfiles/files_test.go

@@ -1395,3 +1395,105 @@ func TestReadFileLines(t *testing.T) {
 		})
 	}
 }
+
+func TestWriteFile_FollowsSymlinks(t *testing.T) {
+	t.Parallel()
+
+	if runtime.GOOS == "windows" {
+		t.Skip("symlinks are not reliably supported on Windows")
+	}
+
+	dir := t.TempDir()
+	logger := slogtest.Make(t, nil).Leveled(slog.LevelDebug)
+	osFs := afero.NewOsFs()
+	api := agentfiles.NewAPI(logger, osFs, nil)
+
+	// Create a real file and a symlink pointing to it.
+	realPath := filepath.Join(dir, "real.txt")
+	err := afero.WriteFile(osFs, realPath, []byte("original"), 0o644)
+	require.NoError(t, err)
+
+	linkPath := filepath.Join(dir, "link.txt")
+	err = os.Symlink(realPath, linkPath)
+	require.NoError(t, err)
+
+	ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitShort)
+	defer cancel()
+
+	// Write through the symlink.
+	w := httptest.NewRecorder()
+	r := httptest.NewRequestWithContext(ctx, http.MethodPost,
+		fmt.Sprintf("/write-file?path=%s", linkPath),
+		bytes.NewReader([]byte("updated")))
+	api.Routes().ServeHTTP(w, r)
+	require.Equal(t, http.StatusOK, w.Code)
+
+	// The symlink must still be a symlink.
+	fi, err := os.Lstat(linkPath)
+	require.NoError(t, err)
+	require.NotZero(t, fi.Mode()&os.ModeSymlink, "symlink was replaced")
+
+	// The real file must have the new content.
+	data, err := os.ReadFile(realPath)
+	require.NoError(t, err)
+	require.Equal(t, "updated", string(data))
+}
+
+func TestEditFiles_FollowsSymlinks(t *testing.T) {
+	t.Parallel()
+
+	if runtime.GOOS == "windows" {
+		t.Skip("symlinks are not reliably supported on Windows")
+	}
+
+	dir := t.TempDir()
+	logger := slogtest.Make(t, nil).Leveled(slog.LevelDebug)
+	osFs := afero.NewOsFs()
+	api := agentfiles.NewAPI(logger, osFs, nil)
+
+	// Create a real file and a symlink pointing to it.
+	realPath := filepath.Join(dir, "real.txt")
+	err := afero.WriteFile(osFs, realPath, []byte("hello world"), 0o644)
+	require.NoError(t, err)
+
+	linkPath := filepath.Join(dir, "link.txt")
+	err = os.Symlink(realPath, linkPath)
+	require.NoError(t, err)
+
+	ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitShort)
+	defer cancel()
+
+	body := workspacesdk.FileEditRequest{
+		Files: []workspacesdk.FileEdits{
+			{
+				Path: linkPath,
+				Edits: []workspacesdk.FileEdit{
+					{
+						Search:  "hello",
+						Replace: "goodbye",
+					},
+				},
+			},
+		},
+	}
+	buf := bytes.NewBuffer(nil)
+	enc := json.NewEncoder(buf)
+	enc.SetEscapeHTML(false)
+	err = enc.Encode(body)
+	require.NoError(t, err)
+
+	w := httptest.NewRecorder()
+	r := httptest.NewRequestWithContext(ctx, http.MethodPost, "/edit-files", buf)
+	api.Routes().ServeHTTP(w, r)
+	require.Equal(t, http.StatusOK, w.Code)
+
+	// The symlink must still be a symlink.
+	fi, err := os.Lstat(linkPath)
+	require.NoError(t, err)
+	require.NotZero(t, fi.Mode()&os.ModeSymlink, "symlink was replaced")
+
+	// The real file must have the edited content.
+	data, err := os.ReadFile(realPath)
+	require.NoError(t, err)
+	require.Equal(t, "goodbye world", string(data))
+}