Implement some primitive cache locking.

internal/cachedirectory/cachedirectory.go

@@ -103,10 +103,43 @@ func (cacheDirectory *CacheDirectory) CheckOrCreateVersionFile(pull bool, versio
 	return errors.New(errorPushNonCache)
 }
 
+func (cacheDirectory *CacheDirectory) Lock() error {
+	file, err := os.Create(cacheDirectory.lockFilePath())
+	if err != nil {
+		return errors.Wrap(err, "Error locking cache directory.")
+	}
+	defer file.Close()
+	// If the cache directory is already locked, it's not really a huge issue since the purpose of the lock is mostly to check whether a `pull` operation was interrupted before pushing.
+	return nil
+}
+
+func (cacheDirectory *CacheDirectory) Unlock() error {
+	err := os.Remove(cacheDirectory.lockFilePath())
+	if err != nil {
+		return errors.Wrap(err, "Error unlocking cache directory.")
+	}
+	return nil
+}
+
+func (cacheDirectory *CacheDirectory) CheckLock() error {
+	_, err := os.Stat(cacheDirectory.lockFilePath())
+	if err == nil {
+		return errors.New("The cache directory is locked, likely due to a `pull` command being interrupted. Please run `pull` again to ensure all required data is downloaded.")
+	}
+	if os.IsNotExist(err) {
+		return nil
+	}
+	return errors.Wrap(err, "Error checking if cache directory is locked.")
+}
+
 func (cacheDirectory *CacheDirectory) versionFilePath() string {
 	return path.Join(cacheDirectory.path, ".codeql-actions-sync-version")
 }
 
+func (cacheDirectory *CacheDirectory) lockFilePath() string {
+	return path.Join(cacheDirectory.path, ".codeql-actions-sync-lock")
+}
+
 func (cacheDirectory *CacheDirectory) GitPath() string {
 	return path.Join(cacheDirectory.path, "git")
 }

internal/cachedirectory/cachedirectory_test.go

@@ -88,3 +88,14 @@ func TestCreateCacheDirectoryWithTrailingSlash(t *testing.T) {
 	err := cacheDirectory.CheckOrCreateVersionFile(true, aVersion)
 	require.NoError(t, err)
 }
+
+func TestLocking(t *testing.T) {
+	temporaryDirectory := test.CreateTemporaryDirectory(t)
+	cacheDirectory := NewCacheDirectory(path.Join(temporaryDirectory, "cache"))
+	require.NoError(t, cacheDirectory.CheckOrCreateVersionFile(true, aVersion))
+	require.NoError(t, cacheDirectory.Lock())
+	require.NoError(t, cacheDirectory.Lock())
+	require.Error(t, cacheDirectory.CheckLock())
+	require.NoError(t, cacheDirectory.Unlock())
+	require.NoError(t, cacheDirectory.CheckLock())
+}

internal/pull/pull.go

@@ -237,6 +237,10 @@ func Pull(ctx context.Context, cacheDirectory cachedirectory.CacheDirectory, sou
 	if err != nil {
 		return err
 	}
+	err = cacheDirectory.Lock()
+	if err != nil {
+		return err
+	}
 
 	var tokenClient *http.Client
 	if sourceToken != "" {
@@ -266,6 +270,11 @@ func Pull(ctx context.Context, cacheDirectory cachedirectory.CacheDirectory, sou
 	if err != nil {
 		return err
 	}
+
+	err = cacheDirectory.Unlock()
+	if err != nil {
+		return err
+	}
 	log.Print("Finished pulling the CodeQL Action repository and bundles!")
 	return nil
 }

internal/push/push.go

@@ -291,6 +291,10 @@ func Push(ctx context.Context, cacheDirectory cachedirectory.CacheDirectory, des
 	if err != nil {
 		return err
 	}
+	err = cacheDirectory.CheckLock()
+	if err != nil {
+		return err
+	}
 
 	destinationURL = strings.TrimRight(destinationURL, "/")
 	tokenSource := oauth2.StaticTokenSource(