storage/inmemory: implement patch operation (#4654)

Add a new Patch() method that updates specific fields of an existing
record's data, based on a field mask.

Extract some logic from the existing Get() and Put() methods so it can
be shared with the new Patch() method.

pkg/storage/patch_test.go

@@ -0,0 +1,45 @@
+package storage_test
+
+import (
+	"testing"
+	"time"
+
+	"google.golang.org/protobuf/proto"
+	"google.golang.org/protobuf/types/known/anypb"
+	"google.golang.org/protobuf/types/known/fieldmaskpb"
+	"google.golang.org/protobuf/types/known/timestamppb"
+
+	"github.com/pomerium/pomerium/internal/testutil"
+	"github.com/pomerium/pomerium/pkg/grpc/databroker"
+	"github.com/pomerium/pomerium/pkg/grpc/session"
+	"github.com/pomerium/pomerium/pkg/storage"
+)
+
+func TestPatchRecord(t *testing.T) {
+	tm := timestamppb.New(time.Date(2023, 10, 31, 12, 0, 0, 0, time.UTC))
+
+	s1 := &session.Session{Id: "session-id"}
+	a1, _ := anypb.New(s1)
+	r1 := &databroker.Record{Data: a1}
+
+	s2 := &session.Session{Id: "new-session-id", AccessedAt: tm}
+	a2, _ := anypb.New(s2)
+	r2 := &databroker.Record{Data: a2}
+
+	originalR1 := proto.Clone(r1).(*databroker.Record)
+
+	m, _ := fieldmaskpb.New(&session.Session{}, "accessed_at")
+
+	storage.PatchRecord(r1, r2, m)
+
+	testutil.AssertProtoJSONEqual(t, `{
+		"data": {
+			"@type": "type.googleapis.com/session.Session",
+			"accessedAt": "2023-10-31T12:00:00Z",
+			"id": "session-id"
+		}
+	}`, r2)
+
+	// The existing record should not be modified.
+	testutil.AssertProtoEqual(t, originalR1, r1)
+}