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/common/git/git.go

@@ -38,9 +38,11 @@ var (
 	ErrNoRepo   = errors.New("unable to find git repo")
 )
 
-// acquireCloneLock returns an unlock function after locking the per-directory mutex for dir.
+// AcquireCloneLock returns an unlock function after locking the per-directory mutex for dir.
-// Only concurrent operations targeting the same directory are erialized; clones into different directories run in parallel.
+// Only concurrent operations targeting the same directory are serialized; clones into different directories run in parallel.
-func acquireCloneLock(dir string) func() {
+// Callers reading files inside dir (e.g. tarring a checked-out action into a job container) must hold this lock too,
+// otherwise a concurrent NewGitCloneExecutor on the same dir can mutate the worktree mid-read.
+func AcquireCloneLock(dir string) func() {
 	v, _ := cloneLocks.LoadOrStore(dir, &sync.Mutex{})
 	mu := v.(*sync.Mutex)
 	mu.Lock()
@@ -308,7 +310,7 @@ func NewGitCloneExecutor(input NewGitCloneExecutorInput) common.Executor {
 		logger.Infof("git clone '%s' # ref=%s", input.URL, input.Ref)
 		logger.Debugf("  cloning %s to %s", input.URL, input.Dir)
 
-		defer acquireCloneLock(input.Dir)()
+		defer AcquireCloneLock(input.Dir)()
 
 		refName := plumbing.ReferenceName("refs/heads/" + input.Ref)
 		r, err := CloneIfRequired(ctx, refName, input, logger)

act/common/git/git_test.go

@@ -310,11 +310,11 @@ func TestAcquireCloneLock(t *testing.T) {
 	t.Run("same directory serializes", func(t *testing.T) {
 		dir := t.TempDir()
 
-		unlock1 := acquireCloneLock(dir)
+		unlock1 := AcquireCloneLock(dir)
 
 		secondAcquired := make(chan struct{})
 		go func() {
-			unlock := acquireCloneLock(dir)
+			unlock := AcquireCloneLock(dir)
 			close(secondAcquired)
 			unlock()
 		}()
@@ -338,12 +338,12 @@ func TestAcquireCloneLock(t *testing.T) {
 		dirA := t.TempDir()
 		dirB := t.TempDir()
 
-		unlockA := acquireCloneLock(dirA)
+		unlockA := AcquireCloneLock(dirA)
 		defer unlockA()
 
 		done := make(chan struct{})
 		go func() {
-			unlock := acquireCloneLock(dirB)
+			unlock := AcquireCloneLock(dirB)
 			unlock()
 			close(done)
 		}()

act/runner/action.go

@@ -19,6 +19,7 @@ import (
 	"strings"
 
 	"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"
 
@@ -44,6 +45,11 @@ type runAction func(step actionStep, actionDir string, remoteAction *remoteActio
 //go:embed res/trampoline.js
 var trampoline embed.FS
 
+var (
+	ContainerImageExistsLocally     = container.ImageExistsLocally
+	ContainerNewDockerBuildExecutor = container.NewDockerBuildExecutor
+)
+
 func readActionImpl(ctx context.Context, step *model.Step, actionDir, actionPath string, readFile actionYamlReader, writeFile fileWriter) (*model.Action, error) {
 	logger := common.Logger(ctx)
 	allErrors := []error{}
@@ -148,6 +154,8 @@ func maybeCopyToActionDir(ctx context.Context, step actionStep, actionDir, actio
 		return rc.JobContainer.CopyTarStream(ctx, containerActionDirCopy, ta)
 	}
 
+	defer git.AcquireCloneLock(actionDir)()
+
 	if err := removeGitIgnore(ctx, actionDir); err != nil {
 		return err
 	}
@@ -197,7 +205,7 @@ func runActionImpl(step actionStep, actionDir string, remoteAction *remoteAction
 			if remoteAction == nil {
 				location = containerActionDir
 			}
-			return execAsDocker(ctx, step, actionName, location, remoteAction == nil)
+			return execAsDocker(ctx, step, actionName, actionDir, location, remoteAction == nil)
 		case x.IsComposite():
 			if err := maybeCopyToActionDir(ctx, step, actionDir, actionPath, containerActionDir); err != nil {
 				return err
@@ -265,7 +273,7 @@ func removeGitIgnore(ctx context.Context, directory string) error {
 }
 
 // TODO: break out parts of function to reduce complexicity
-func execAsDocker(ctx context.Context, step actionStep, actionName, basedir string, localAction bool) error {
+func execAsDocker(ctx context.Context, step actionStep, actionName, actionDir, basedir string, localAction bool) error {
 	logger := common.Logger(ctx)
 	rc := step.getRunContext()
 	action := step.getActionModel()
@@ -284,12 +292,12 @@ func execAsDocker(ctx context.Context, step actionStep, actionName, basedir stri
 		image = strings.ToLower(image)
 		contextDir, fileName := filepath.Split(filepath.Join(basedir, action.Runs.Image))
 
-		anyArchExists, err := container.ImageExistsLocally(ctx, image, "any")
+		anyArchExists, err := ContainerImageExistsLocally(ctx, image, "any")
 		if err != nil {
 			return err
 		}
 
