fix: serialize action-cache reads to prevent worktree race (#938)

`NewGitCloneExecutor` holds a per-directory mutex while it `git checkout --force`s a remote action into the shared `<ActionCacheDir>/<UsesHash>`, but four read sites ran unlocked:

- `maybeCopyToActionDir`'s tar walk via `JobContainer.CopyDir`
- `prepareActionExecutor`'s `readAction` parse of `action.yml`
- `newReusableWorkflowExecutor`'s `model.NewWorkflowPlanner` after `cloneRemoteReusableWorkflow` released its lock
- `execAsDocker` when `ActionCache == nil`: `docker build` walks `contextDir` for the daemon-side build context

When two matrix jobs share a `uses:`, a read interleaved with a peer's checkout produces partial state — observed as `Cannot find module .../dist/index.js` and `setup-uv` failing on a half-written `action.yml`.

Exports `acquireCloneLock` as `AcquireCloneLock` and takes it at all four sites. `container.ImageExistsLocally` / `NewDockerBuildExecutor` and `model.NewWorkflowPlanner` are indirected through package-level vars so the docker-action build path and the reusable-workflow read site are testable without a real daemon, mirroring `ContainerNewContainer`. Three regression tests cover the higher-risk sites (`maybeCopyToActionDir`, `execAsDocker`, `newReusableWorkflowExecutor`); each fails if its `AcquireCloneLock` is removed.

Subsumed by https://gitea.com/gitea/runner/pulls/814 once that lands. Related: https://gitea.com/gitea/runner/pulls/930

---
This PR was written with the help of Claude Opus 4.7

---------

Co-authored-by: Nicolas <bircni@icloud.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/938
Reviewed-by: Nicolas <bircni@icloud.com>
Reviewed-by: Zettat123 <39446+zettat123@noreply.gitea.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

act/runner/action_test.go

@@ -9,8 +9,13 @@ import (
 	"io"
 	"io/fs"
 	"strings"
+	"sync"
 	"testing"
+	"time"
 
+	"gitea.com/gitea/runner/act/common"
+	"gitea.com/gitea/runner/act/common/git"
+	"gitea.com/gitea/runner/act/container"
 	"gitea.com/gitea/runner/act/model"
 
 	"github.com/stretchr/testify/assert"
@@ -252,3 +257,153 @@ func TestActionRunner(t *testing.T) {
 		})
 	}
 }
+
+func TestMaybeCopyToActionDirHoldsCloneLock(t *testing.T) {
+	ctx := context.Background()
+
+	actionDir := t.TempDir()
+
+	releaseCopy := make(chan struct{})
+	release := sync.OnceFunc(func() { close(releaseCopy) })
+	defer release()
+
+	copyEntered := make(chan struct{})
+
+	cm := &containerMock{}
+	cm.On("CopyDir", "/var/run/act/actions/", actionDir+"/", false).Return(func(ctx context.Context) error {
+		close(copyEntered)
+		<-releaseCopy
+		return nil
+	})
+
+	step := &stepActionRemote{
+		Step: &model.Step{Uses: "remote/action@v1"},
+		RunContext: &RunContext{
+			Config:       &Config{},
+			JobContainer: cm,
+		},
+	}
+
+	copyDone := make(chan error, 1)
+	go func() {
+		copyDone <- maybeCopyToActionDir(ctx, step, actionDir, "", "/var/run/act/actions/")
+	}()
+
+	select {
+	case <-copyEntered:
+	case err := <-copyDone:
+		t.Fatalf("maybeCopyToActionDir returned before CopyDir was entered: %v", err)
+	case <-time.After(time.Second):
+		t.Fatal("CopyDir was not entered within 1 second")
+	}
+
+	peerAcquired := make(chan struct{})
+	go func() {
+		unlock := git.AcquireCloneLock(actionDir)
+		close(peerAcquired)
+		unlock()
+	}()
+
+	select {
+	case <-peerAcquired:
+		t.Fatal("peer AcquireCloneLock returned while CopyDir was running")
+	case <-time.After(50 * time.Millisecond):
+	}
+
+	release()
+
+	select {
+	case err := <-copyDone:
+		if err != nil {
+			t.Fatalf("maybeCopyToActionDir returned error: %v", err)
+		}
+	case <-time.After(time.Second):
+		t.Fatal("maybeCopyToActionDir did not return after CopyDir was unblocked")
+	}
+
+	select {
+	case <-peerAcquired:
+	case <-time.After(time.Second):
+		t.Fatal("peer AcquireCloneLock did not proceed after lock released")
+	}
+
+	cm.AssertExpectations(t)
+}
+
+func TestExecAsDockerHoldsCloneLockForRemoteUncached(t *testing.T) {
+	actionDir := t.TempDir()
+
+	unlockOnce := sync.OnceFunc(git.AcquireCloneLock(actionDir))
+	defer unlockOnce()
+
+	innerEntered := make(chan struct{})
+	releaseInner := make(chan struct{})
+	releaseOnce := sync.OnceFunc(func() { close(releaseInner) })
+	defer releaseOnce()
+
+	origImageExists := ContainerImageExistsLocally
+	ContainerImageExistsLocally = func(_ context.Context, _, _ string) (bool, error) {
+		return false, nil
+	}
+	defer func() { ContainerImageExistsLocally = origImageExists }()
+
+	origBuildExec := ContainerNewDockerBuildExecutor
+	ContainerNewDockerBuildExecutor = func(_ container.NewDockerBuildExecutorInput) common.Executor {
+		return func(_ context.Context) error {
+			close(innerEntered)
+			<-releaseInner
+			return nil
+		}
+	}
+	defer func() { ContainerNewDockerBuildExecutor = origBuildExec }()
+
+	step := &stepActionRemote{
+		Step: &model.Step{ID: "1", Uses: "remote/action@v1", With: map[string]string{}},
+		RunContext: &RunContext{
+			Config: &Config{},
+			Run: &model.Run{
+				JobID: "1",
+				Workflow: &model.Workflow{
+					Name: "wf",
+					Jobs: map[string]*model.Job{"1": {}},
+				},
+			},
+			JobContainer: &containerMock{},
+		},
+		action: &model.Action{Runs: model.ActionRuns{Using: "docker", Image: "Dockerfile"}},
+		env:    map[string]string{},
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	done := make(chan error, 1)
+	go func() { done <- execAsDocker(ctx, step, "test-action", actionDir, actionDir, false) }()
+
+	select {
+	case <-innerEntered:
+		t.Fatal("inner build executor ran before clone lock was released")
+	case err := <-done:
+		t.Fatalf("execAsDocker returned before inner was entered: %v", err)
+	case <-time.After(50 * time.Millisecond):
+	}
+
+	unlockOnce()
+
+	select {
+	case <-innerEntered:
+	case err := <-done:
+		t.Fatalf("execAsDocker returned without entering inner: %v", err)
+	case <-time.After(time.Second):
+		t.Fatal("inner build executor not entered after lock released")
+	}
+
+	cancel()
+	releaseOnce()
+
+	select {
+	case <-done:
+	case <-time.After(time.Second):
+		t.Fatal("execAsDocker did not return after inner was released and ctx was canceled")
+	}
+}