pointer: add Merge helper function for merging pointers (#15499)

This PR adds Merge() helper function for choosing which value of two pointers
to use during a larger merge operation.

If 'next' is not nil, use that value, otherwise use the 'previous' value.

helper/pointer/pointer.go

@@ -5,6 +5,12 @@ import (
 	"golang.org/x/exp/constraints"
 )
 
+// Primitive represents basic types that are safe to do basic comparisons by
+// pointer dereference (checking nullity first).
+type Primitive interface {
+	constraints.Ordered | bool
+}
+
 // Of returns a pointer to a.
 func Of[A any](a A) *A {
 	return &a
@@ -19,10 +25,12 @@ func Copy[A any](a *A) *A {
 	return &na
 }
 
-// Primitive represents basic types that are safe to do basic comparisons by
-// pointer dereference (checking nullity first).
-type Primitive interface {
-	constraints.Ordered // just so happens to be the types we want
+// Merge will return Copy(next) if next is not nil, otherwise return Copy(previous).
+func Merge[P Primitive](previous, next *P) *P {
+	if next != nil {
+		return Copy(next)
+	}
+	return Copy(previous)
 }
 
 // Eq returns whether a and b are equal in underlying value.

helper/pointer/pointer_test.go

@@ -4,10 +4,13 @@ import (
 	"testing"
 	"time"
 
+	"github.com/hashicorp/nomad/ci"
 	"github.com/shoenig/test/must"
 )
 
 func Test_Of(t *testing.T) {
+	ci.Parallel(t)
+
 	s := "hello"
 	sPtr := Of(s)
 
@@ -19,6 +22,8 @@ func Test_Of(t *testing.T) {
 }
 
 func Test_Copy(t *testing.T) {
+	ci.Parallel(t)
+
 	orig := Of(1)
 	dup := Copy(orig)
 	orig = Of(7)
@@ -27,6 +32,8 @@ func Test_Copy(t *testing.T) {
 }
 
 func Test_Compare(t *testing.T) {
+	ci.Parallel(t)
+
 	t.Run("int", func(t *testing.T) {
 		a := 1
 		b := 2
@@ -65,3 +72,23 @@ func Test_Compare(t *testing.T) {
 		must.True(t, Eq(n, nil))
 	})
 }
+
+func Test_Merge(t *testing.T) {
+	ci.Parallel(t)
+	
+	a := 1
+	b := 2
+
+	ptrA := &a
+	ptrB := &b
+
+	t.Run("exists", func(t *testing.T) {
+		result := Merge(ptrA, ptrB)
+		must.Eq(t, 2, *result)
+	})
+
+	t.Run("nil", func(t *testing.T) {
+		result := Merge(ptrA, nil)
+		must.Eq(t, 1, *result)
+	})
+}