-		correctArchExists, err := container.ImageExistsLocally(ctx, image, rc.Config.ContainerArchitecture)
+		correctArchExists, err := ContainerImageExistsLocally(ctx, image, rc.Config.ContainerArchitecture)
 		if err != nil {
 			return err
 		}
@@ -321,13 +329,21 @@ func execAsDocker(ctx context.Context, step actionStep, actionName, basedir stri
 				}
 				defer buildContext.Close()
 			}
-			prepImage = container.NewDockerBuildExecutor(container.NewDockerBuildExecutorInput{
+			prepImage = ContainerNewDockerBuildExecutor(container.NewDockerBuildExecutorInput{
 				ContextDir:   contextDir,
 				Dockerfile:   fileName,
 				ImageTag:     image,
 				BuildContext: buildContext,
 				Platform:     rc.Config.ContainerArchitecture,
 			})
+			if buildContext == nil {
+				// Held across the whole build: the daemon drains contextDir lazily.
+				inner := prepImage
+				prepImage = func(ctx context.Context) error {
+					defer git.AcquireCloneLock(actionDir)()
+					return inner(ctx)
+				}
+			}
 		} else {
 			logger.Debugf("image '%s' for architecture '%s' already exists", image, rc.Config.ContainerArchitecture)
 		}

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")
+	}
+}

act/runner/reusable_workflow.go

@@ -142,9 +142,16 @@ func cloneRemoteReusableWorkflow(rc *RunContext, cloneURL, ref, targetDirectory,
 	}
 }
 
+var modelNewWorkflowPlanner = model.NewWorkflowPlanner
+
 func newReusableWorkflowExecutor(rc *RunContext, directory, workflow string) common.Executor {
 	return func(ctx context.Context) error {
-		planner, err := model.NewWorkflowPlanner(path.Join(directory, workflow), true)
+		// Scoped to the yaml read so concurrent invocations don't serialize
+		// on the whole job run.
+		planner, err := func() (model.WorkflowPlanner, error) {
+			defer git.AcquireCloneLock(directory)()
+			return modelNewWorkflowPlanner(path.Join(directory, workflow), true)
+		}()
 		if err != nil {
 			return err
 		}

act/runner/reusable_workflow_test.go

@@ -5,11 +5,15 @@ package runner
 
 import (
 	"context"
+	"errors"
 	"os"
 	"os/exec"
 	"path/filepath"
+	"sync"
 	"testing"
+	"time"
 
+	"gitea.com/gitea/runner/act/common/git"
 	"gitea.com/gitea/runner/act/model"
 
 	"github.com/stretchr/testify/require"
@@ -71,6 +75,54 @@ func TestReusableWorkflowCachedBranchRefRefreshes(t *testing.T) {
 	require.Equal(t, tmpl("v2"), string(got), "cached workflow file must reflect the updated branch tip")
 }
 
+func TestNewReusableWorkflowExecutorHoldsCloneLock(t *testing.T) {
+	workflowDir := t.TempDir()
+
+	unlockOnce := sync.OnceFunc(git.AcquireCloneLock(workflowDir))
+	defer unlockOnce()
+
+	plannerCalled := make(chan struct{})
+
+	origPlanner := modelNewWorkflowPlanner
+	modelNewWorkflowPlanner = func(string, bool) (model.WorkflowPlanner, error) {
+		close(plannerCalled)
+		return nil, errors.New("stop")
+	}
+	defer func() { modelNewWorkflowPlanner = origPlanner }()
+
+	rc := &RunContext{
+		Config: &Config{},
+		Run:    &model.Run{Workflow: &model.Workflow{Jobs: map[string]*model.Job{}}},
+	}
+	exec := newReusableWorkflowExecutor(rc, workflowDir, "reusable.yml")
+
+	done := make(chan error, 1)
+	go func() { done <- exec(context.Background()) }()
+
+	select {
+	case <-plannerCalled:
+		t.Fatal("planner ran while clone lock was held")
+	case err := <-done:
+		t.Fatalf("executor returned before planner was reached: %v", err)
+	case <-time.After(50 * time.Millisecond):
+	}
+
+	unlockOnce()
+
+	select {
+	case <-plannerCalled:
+	case <-time.After(time.Second):
+		t.Fatal("planner not called after lock was released")
+	}
+
+	select {
+	case err := <-done:
+		require.Error(t, err)
+	case <-time.After(time.Second):
+		t.Fatal("executor did not return after planner ran")
+	}
+}
+
 func gitMust(t *testing.T, dir string, args ...string) {
 	t.Helper()
 	cmd := exec.Command("git", args...)

act/runner/step_action_remote.go

@@ -145,6 +145,7 @@ func (sar *stepActionRemote) prepareActionExecutor() common.Executor {
 		return common.NewPipelineExecutor(
 			ntErr,
 			func(ctx context.Context) error {
+				defer git.AcquireCloneLock(actionDir)()
 				actionModel, err := sar.readAction(ctx, sar.Step, actionDir, sar.remoteAction.Path, remoteReader(ctx), os.WriteFile)
 				sar.action = actionModel
 				return err