add missing bazel build file

beacon-chain/forkchoice/borrower/BUILD.bazel

@@ -0,0 +1,19 @@
+load("@prysm//tools/go:def.bzl", "go_library", "go_test")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["borrower.go"],
+    importpath = "github.com/prysmaticlabs/prysm/v4/beacon-chain/forkchoice/borrower",
+    visibility = ["//visibility:public"],
+    deps = ["//beacon-chain/forkchoice:go_default_library"],
+)
+
+go_test(
+    name = "go_default_test",
+    srcs = ["borrower_test.go"],
+    deps = [
+        ":go_default_library",
+        "//beacon-chain/forkchoice/doubly-linked-tree:go_default_library",
+        "//testing/require:go_default_library",
+    ],
+)

beacon-chain/forkchoice/borrower/borrower.go

@@ -0,0 +1,42 @@
+package borrower
+
+import "github.com/prysmaticlabs/prysm/v4/beacon-chain/forkchoice"
+
+func NewForkchoiceBorrower(fc forkchoice.ForkChoicer) *ForkchoiceBorrower {
+	return &ForkchoiceBorrower{fc: fc}
+}
+
+type ForkchoiceBorrower struct {
+	fc forkchoice.ForkChoicer
+}
+
+type BorrowedForkChoicer struct {
+	forkchoice.ForkChoicer
+	unlock func()
+}
+
+// Return ends the borrow lifecycle, calling the (Unlock/RUlock) method and nilling the
+// pointer to the underlying forkchoice object so that it can no longer be used by the caller.
+func (b *BorrowedForkChoicer) Return() {
+	if b.unlock != nil {
+		b.unlock()
+	}
+	b.ForkChoicer = nil
+}
+
+// Borrow is used to borrow the Forkchoicer. When the borrower is done with it, they must
+// call the function, passing in the returned Forkchoicer, to invalidate the pointer.
+func (fb *ForkchoiceBorrower) Borrow() (*BorrowedForkChoicer, func()) {
+	fb.fc.Lock()
+	b := &BorrowedForkChoicer{ForkChoicer: fb.fc, unlock: fb.fc.Unlock}
+	return b, b.Return
+}
+
+// BorrowRO is an implementation of Borrow that uses read locks rather than write locks.
+// Borrow is used to borrow the Forkchoicer. When the borrower is done with it, they must
+// call the function, passing in the returned Forkchoicer, to invalidate the pointer.
+func (fb *ForkchoiceBorrower) BorrowRO() (*BorrowedForkChoicer, func()) {
+	fb.fc.RLock()
+	b := &BorrowedForkChoicer{ForkChoicer: fb.fc, unlock: fb.fc.RUnlock}
+	return b, b.Return
+}

beacon-chain/forkchoice/borrower/borrower_test.go

@@ -0,0 +1,19 @@
+package borrower_test
+
+import (
+	"testing"
+
+	"github.com/prysmaticlabs/prysm/v4/beacon-chain/forkchoice/borrower"
+	doublylinkedtree "github.com/prysmaticlabs/prysm/v4/beacon-chain/forkchoice/doubly-linked-tree"
+	"github.com/prysmaticlabs/prysm/v4/testing/require"
+)
+
+func TestBorrowNilness(t *testing.T) {
+	realFC := doublylinkedtree.New()
+	fb := borrower.NewForkchoiceBorrower(realFC)
+	fc, r := fb.Borrow()
+	// just calling a method to make sure this doesn't panic
+	require.Equal(t, 0, fc.NodeCount())
+	r()
+	require.Equal(t, nil, fc.ForkChoicer)
+}