fix: overwrite read-only files when copying action directories (#942)

## Summary

  - `CopyCollector.WriteFile` now removes any existing destination file
    before writing, handling read-only modes (e.g. git pack files at
    `0444`) that cause `EACCES`/`ERROR_ACCESS_DENIED` on macOS and Windows.
  - Added `O_TRUNC` to the `OpenFile` flags as a safety net.

  ## Root cause

  When a composite action with a post step runs on a host runner,
  `runPostStep` calls `maybeCopyToActionDir`, which re-copies the action
  into `miscpath/act/actions/<name>/`. The first copy (main step) writes
  `.git/objects/pack/*.idx` at the destination with mode `0444` (as set
  by go-git). The second copy (post step) calls
  `os.OpenFile(dest, O_CREATE|O_WRONLY, …)` on that existing `0444` file,
  which fails immediately:

  - macOS: `open <path>: permission denied`
  - Windows: `open <path>: Access is denied`

Fixes: https://gitea.com/gitea/runner/issues/941
Fixes: https://gitea.com/gitea/runner/issues/876

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: silverwind <2021+silverwind@noreply.gitea.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/942
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>
Co-authored-by: Nicolas <bircni@icloud.com>
Co-committed-by: Nicolas <bircni@icloud.com>

act/filecollector/file_collector.go

@@ -73,10 +73,16 @@ func (cc *CopyCollector) WriteFile(fpath string, fi fs.FileInfo, linkName string
 	if err := os.MkdirAll(filepath.Dir(fdestpath), 0o777); err != nil {
 		return err
 	}
+	// Remove any existing destination so we can overwrite read-only files
+	// (e.g. git pack files at mode 0444 trip EACCES on macOS and "Access is
+	// denied" on Windows when reopened with O_WRONLY) and so os.Symlink does
+	// not fail with EEXIST. os.Remove clears the Windows read-only attribute
+	// internally; on Unix unlink only needs write permission on the parent.
+	_ = os.Remove(fdestpath)
 	if f == nil {
 		return os.Symlink(linkName, fdestpath)
 	}
-	df, err := os.OpenFile(fdestpath, os.O_CREATE|os.O_WRONLY, fi.Mode())
+	df, err := os.OpenFile(fdestpath, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, fi.Mode())
 	if err != nil {
 		return err
 	}

act/filecollector/file_collector_test.go

@@ -8,7 +8,9 @@ import (
 	"archive/tar"
 	"context"
 	"io"
+	"os"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"testing"
 
@@ -20,6 +22,7 @@ import (
 	"github.com/go-git/go-git/v5/plumbing/format/index"
 	"github.com/go-git/go-git/v5/storage/filesystem"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 type memoryFs struct {
@@ -174,3 +177,47 @@ func TestSymlinks(t *testing.T) {
 	assert.Equal(t, ".env", files["test.env"].Linkname)
 	assert.ErrorIs(t, err, io.EOF, "tar must be read cleanly to EOF")
 }
+
+// Regression for https://gitea.com/gitea/runner/issues/876 and /941:
+// re-copying an action directory must overwrite a pre-existing read-only
+// file (e.g. a git pack .idx at mode 0444) instead of failing with EACCES
+// on macOS or "Access is denied" on Windows.
+func TestCopyCollectorWriteFileOverwritesReadOnlyFile(t *testing.T) {
+	dst := t.TempDir()
+	target := filepath.Join(dst, "sub", "pack.idx")
+	require.NoError(t, os.MkdirAll(filepath.Dir(target), 0o755))
+	require.NoError(t, os.WriteFile(target, []byte("old"), 0o444))
+
+	src := filepath.Join(t.TempDir(), "pack.idx")
+	require.NoError(t, os.WriteFile(src, []byte("new"), 0o444))
+	fi, err := os.Stat(src)
+	require.NoError(t, err)
+
+	cc := &CopyCollector{DstDir: dst}
+	require.NoError(t, cc.WriteFile("sub/pack.idx", fi, "", strings.NewReader("new")))
+
+	got, err := os.ReadFile(target)
+	require.NoError(t, err)
+	assert.Equal(t, "new", string(got))
+}
+
+// Without the destination removal, os.Symlink fails with EEXIST when the
+// path already holds a regular file from an earlier copy of the action.
+func TestCopyCollectorWriteFileOverwritesFileWithSymlink(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("creating symlinks requires elevated privileges on Windows")
+	}
+	dst := t.TempDir()
+	target := filepath.Join(dst, "link")
+	require.NoError(t, os.WriteFile(target, []byte("stale"), 0o644))
+
+	fi, err := os.Lstat(target)
+	require.NoError(t, err)
+
+	cc := &CopyCollector{DstDir: dst}
+	require.NoError(t, cc.WriteFile("link", fi, "target", nil))
+
+	resolved, err := os.Readlink(target)
+	require.NoError(t, err)
+	assert.Equal(t, "target", resolved)
+}